Scripting Languages I: Node.js, Python, PHP, Ruby

a side-by-side reference sheet

sheet one: version | grammar and execution | variables and expressions | arithmetic and logic | strings | regexes | dates and time | arrays | dictionaries | functions | execution control | exceptions | threads

sheet two: streams | asynchronous events | files | file formats | directories | processes and environment | option parsing | libraries and namespaces | objects | reflection | net and web | databases | unit tests | debugging

sheet two: streams | asynchronous events | files | directories | processes and environment | option parsing | libraries and namespaces | objects | inheritance and polymorphism | reflection | net and web | gui | databases | unit tests | logging | debugging

The versions used for testing code in the reference sheet.

How to get the version.

php:

The function phpversion() will return the version number as a string.

python:

The following function will return the version number as a string:

import platform platform.python_version()

ruby:

Also available in the global constant RUBY_VERSION .

Code which examples in the sheet assume to have already been executed.

javascript:

underscore.js adds some convenience functions as attributes of an object which is normally stored in the underscore _ variable. E.g.:

_.map([1, 2, 3], function(n){ return n * n; });

cdnjs hosts underscore.js and other JavaScript libraries for situations where it is inconvenient to have the webserver host the libraries.

When using underscore.js with the Node REPL, there is a conflict, since the Node REPL uses the underscore _ variable to store the result of the last evaluation.

$ npm install underscore $ node > var us = require('underscore'); _ > us.keys({"one": 1, "two": 2}); [ 'one', 'two' ]

php:

The mbstring package adds UTF-8 aware string functions with mb_ prefixes.

python:

We assume that os , re , and sys are always imported.

The customary name of the interpreter and how to invoke it.

php:

php -f will only execute portions of the source file within a <?php php code ?> tag as php code. Portions of the source file outside of such tags is not treated as executable code and is echoed to standard out.

If short tags are enabled, then php code can also be placed inside <? php code ?> and <?= php code ?> tags.

<?= php code ?> is identical to <?php echo php code ?>.

The customary name of the repl.

php:

The php -a REPL does not save or display the result of an expression.

python:

The python repl saves the result of the last statement in _.

ruby:

irb saves the result of the last statement in _.

How to pass the code to be executed to the interpreter as a command line argument.

How blocks are delimited.

python:

Python blocks begin with a line that ends in a colon. The block ends with the first line that is not indented further than the initial line. Python raises an IndentationError if the statements in the block that are not in a nested block are not all indented the same. Using tabs in Python source code is unrecommended and many editors replace them automatically with spaces. If the Python interpreter encounters a tab, it is treated as 8 spaces.

The python repl switches from a >>> prompt to a … prompt inside a block. A blank line terminates the block.

Colons are also used to separate keys from values in dictionary literals and in sequence slice notation.

ruby:

Curly brackets {} delimit blocks. A matched curly bracket pair can be replaced by the do and end keywords. By convention curly brackets are used for one line blocks.

The end keyword also terminates blocks started by def , class , or module .

Curly brackets are also used for hash literals, and the #{ } notation is used to interpolate expressions into strings.

How the parser determines the end of a statement.

php:

Inside braces statements must be terminated by a semicolon. The following causes a parse error:

<? if (true) { echo "true" } ?>

The last statement inside <?= ?> or <? ?> tags does not need to be semicolon terminated, however. The following code is legal:

<?= $a = 1 ?> <? echo $a ?>

python:

Newline does not terminate a statement when:

inside parens

inside list [] or dictionary {} literals

Python single quote '' and double quote "" strings cannot contain newlines except as the two character escaped form

. Putting a newline in these strings results in a syntax error. There is however a multi-line string literal which starts and ends with three single quotes ''' or three double quotes: """.

A newline that would normally terminate a statement can be escaped with a backslash.

ruby:

Newline does not terminate a statement when:

inside single quotes '', double quotes "", backticks ``, or parens ()

after an operator such as + or , that expects another argument

Ruby permits newlines in array [] or hash literals, but only after a comma , or associator =>. Putting a newline before the comma or associator results in a syntax error.

A newline that would normally terminate a statement can be escaped with a backslash.

How to identify the character encoding for a source code file.

Setting the source code encoding makes it possible to safely use non-ASCII characters in string literals and regular expression literals.

How to create a comment that ends at the next newline.

How to comment out multiple lines.

python:

The triple single quote ''' and triple double quote """ syntax is a syntax for string literals.

How to declare variables which are local to the scope defining region which immediately contain them.

php:

Variables do not need to be declared and there is no syntax for declaring a local variable. If a variable with no previous reference is accessed, its value is NULL.

python:

A variable is created by assignment if one does not already exist. If the variable is inside a function or method, then its scope is the body of the function or method. Otherwise it is a global.

ruby:

Variables are created by assignment. If the variable does not have a dollar sign ($) or ampersand (@) as its first character then its scope is scope defining region which most immediately contains it.

A lower case name can refer to a local variable or method. If both are defined, the local variable takes precedence. To invoke the method make the receiver explicit: e.g. self.name. However, outside of class and modules local variables hide functions because functions are private methods in the class Object. Assignment to name will create a local variable if one with that name does not exist, even if there is a method name.

How to define a variable with scope bound by the source file.

How to declare and access a variable with global scope.

php:

A variable is global if it is used at the top level (i.e. outside any function definition) or if it is declared inside a function with the global keyword. A function must use the global keyword to access the global variable.

python:

A variable is global if it is defined at the top level of a file (i.e. outside any function definition). Although the variable is global, it must be imported individually or be prefixed with the module name prefix to be accessed from another file. To be accessed from inside a function or method it must be declared with the global keyword.

ruby:

A variable is global if it starts with a dollar sign: $.

How to declare a constant.

php:

A constant can be declared inside a class:

class Math { const pi = 3.14; }

Refer to a class constant like this:

Math::pi

ruby:

Capitalized variables contain constants and class/module names. By convention, constants are all caps and class/module names are camel case. The ruby interpreter does not prevent modification of constants, it only gives a warning. Capitalized variables are globally visible, but a full or relative namespace name must be used to reach them: e.g. Math::PI.

How to assign a value to a variable.

python:

If the variable on the left has not previously been defined in the current scope, then it is created. This may hide a variable in a containing scope.

Assignment does not return a value and cannot be used in an expression. Thus, assignment cannot be used in a conditional test, removing the possibility of using assignment (=) when an equality test (==) was intended. Assignments can nevertheless be chained to assign a value to multiple variables:

a = b = 3

ruby:

Assignment operators have right precedence and evaluate to the right argument, so they can be chained. If the variable on the left does not exist, then it is created.

How to assign values to variables in parallel.

python:

The r-value can be a list or tuple:

nums = [1, 2, 3] a, b, c = nums more_nums = (6, 7, 8) d, e, f = more_nums

Nested sequences of expression can be assigned to a nested sequences of l-values, provided the nesting matches. This assignment will set a to 1, b to 2, and c to 3:

(a,[b,c]) = [1,(2,3)]

This assignment will raise a TypeError :

(a,(b,c)) = ((1,2),3)

In Python 3 the splat operator * can be used to collect the remaining right side elements in a list:

x, y, *z = 1, 2 # assigns [] to z x, y, *z = 1, 2, 3 # assigns [3] to z x, y, *z = 1, 2, 3, 4 # assigns [3, 4] to z

ruby:

The r-value can be an array:

nums = [1, 2, 3] a,b,c = nums

How to swap the values held by two variables.

Compound assignment operators mutate a variable, setting it to the value of an operation which takes the previous value of the variable as an argument.

If <OP> is a binary operator and the language has the compound assignment operator <OP>= , then the following are equivalent:

x <OP>= y x = x <OP> y

The compound assignment operators are displayed in this order:

First row: arithmetic operator assignment: addition, subtraction, multiplication, (float) division, integer division, modulus, and exponentiation.

Second row: string concatenation assignment and string replication assignment

Third row: logical operator assignment: and, or, xor

Fourth row: bit operator assignment: left shift, right shift, and, or, xor.

python:

Python compound assignment operators do not return a value and hence cannot be used in expressions.

The C-style increment and decrement operators can be used to increment or decrement values. They return values and thus can be used in expressions. The prefix versions return the value in the variable after mutation, and the postfix version return the value before mutation.

Incrementing a value two or more times in an expression makes the order of evaluation significant:

x = 1; foo(++x, ++x); // foo(2, 3) or foo(3, 2)? x = 1; y = ++x/++x; // y = 2/3 or y = 3/2?

Python avoids the problem by not having an in-expression increment or decrement.

Ruby mostly avoids the problem by providing a non-mutating increment and decrement. However, here is a Ruby expression which is dependent on order of evaluation:

x = 1 y = (x += 1)/(x += 1)

php:

The increment and decrement operators also work on strings. There are postfix versions of these operators which evaluate to the value before mutation:

$x = 1; $x++; $x--;

ruby:

The Integer class defines succ , pred , and next , which is a synonym for succ .

The String class defines succ , succ! , next , and next! . succ! and next! mutate the string.

The null literal.

How to test if a variable contains null.

php:

$v == NULL does not imply that $v is NULL, since any comparison between NULL and a falsehood will return true. In particular, the following comparisons are true:

$v = NULL; if ($v == NULL) { echo "true"; } $v = 0; if ($v == NULL) { echo "sadly true"; } $v = ''; if ($v == NULL) { echo "sadly true"; }

The result of attempting to access an undefined variable.

python:

Because a class can implement an eq method to change the implementation of v == None , the expression can be True when v is not None .

php:

PHP does not provide the programmer with a mechanism to distinguish an undefined variable from a variable which has been set to NULL.

A test showing that isset is the logical negation of is_null .

python:

How to test if a variable is defined:

not_defined = False try: v except NameError: not_defined = True

ruby:

How to test if a variable is defined:

! defined?(v)

How to write a conditional expression. A ternary operator is an operator which takes three arguments. Since

condition ? true value : false value

is the only ternary operator in C, it is unambiguous to refer to it as the ternary operator.

python:

The Python conditional expression comes from Algol.

ruby:

The Ruby if statement is also an expression:

x = if x > 0 x else -x end

Literals for the booleans.

These are the return values of the relational operators.

php:

Any identifier which matches TRUE case-insensitive can be used for the TRUE boolean. Similarly for FALSE.

In general, PHP variable names are case-sensitive, but function names are case-insensitive.

When converted to a string for display purposes, TRUE renders as "1" and FALSE as "". The equality tests TRUE == 1 and FALSE == "" evaluate as TRUE but the equality tests TRUE === 1 and FALSE === "" evaluate as FALSE.

Values which behave like the false boolean in a conditional context.

Examples of conditional contexts are the conditional clause of an if statement and the test of a while loop.

python:

Whether a object evaluates to True or False in a boolean context can be customized by implementing a __nonzero__ (Python 2) or __bool__ (Python 3) instance method for the class.

Logical and, or, and not.

php, ruby:

&& and || have higher precedence than assignment, compound assignment, and the ternary operator (?:), which have higher precedence than and and or.

Equality, inequality, greater than, less than, greater than or equal, less than or equal.

php:

Most of the relational operators will convert a string to a number if the other operand is a number. Thus 0 == "0" is true. The operators === and !== do not perform this conversion, so 0 === "0" is false.

python:

Relational operators can be chained. The following expressions evaluate to true:

1 < 2 < 3 1 == 1 != 2

In general if A i are expressions and op i are relational operators, then

A 1 op 1 A 2 op 2 A 3 … A n op n A n+1

is true if and only if each of the following is true

A 1 op 1 A 2

A 2 op 2 A 3

…

A n op n A n+1

How to get the min and max.

The operators for addition, subtraction, multiplication, float division, integer division, modulus, and exponentiation.

How to get the integer quotient of two integers.

How to get the quotient and remainder with single function call.

What happens when an integer is divided by zero.

How to perform floating point division, even if the operands might be integers.

What happens when a float is divided by zero.

How to get the value of a number raised to a power.

The square root function.

The result of taking the square root of negative one.

Some mathematical functions. Trigonometric functions are in radians unless otherwise noted. Logarithms are natural unless otherwise noted.

python:

Python also has math.log10. To compute the log of x for base b, use:

math.log(x)/math.log(b)

ruby:

Ruby also has Math.log2, Math.log10. To compute the log of x for base b, use

Math.log(x)/Math.log(b)

Constants for π and Euler's constant.

How to truncate a float to the nearest integer towards zero; how to round a float to the nearest integer; how to find the nearest integer above a float; how to find the nearest integer below a float; how to take the absolute value.

How to get the absolute value of a number.

What happens when the largest representable integer is exceeded.

What happens when the largest representable float is exceeded.

How to create rational numbers and get the numerator and denominator.

ruby:

Require the library mathn and integer division will yield rationals instead of truncated integers.

python:

Most of the functions in math have analogues in cmath which will work correctly on complex numbers.

How to generate a random integer between 0 and 99, include, float between zero and one in a uniform distribution, or a float in a normal distribution with mean zero and standard deviation one.

How to set the random seed; how to get the current random seed and later restore it.

All the languages in the sheet set the seed automatically to a value that is difficult to predict. The Ruby MRI interpreter uses the current time and process ID, for example. As a result there is usually no need to set the seed.

Setting the seed to a hardcoded value yields a random but repeatable sequence of numbers. This can be used to ensure that unit tests which cover code using random numbers doesn't intermittently fail.

The seed is global state. If multiple functions are generating random numbers then saving and restoring the seed may be necessary to produce a repeatable sequence.

The bit operators for left shift, right shift, and, inclusive or, exclusive or, and negation.

Binary, octal, and hex integer literals

How to convert integers to strings of digits of a given base. How to convert such strings into integers.

python

Python has the functions bin , oct , and hex which take an integer and return a string encoding the integer in base 2, 8, and 16.

bin(42) oct(42) hex(42)

The type for a string of Unicode characters.

php:

PHP assumes all strings have single byte characters.

python:

In Python 2.7 the str type assumes single byte characters. A separate unicode type is available for working with Unicode strings.

In Python 3 the str type supports multibtye characters and the unicode type has been removed.

There is a mutable bytearray type and an immutable bytes type for working with sequences of bytes.

ruby:

The String type supports multibtye characters. All strings have an explicit Encoding .

The syntax for string literals.

python:

String literals may have a u prefix

u'lorem ipsum' u"lorem ipsum" u'''lorem ipsum''' u"""lorem ipsum"""

In Python 3, these are identical to literals without the u prefix.

In Python 2, these create unicode strings instead of str strings. Since the Python 2 unicode type corresponds to the Python 3 str type, portable code will use the u prefix.

ruby:

How to specify custom delimiters for single and double quoted strings. These can be used to avoid backslash escaping. If the left delimiter is (, [, or { the right delimiter must be ), ], or }, respectively.

s1 = %q(lorem ipsum) s2 = %Q(#{s1} dolor sit amet)

Whether newlines are permitted in string literals.

python:

Newlines are not permitted in single quote and double quote string literals. A string can continue onto the following line if the last character on the line is a backslash. In this case, neither the backslash nor the newline are taken to be part of the string.

Triple quote literals, which are string literals terminated by three single quotes or three double quotes, can contain newlines:

'''This is two lines''' """This is also two lines"""

Backslash escape sequences for inserting special characters into string literals.

unrecognized backslash escape sequence double quote single quote JavaScript PHP preserve backslash preserve backslash Python preserve backslash preserve backslash Ruby drop backslash preserve backslash

python:

When string literals have an r or R prefix there are no backslash escape sequences and any backslashes thus appear in the created string. The delimiter can be inserted into a string if it is preceded by a backslash, but the backslash is also inserted. It is thus not possible to create a string with an r or R prefix that ends in a backslash. The r and R prefixes can be used with single or double quotes:

r'C:\Documents and Settings\Admin' r"C:\Windows\System32"

The \uhhhh escapes are also available inside Python 2 Unicode literals. Unicode literals have a u prefiix:

u'lambda: \u03bb'

This syntax is also available in Python 3.3, but not Python 3.2. In Python 3.3 it creates a string of type str which has the same features as the unicode type of Python 2.7.

Here documents are strings terminated by a custom identifier. They perform variable substitution and honor the same backslash escapes as double quoted strings.

python:

Triple quotes honor the same backslash escape sequences as regular quotes, so triple quotes can otherwise be used like here documents:

s = '''here document there computer '''

ruby:

Put the customer identifier in single quotes to prevent variable interpolation and backslash escape interpretation:

s = <<'EOF' Ruby code uses #{var} type syntax to interpolate variables into strings. EOF

How to interpolate variables into strings.

python:

The f'1 + 1 = {1 + 1}' and f"1 + 1 = {1 + 1}" literals, which support variable interpolation and expression interpolation, are new in Python 3.6.

str.format will take named or positional parameters. When used with named parameters str.format can mimic the variable interpolation feature of the other languages.

A selection of variables in scope can be passed explicitly:

count = 3 item = 'ball' print('{count} {item}s'.format( count=count, item=item))

Python 3 has format_map which accepts a dict as an argument:

count = 3 item = 'ball' print('{count} {item}s'.format_map(locals()))

How to interpolate the result of evaluating an expression into a string.

How to create a string using a printf style format.

python:

The % operator will interpolate arguments into printf-style format strings.

The str.format with positional parameters provides an alternative format using curly braces {0}, {1}, … for replacement fields.

The curly braces are escaped by doubling:

'to insert parameter {0} into a format, use {{{0}}}'.format(3)

If the replacement fields appear in sequential order and aren't repeated, the numbers can be omitted:

'lorem {} {} {}'.format('ipsum', 13, 3.7)

Are strings mutable?

How to copy a string such that changes to the original do not modify the copy.

The string concatenation operator.

The string replication operator.

How to put a string into all caps or all lower case letters.

How to capitalize a string and the words in a string.

The examples lowercase non-initial letters.

php:

How to define a UTF-8 aware version of ucfirst . This version also puts the rest of the string in lowercase:

function mb_ucfirst($string, $encoding = "UTF-8") { $strlen = mb_strlen($string, $encoding); $firstChar = mb_substr($string, 0, 1, $encoding); $then = mb_substr(mb_strtolower($string), 1, $strlen - 1, $encoding); return mb_strtoupper($firstChar, $encoding) . $then; }

ruby:

Rails monkey patches the String class with the titleize method for capitalizing the words in a string.

How to remove whitespace from the ends of a string.

How to pad the edge of a string with spaces so that it is a prescribed length.

How to convert numeric data to string data.

How to control the number of digits in a float when converting it to a string.

python:

The number after the decimal controls the number of digits after the decimal:

>>> '%.2f' % math.pi '3.14'

The number after the decimal controls the total number of digits:

>>> '{:.3}'.format(math.pi) '3.14'

How to convert string data to numeric data.

php:

PHP converts a scalar to the desired type automatically and does not raise an error if the string contains non-numeric data. If the start of the string is not numeric, the string evaluates to zero in a numeric context.

python:

float and int raise an error if called on a string and any part of the string is not numeric.

ruby:

to_i and to_f always succeed on a string, returning the numeric value of the digits at the start of the string, or zero if there are no initial digits.

How to concatenate the elements of an array into a string with a separator.

How to split a string containing a separator into an array of substrings.

See also scan.

python:

str.split() takes simple strings as delimiters; use re.split() to split on a regular expression:

re.split('\s+', 'do re mi fa') re.split('\s+', 'do re mi fa', 1)

How to split a string in two.

javascript:

A regular expression is probably the best method for splitting a string in two:

var m = /^([^ ]+) (.+)/.exec("do re mi"); var first = m[1]; var rest = m[2];

This technique works when the delimiter is a fixed string:

var a = "do re mi".split(" "); var first = a[0]; var rest = a.splice(1).join(" ");

python:

Methods for splitting a string into three parts using the first or last occurrence of a substring:

'do re mi'.partition(' ') # returns ('do', ' ', 're mi') 'do re mi'.rpartition(' ') # returns ('do re', ' ', 'mi')

How to split a string with the delimiters preserved as separate elements.

How to test whether a string begins or ends with a substring.

How to get the length in characters of a string.

How to find the index of the leftmost occurrence of a substring in a string; how to find the index of the rightmost occurrence.

How to extract a substring from a string by index.

The type for an array of bytes.

How to convert an array of bytes to a string of Unicode characters.

How to convert a string of Unicode characters to an array of bytes.

How to look up the character in a string at an index.

Converting characters to ASCII codes and back.

The languages in this reference sheet do not have character literals, so characters are represented by strings of length one.

How to split a string into an array of single character strings.

How to apply a character mapping to a string.

python:

In Python 2, the string of lowercase letters is in string.lowercase instead of string.ascii_lowercase .

In Python 2, the maketrans function is in the module string instead of str .

How to remove all specified characters from a string; how to remove all but the specified characters from a string.

How to replace multiple adjacent occurrences of a character with a single occurrence.

Regular expressions or regexes are a way of specifying sets of strings. If a string belongs to the set, the string and regex "match". Regexes can also be used to parse strings.

The modern notation for regexes was introduced by Unix command line tools in the 1970s. POSIX standardized the notation into two types: extended regexes and the more archaic basic regexes. Perl regexes are extended regexes augmented by new character class abbreviations and a few other features introduced by the Perl interpreter in the 1990s. All the languages in this sheet use Perl regexes.

Any string that doesn't contain regex metacharacters is a regex which matches itself. The regex metacharacters are: [ ] . | ( ) * + ? { } ^ $ \

character classes: [ ] .

A character class is a set of characters in brackets: [ ]. When used in a regex it matches any character it contains.

Character classes have their own set of metacharacters: ^ - \ ]

The ^ is only special when it is the first character in the character class. Such a character class matches its complement; that is, any character not inside the brackets. When not the first character the ^ refers to itself.

The hyphen is used to specify character ranges: e.g. 0-9 or A-Z . When the hyphen is first or last inside the brackets it matches itself.

The backslash can be used to escape the above characters or the terminal character class delimiter: ] . It can be used in character class abbreviations or string backslash escapes.

The period . is a character class abbreviation which matches any character except for newline. In all languages the period can be made to match all characters. PHP uses the m modifier. Python uses the re.M flag. Ruby uses the s modifier.



character class abbreviations:

abbrev name character class \d digit [0-9] \D nondigit [^0-9] \h PHP: horizontal whitespace character

Ruby: hex digit PHP: [ \t]

Ruby: [0-9a-fA-F] \H PHP: not a horizontal whitespace character

Ruby: not a hex digit PHP: [^ \t]

Ruby: [^0-9a-fA-F] \s whitespace character [ \t\r

\f] \S non whitespace character [^ \t\r

\f] \v vertical whitespace character [\r

\f] \V not a vertical whitespace character [^\r

\f] \w word character [A-Za-z0-9_] \W non word character [^A-Za-z0-9_]

alternation and grouping: | ( )

The vertical pipe | is used for alternation and parens () for grouping.

A vertical pipe takes as its arguments everything up to the next vertical pipe, enclosing paren, or end of string.

Parentheses control the scope of alternation and the quantifiers described below. They are also used for capturing groups, which are the substrings which matched parenthesized parts of the regular expression. Each language numbers the groups and provides a mechanism for extracting them when a match is made. A parenthesized subexpression can be removed from the groups with this syntax: (?:expr)

quantifiers: * + ? { }

As an argument quantifiers take the preceding regular character, character class, or group. The argument can itself be quantified, so that ^a{4}*$ matches strings with the letter a in multiples of 4.

quantifier # of occurrences of argument matched * zero or more, greedy + one or more, greedy ? zero or one, greedy {m,n} m to n, greedy {n} exactly n {m,} m or more, greedy {,n} zero to n, greedy *? zero or more, lazy +? one or more, lazy {m,n}? m to n, lazy {m,}? m or more, lazy {,n}? zero to n, lazy

When there is a choice, greedy quantifiers will match the maximum possible number of occurrences of the argument. Lazy quantifiers match the minimum possible number.

anchors: ^ $

anchor matches ^ beginning of a string. In Ruby or when m modifier is used also matches right side of a newline $ end of a string. In Ruby or when m modifier is used also matches left side of a newline \A beginning of the string \b word boundary. In between a \w and a \W character or in between a \w character and the edge of the string \B not a word boundary. In between two \w characters or two \W characters \z end of the string \Z end of the string unless it is a newline, in which case it matches the left side of the terminal newline

escaping: \

To match a metacharacter, put a backslash in front of it. To match a backslash use two backslashes.

php:

PHP 5.3 still supports the EREG engine, though the functions which use it are deprecated. These include the split function and functions which start with ereg . The preferred functions are preg_split and the other functions with a preg prefix.

The literal for a regular expression; the literal for a regular expression with a custom delimiter.

javascript:

The constructor for a regular expression is:

var rx = RegExp("lorem|ipsum");

php:

PHP regex literals are strings. The first character is the delimiter and it must also be the last character. If the start delimiter is (, {, or [ the end delimiter must be ), }, or ], respectively.

Here are the signatures from the PHP manual for the preg functions used in this sheet:

array preg_split ( string $pattern , string $subject [, int $limit = -1 [, int $flags = 0 ]] ) int preg_match ( string $pattern , string $subject [, array &$matches [, int $flags = 0 [, int $offset = 0 ]]] ) mixed preg_replace ( mixed $pattern , mixed $replacement , mixed $subject [, int $limit = -1 [, int &$count ]] ) int preg_match_all ( string $pattern , string $subject [, array &$matches [, int $flags = PREG_PATTERN_ORDER [, int $offset = 0 ]]] )

python:

Python does not have a regex literal, but the re.compile function can be used to create regex objects.

Compiling regexes can always be avoided:

re.compile('\d{4}').search('1999') re.search('\d{4}', '1999') re.compile('foo').sub('bar', 'foo bar') re.sub('foo', 'bar', 'foo bar') re.compile('\w+').findall('do re me') re.findall('\w+', 'do re me')

The supported character class abbreviations.

Note that \h refers to horizontal whitespace (i.e. a space or tab) in PHP and a hex digit in Ruby. Similarly \H refers to something that isn't horizontal whitespace in PHP and isn't a hex digit in Ruby.

The supported character class abbreviations for sets of Unicode characters.

Each Unicode character belongs to one of these major categories:

C Other L Letter M Mark N Number P Punctuation S Symbol Z Separator

Each major category is subdivided into multiple minor categories. Each minor category has a two letter code, where the first letter is the major category. For example, Nd is "Number, decimal digit".

Download UnicodeData.txt to find out which major and minor category and character belongs to.

The supported anchors.

How to test whether a string matches a regular expression.

python:

The re.match function returns true only if the regular expression matches the beginning of the string. re.search returns true if the regular expression matches any substring of the of string.

ruby:

match is a method of both Regexp and String so can match with both

/1999/.match("1999")

and

"1999".match(/1999/)

When variables are involved it is safer to invoke the Regexp method because string variables are more likely to contain nil .

How to perform a case insensitive match test.

Modifiers that can be used to adjust the behavior of a regular expression.

The lists are not comprehensive. For all languages except Ruby there are additional modifiers.

modifier behavior e PHP: when used with preg_replace, the replacement string, after backreferences are substituted, is eval'ed as PHP code and the result is used as the replacement. g JavaScript: read all non-overlapping matches into an array. i, re.I all: ignores case. Upper case letters match lower case letters and vice versa. m, re.M JavaScript, PHP, Python: makes the ^ and $ match the right and left edge of newlines in addition to the beginning and end of the string.

Ruby: makes the period . match newline characters. o Ruby: performs variable interpolation #{ } only once per execution of the program. s, re.S PHP, Python: makes the period . match newline characters. x, re.X all: ignores whitespace (outside of [] character classes) and #-style comments in the regex.

python:

Python modifiers are bit flags. To use more than one flag at the same time, join them with bit or: |

There are alternative identifiers for the modifiers:

re.A re.ASCII re.I re.IGNORECASE re.M re.MULTILINE re.S re.DOTALL re.X re.VERBOSE

How to replace all occurrences of a matching pattern in a string with the provided substitution string.

php:

The number of occurrences replaced can be controlled with a 4th argument to preg_replace :

$s = "foo bar bar"; preg_replace('/bar/', "baz", $s, 1);

If no 4th argument is provided, all occurrences are replaced.

python:

The 3rd argument to sub controls the number of occurrences which are replaced.

s = 'foo bar bar' re.compile('bar').sub('baz', s, 1)

If there is no 3rd argument, all occurrences are replaced.

ruby:

The gsub operator returns a copy of the string with the substitution made, if any. The gsub! performs the substitution on the original string and returns the modified string.

The sub and sub! operators only replace the first occurrence of the match pattern.

How to get the substring that matched the regular expression, as well as the part of the string before and after the matching substring.

ruby:

The special variables $& , $` , and $' also contain the match, prematch, and postmatch.

How to get the substrings which matched the parenthesized parts of a regular expression.

ruby:

Ruby has syntax for extracting a group from a match in a single expression. The following evaluates to "1999":

"1999-07-08"[/(\d{4})-(\d{2})-(\d{2})/, 1]

How to get the substrings which matched the parenthesized parts of a regular expression and put them into a dictionary.

For reference, we call the (?P<foo>...) notation Python-style and the (?<foo>...) notation Perl-style.

php:

PHP originally supported Python-style named groups since that was the style that was added to the PCRE regex engine. Perl-style named groups were added to PHP 5.2.

python:

The Python interpreter was the first to support named groups.

How to return all non-overlapping substrings which match a regular expression as an array.

How to use backreferences in a regex; how to use backreferences in the replacement string of substitution.

An examples of a recursive regex.

The example matches substrings containing balanced parens.

In ISO 8601 terminology, a date specifies a day in the Gregorian calendar and a time does not contain date information; it merely specifies a time of day. A data type which combines both date and time information is convenient, but ISO 8601 doesn't provide a name for such an entity. PHP, Python, and C# use the compound noun datetime for combined date and time values and we adopt it here as a generic term.

An useful property of ISO 8601 dates, times, and datetimes is that they are correctly ordered by a lexical sort on their string representations. This is because they are big-endian (the year is the leftmost element) and they used fixed-length, zero-padded fields with numerical values for each term in the string representation.

The C standard library provides two methods for representing dates. The first is the Unix epoch, which is the seconds since the beginning of January 1, 1970 in UTC. If such a time were stored in a 32-bit signed integer, the rollover would happen on January 18, 2038. The Unix epoch is an example of a serial datetime, in which the value is stored as a single numeric value representing the difference in time in some unit from a specially designated datetime called the epoch.

Another serial datetime is the Windows file time, which is the number of 100 nanosecond intervals since the beginning of January 1, 1601 UTC. It was introduced when journaling was added to NTFS as part of the Windows 2000 launch.

Some serial datetimes use days as the unit. The Excel serial number is the number of days since December 31, 1899. The Julian day number, used in astronomy, is the number of days since November 24, 4714 BCE in the proleptic Gregorian calendar. Julian days start at noon GMT.

A broken-down datetime uses multiple numeric values to represent the components of a calendar date and time. An example from the C standard library is the tm struct, a definition of which can be found on Unix systems in /usr/include/time.h :

struct tm { int tm_sec; /* seconds after the minute [0-60] */ int tm_min; /* minutes after the hour [0-59] */ int tm_hour; /* hours since midnight [0-23] */ int tm_mday; /* day of the month [1-31] */ int tm_mon; /* months since January [0-11] */ int tm_year; /* years since 1900 */ int tm_wday; /* days since Sunday [0-6] */ int tm_yday; /* days since January 1 [0-365] */ int tm_isdst; /* Daylight Savings Time flag */ long tm_gmtoff; /* offset from CUT in seconds */ char *tm_zone; /* timezone abbreviation */ };

The Linux man pages call the tm struct a "broken-down" date and time, whereas the BSD man pages call it a "broken-out" date and time.

The first day in the Gregorian calendar was 15 October 1582. The proleptic Gregorian calendar is an extension of the Gregorian calendar to earlier dates. When such dates are used, they should be called out to be precise. The epoch in the proleptic Gregorian calendar is the year 0001, also written 1 AD or 1 CE. The previous year is the year 0000, also written 1 BC or 1 BCE. The year before that is the year -0001, also written 2 BC or 2 BCE. The ISO 8601 standard recommends that years before 0000 or after 9999 be written with a plus or minus sign prefix.

An ordinal date is a broken-down date consisting of a year, followed by the day of the year. The ISO 8601 standard recommends that it be written in YYYY-DDD or YYYYDDD format. The corresponding strftime formats are %Y-%j and %Y%j .

A week date is a type of calendar which uses the year, week of the year, and day of the week to refer to to dates. In the ISO 8601 week date, the first week of the year is the week starting from Monday which contains January 4th. An ISO 8601 week date can thus have a different year number than the corresponding Gregorian date. The first week of the year is numbered 01 , and the first day of the week, Monday, is numbered 1 . Weeks are written in YYYY-Www-D or YYYYWwwD format, where the upper case W is literal. The corresponding strftime literals are %G-W%V-%u and %GW%V%u .

Common years have 365 days and leap years have 366 days. The extra day in February 29th. Leap years are years divisible by 4 but not 100, or years divisible by 400.

In 1967, the definition of a second was changed from 1/86,400 of a solar day to a value expressed in terms of radiation produced by 133Cs. Because the length of a solar day is irregular, leap seconds are occasionally used to keep things in sync. This is accomplished by occasionally adding a leap second to the end of June 30th or December 31st. The system also allows for removing the last second of June 30th or December 31st, though as of 2014 this hasn't been done.

The data type used to hold a combined date and time.

python:

Python uses and exposes the tm struct of the C standard library. Python has a module called time which is a thin wrapper to the standard library functions which operate on this struct. Here is how get a tm struct in Python:

import time utc = time.gmtime(time.time()) t = time.localtime(time.time())

How to get the combined date and time for the present moment in both local time and UTC.

How to get the current time as a Unix epoch timestamp.

How to convert a datetime type to the Unix epoch which is the number of seconds since the start of January 1, 1970 UTC.

python:

The Python datetime object created by now() and utcnow() has no timezone information associated with it. The strftime() method assumes a receiver with no time zone information represents a local time. Thus it is an error to call strftime() on the return value of utcnow() .

Here are two different ways to get the current Unix epoch. The second way is faster:

import calendar import datetime int(datetime.datetime.now().strftime('%s')) calendar.timegm(datetime.datetime.utcnow().utctimetuple())

Replacing now() with utcnow() in the first way, or utcnow() with now() in the second way produces an incorrect value.

How to convert the Unix epoch to a broken-down datetime.

How to format a datetime as a string using using a string of format specifiers.

The format specifiers used by the strftime function from the standard C library and the Unix date command:

numeric alphanumeric notes year %Y %C%y %C and %y are the first two and last two digits of a 4 digit year month %m %B %b %h %m is zero padded in {01, …, 12}

%h is blank padded in {1, …, 12} day of month %d %e %d is zero padded in {01, …, 31}

%e is blank padded in {1, …, 31} hour %H %k %I%p %l%p %H and %k are in zero and blank padded minute %M %M is zero padded in the range {00, …, 59} second %S %S is zero padded, due to leap seconds it is in the range {00, …, 60} day of year %j %j is zero padded in the range {000, …, 366} week date year %G %g the ISO 8601 week date year. Used with %V and %u. week of year %V %U %W %V is the ISO 8601 week of year. In {01, 53}. Used with %G

%U is the week number when Sunday starts the week. In {00, 53}. Used with %Y and %C%y.

%W is the week number when Monday starts the week. In {00, 53}. Used with %Y and %C%y. day of week %u %w %A %a %u is in {{1, …, 7} starting at Monday

%w is in {0, …, 6} starting at Sunday unix epoch %s date %D %F %x %v %D is %m/%d/%y

%F is %Y-%m-%d

%x locale dependent; same as %D in US time %T %R %X %r %T is %H:%M:%S

%R is %H:%M

%X is locale dependent; same as %T in US

%r is %I:%M:%S %p date and time %c locale dependent date, time, and tmz %+ locale dependent time zone name %Z the ambiguous 3 letter abbrevation; e.g. "PST" time zone offset %z "-0800" for Pacific Standard Time percent sign %% newline %n tab %t

php:

PHP supports strftime but it also has its own time formatting system used by date , DateTime::format , and DateTime::createFromFormat . The letters used in the PHP time formatting system are described here.

How to parse a datetime using the format notation of the strptime function from the standard C library.

How to parse a date without providing a format string.

How to get the year, month, and day of month from a datetime.

How to the hour, minute, and second from a datetime.

How to build a broken-down datetime from the date parts and the time parts.

The data type that results when subtraction is performed on two combined date and time values.

How to add a duration to a datetime.

A duration can easily be added to a datetime value when the value is a Unix epoch value.

ISO 8601 distinguishes between a time interval, which is defined by two datetime endpoints, and a duration, which is the length of a time interval and can be defined by a unit of time such as '10 minutes'. A time interval can also be defined by date and time representing the start of the interval and a duration.

ISO 8601 defines notation for durations. This notation starts with a 'P' and uses a 'T' to separate the day and larger units from the hour and smaller units. Observing the location relative to the 'T' is important for interpreting the letter 'M', which is used for both months and minutes.

Do datetime values include time zone information. When a datetime value for the local time is created, how the local time zone is determined.

On Unix systems processes determine the local time zone by inspecting the binary file /etc/localtime . To examine it from the command line use zdump :

$ zdump /etc/localtime /etc/localtime Tue Dec 30 10:03:27 2014 PST

On Windows the time zone name is stored in the registry at HKLM\SYSTEM\CurrentControlSet\Control\TimeZoneInformation\TimeZoneKeyName .

php:

The default time zone can also be set in the php.ini file.

date.timezone = "America/Los_Angeles"

Here is the list of timezones supported by PHP.

How to convert a datetime to the equivalent datetime in an arbitrary time zone.

How to get the name of the time zone and the offset in hours from UTC.

Timezones are often identified by three or four letter abbreviations. Many of the abbreviations do not uniquely identify a time zone. Furthermore many of the time zones have been altered in the past. The Olson database (aka Tz database) decomposes the world into zones in which the local clocks have all been set to the same time since 1970; it gives these zones unique names.

ruby:

The Time class has a zone method which returns the time zone abbreviation for the object. There is a tzinfo gem which can be used to create time zone objects using the Olson database name. This can in turn be used to convert between UTC times and local times which are daylight saving aware.

Is a datetime in daylight savings time?

How to get the microseconds component of a combined date and time value. The SI abbreviations for milliseconds and microseconds are ms and μs , respectively. The C standard library uses the letter u as an abbreviation for micro . Here is a struct defined in /usr/include/sys/time.h :

struct timeval { time_t tv_sec; /* seconds since Jan. 1, 1970 */ suseconds_t tv_usec; /* and microseconds */ };

How to put the process to sleep for a specified number of seconds. In Python and Ruby the default version of sleep supports a fractional number of seconds.

php:

PHP provides usleep which takes an argument in microseconds:

usleep(500000);

How to cause a process to timeout if it takes too long.

Techniques relying on SIGALRM only work on Unix systems.

What the languages call their basic container types:

javascript php python ruby array array list, tuple, sequence Array, Enumerable dictionary array dict, mapping Hash

javascript:

php:

PHP uses the same data structure for arrays and dictionaries.

python:

Python has the mutable list and the immutable tuple. Both are sequences. To be a sequence, a class must implement __getitem__, __setitem__, __delitem__, __len__, __contains__, __iter__, __add__, __mul__, __radd__, and __rmul__.

ruby:

Ruby provides an Array datatype. If a class defines an each iterator and a comparison operator <=>, then it can mix in the Enumerable module.

Array literal syntax.

ruby:

The %w operator splits the following string on whitespace and creates an array of strings from the words. The character following the %w is the string delimiter. If the following character is (, [, or {, then the character which terminates the string must be ), ], or }.

The %W operator is like the %w operator, except that double-quote style #{ } expressions will be interpolated.

The quote words operator, which is a literal for arrays of strings where each string contains a single word.

How to get the number of elements in an array.

How to test whether an array is empty.

How to access a value in an array by index.

python:

A negative index refers to the length - index element.

>>> a = [1, 2, 3] >>> a[-1] 3

ruby:

A negative index refers to to the length - index element.

How to update the value at an index.

What happens when the value at an out-of-bounds index is referenced.

How to get the index of an element in an array.

php:

Setting the 3rd argument of array_search to true makes the search use === for an equality test. Otherwise the == test is performed, which makes use of implicit type conversions.

How to slice a subarray from an array by specifying a start index and an end index; how to slice a subarray from an array by specifying an offset index and a length index.

python:

Slices can leave the first or last index unspecified, in which case the first or last index of the sequence is used:

>>> a=[1, 2, 3, 4, 5] >>> a[:3] [1, 2, 3]

Python has notation for taking every nth element:

>>> a=[1, 2, 3, 4, 5] >>> a[::2] [1, 3, 5]

The third argument in the colon-delimited slice argument can be negative, which reverses the order of the result:

>>> a = [1, 2, 3, 4] >>> a[::-1] [4, 3, 2, 1]

How to slice to the end of an array.

The examples take all but the first element of the array.

How to add and remove elements from the back or high index end of an array.

These operations can be used to use the array as a stack.

How to add and remove elements from the front or low index end of an array.

These operations can be used to use the array as a stack. They can be used with the operations that manipulate the back of the array to use the array as a queue.

How to create an array by concatenating two arrays; how to modify an array by concatenating another array to the end of it.

How to create an array containing the same value replicated n times.

How to make an address copy, a shallow copy, and a deep copy of an array.

After an address copy is made, modifications to the copy also modify the original array.

After a shallow copy is made, the addition, removal, or replacement of elements in the copy does not modify of the original array. However, if elements in the copy are modified, those elements are also modified in the original array.

A deep copy is a recursive copy. The original array is copied and a deep copy is performed on all elements of the array. No change to the contents of the copy will modify the contents of the original array.

python:

The slice operator can be used to make a shallow copy:

a2 = a[:]

list(v) always returns a list, but v[:] returns a value of the same as v . The slice operator can be used in this manner on strings and tuples but there is little incentive to do so since both are immutable.

copy.copy can be used to make a shallow copy on types that don't support the slice operator such as a dictionary. Like the slice operator copy.copy returns a value with the same type as the argument.

How an array is passed to a function when provided as an argument.

How to iterate over the elements of an array.

How to iterate over the element-index pairs.

Iterate over a range without instantiating it as a list.

How to convert a range to an array.

Python 3 ranges and Ruby ranges implement some of the functionality of arrays without allocating space to hold all the elements.

python:

In Python 2 range() returns a list.

In Python 3 range() returns an object which implements the immutable sequence API.

ruby:

The Range class includes the Enumerable module.

How to create a reversed copy of an array, and how to reverse an array in place.

python:

reversed returns an iterator which can be used in a for/in construct:

print("counting down:") for i in reversed([1, 2, 3]): print(i)

reversed can be used to create a reversed list:

a = list(reversed([1, 2, 3]))

How to create a sorted copy of an array, and how to sort an array in place. Also, how to set the comparison function when sorting.

php:

usort sorts an array in place and accepts a comparison function as a 2nd argument:

function cmp($x, $y) { $lx = strtolower($x); $ly = strtolower($y); if ( $lx < $ly ) { return -1; } if ( $lx == $ly ) { return 0; } return 1; } $a = ["b", "A", "a", "B"]; usort($a, "cmp");

python:

In Python 2 it is possible to specify a binary comparision function when calling sort :

a = [(1, 3), (2, 2), (3, 1)] a.sort(cmp=lambda a, b: -1 if a[1] < b[1] else 1) # a now contains: [(3, 1), (2, 2), (1, 3)]

In Python 3 the cmp parameter was removed. One can achieve the same effect by defining cmp method on the class of the list element.

How to remove extra occurrences of elements from an array.

python:

Python sets support the len , in , and for operators. It may be more efficient to work with the result of the set constructor directly rather than convert it back to a list.

How to test for membership in an array.

How to compute an intersection.

python:

Python has literal notation for sets:

{1, 2, 3}

Use set and list to convert lists to sets and vice versa:

a = list({1, 2, 3}) ensemble = set([1, 2, 3])

ruby:

The intersect operator & always produces an array with no duplicates.

ruby:

The union operator | always produces an array with no duplicates.

How to compute the relative complement of two arrays or sets; how to compute the symmetric difference.

ruby:

If an element is in the right argument, then it will not be in the return value even if it is contained in the left argument multiple times.

Create an array by applying a function to each element of a source array.

ruby:

The map! method applies the function to the elements of the array in place.

collect and collect! are synonyms for map and map! .

Create an array containing the elements of a source array which match a predicate.

ruby:

The in place version is select! .

reject returns the complement of select . reject! is the in place version.

The partition method returns two arrays:

a = [1, 2, 3] lt2, ge2 = a.partition { |n| n < 2 }

Return the result of applying a binary operator to all the elements of the array.

python:

reduce is not needed to sum a list of numbers:

sum([1, 2, 3])

ruby:

The code for the reduction step can be provided by name. The name can be a symbol or a string:

[1, 2, 3].inject(:+) [1, 2, 3].inject("+") [1, 2, 3].inject(0, :+) [1, 2, 3].inject(0, "+")

How to test whether a condition holds for all members of an array; how to test whether a condition holds for at least one member of any array.

A universal test is always true for an empty array. An existential test is always false for an empty array.

A existential test can readily be implemented with a filter. A universal test can also be implemented with a filter, but it is more work: one must set the condition of the filter to the negation of the predicate and test whether the result is empty.

How to shuffle an array. How to extract a random sample from an array.

php:

The array_rand function returns a random sample of the indices of an array. The result can easily be converted to a random sample of array values:

$a = [1, 2, 3, 4]; $sample = []; foreach (array_rand($a, 2) as $i) { array_push($sample, $a[$i]); }

How to flatten nested arrays by one level or completely.

When nested arrays are flattened by one level, the depth of each element which is not in the top level array is reduced by one.

Flattening nested arrays completely leaves no nested arrays. This is equivalent to extracting the leaf nodes of a tree.

php, python:

To flatten by one level use reduce. Remember to handle the case where an element is not array.

To flatten completely write a recursive function.

How to interleave arrays. In the case of two arrays the result is an array of pairs or an associative list.

The syntax for a dictionary literal.

How to get the number of dictionary keys in a dictionary.

How to lookup a dictionary value using a dictionary key.

What happens when a lookup is performed on a key that is not in a dictionary.

python:

Use dict.get() to avoid handling KeyError exceptions:

d = {} d.get('lorem') # returns None d.get('lorem', '') # returns ''

How to check for the presence of a key in a dictionary without raising an exception. Distinguishes from the case where the key is present but mapped to null or a value which evaluates to false.

How to remove a key/value pair from a dictionary.

How to create a dictionary from an array of pairs; how to create a dictionary from an even length array.

How to merge the values of two dictionaries.

In the examples, if the dictionaries d1 and d2 share keys then the values from d2 will be used in the merged dictionary.

How to turn a dictionary into its inverse. If a key 'foo' is mapped to value 'bar' by a dictionary, then its inverse will map the key 'bar' to the value 'foo'. However, if multiple keys are mapped to the same value in the original dictionary, then some of the keys will be discarded in the inverse.

How to iterate through the key/value pairs in a dictionary.

python:

In Python 2.7 dict.items() returns a list of pairs and dict.iteritems() returns an iterator on the list of pairs.

In Python 3 dict.items() returns an iterator and dict.iteritems() has been removed.

How to convert the keys of a dictionary to an array; how to convert the values of a dictionary to an array.

python:

In Python 3 dict.keys() and dict.values() return read-only views into the dict. The following code illustrates the change in behavior:

d = {} keys = d.keys() d['foo'] = 'bar' if 'foo' in keys: print('running Python 3') else: print('running Python 2')

How to iterate through the key-value pairs in the order of the values.

How to create a dictionary with a default value for missing keys; how to compute and store the value on lookup.

php:

Extend ArrayObject to compute values on lookup:

class Factorial extends ArrayObject { public function offsetExists($i) { return true; } public function offsetGet($i) { if(!parent::offsetExists($i)) { if ( $i < 2 ) { parent::offsetSet($i, 1); } else { $n = $this->offsetGet($i-1); parent::offsetSet($i, $i*$n); } } return parent::offsetGet($i); } } $factorial = new Factorial();

Python has both functions and methods. Ruby only has methods: functions defined at the top level are in fact methods on a special main object. Perl subroutines can be invoked with a function syntax or a method syntax.

How to define a function.

How to invoke a function.

python:

Parens are mandatory, even for functions which take no arguments. Omitting the parens returns the function or method as an object. Whitespace can occur between the function name and the following left paren.

In Python 3 print is a function instead of a keyword; parens are mandatory around the argument.

ruby:

Ruby parens are optional. Leaving out the parens results in ambiguity when function invocations are nested. The interpreter resolves the ambiguity by assigning as many arguments as possible to the innermost function invocation, regardless of its actual arity. It is mandatory that the left paren not be separated from the method name by whitespace.

How to apply a function to an array.

perl:

Perl passes the elements of arrays as individual arguments. In the following invocation, the function foo() does not know which arguments came from which array. For that matter it does not know how many arrays were used in the invocation:

foo(@a, @b);

If the elements must be kept in their respective arrays the arrays must be passed by reference:

sub foo { my @a = @{$_[0]}; my @b = @{$_[1]}; } foo(\@a, \@b);

When hashes are used as arguments, each key and value becomes its own argument.

What happens when a function is invoked with too few arguments.

What happens when a function is invoked with too many arguments.

How to declare a default value for an argument.

How to write a function which accepts a variable number of argument.

python:

This function accepts one or more arguments. Invoking it without any arguments raises a TypeError :

def poker(dealer, *players): ...

ruby:

This function accepts one or more arguments. Invoking it without any arguments raises an ArgumentError :

def poker(dealer, *players) ... end

How to make a parameter an alias of a variable in the caller.

How to write a function which uses named parameters and how to invoke it.

python:

The caller can use named parameter syntax at the point of invocation even if the function was defined using positional parameters.

The splat operator * collects the remaining arguments into a list. In a function invocation, the splat can be used to expand an array into separate arguments.

The double splat operator ** collects named parameters into a dictionary. In a function invocation, the double splat expands a dictionary into named parameters.

A double splat operator can be used to force the caller to use named parameter syntax. This method has the disadvantage that spelling errors in the parameter name are not caught:

def fequal(x, y, **kwargs): eps = opts.get('eps') or 0.01 return abs(x - y) < eps

In Python 3 named parameters can be made mandatory:

def fequal(x, y, *, eps): return abs(x-y) < eps fequal(1.0, 1.001, eps=0.01) # True fequal(1.0, 1.001) # raises TypeError

ruby:

In Ruby 2.1 named parameters can be made mandatory:

def fequals(x, y, eps:) (x - y).abs < eps end # false: fequals(1.0, 1.001, eps: 0.1**10) # ArgumentError: fequals(1.0, 1.001)

How the return value of a function is determined.

How to return multiple values from a function.

The syntax for an anonymous function literal; i.e. a lambda function.

python:

Python lambdas cannot contain newlines or semicolons, and thus are limited to a single statement or expression. Unlike named functions, the value of the last statement or expression is returned, and a return is not necessary or permitted. Lambdas are closures and can refer to local variables in scope, even if they are returned from that scope.

If a closure function is needed that contains more than one statement, use a nested function:

def make_nest(x): b = 37 def nest(y): c = x*y c *= b return c return nest n = make_nest(12*2) print(n(23))

Python closures are read only.

A nested function can be returned and hence be invoked outside of its containing function, but it is not visible by its name outside of its containing function.

ruby:

The following lambda and Proc object behave identically:

sqr = lambda { |x| x * x } sqr = Proc.new {|x| x * x }

With respect to control words, Proc objects behave like blocks and lambdas like functions. In particular, when the body of a Proc object contains a return or break statement, it acts like a return or break in the code which invoked the Proc object. A return in a lambda merely causes the lambda to exit, and a break inside a lambda must be inside an appropriate control structure contained with the lambda body.

Ruby are alternate syntax for defining lambdas and invoking them:

sqr = ->(x) {x*x} sqr.(2)

The syntax for invoking an anonymous function.

How to store a function in a variable and pass it as an argument.

php:

If a variable containing a string is used like a function then PHP will look for a function with the name in the string and attempt to invoke it.

python:

Python function are stored in variables by default. As a result a function and a variable with the same name cannot share the same scope. This is also the reason parens are mandatory when invoking Python functions.

How to create a function with private state which persists between function invocations.

python:

Here is a technique for creating private state which exploits the fact that the expression for a default value is evaluated only once:

def counter(_state=[0]): _state[0] += 1 return _state[0] print(counter())

How to create a first class function with access to the local variables of the local scope in which it was created.

python:

Python 2 has limited closures: access to local variables in the containing scope is read only and the bodies of anonymous functions must consist of a single expression.

Python 3 permits write access to local variables outside the immediate scope when declared with nonlocal .

How to create a function which can yield a value back to its caller and suspend execution.

python:

A Python generator is a function which returns an iterator.

An iterator is an object with two methods: iter() , which returns the iterator itself, and next() , which returns the next item or raises a StopIteration exception.

Python sequences, of which lists are an example, define an iter() for returned an iterator which traverses the sequence.

Python iterators can be used in for/in statements and list comprehensions.

In the table below, p and q are variables for iterators.

itertools generator description count(start=0, step=1) arithmetic sequence of integers cyle(p) cycle over p endlessly repeat(v, [n]) return v n times, or endlessly chain(p, q) p followed by q compress(p, q) p if q groupby(p, func) ifilter(pred, p) p if pred(p) ifilterfalse(pred, p) p if not pred(p) islice(p, [start], stop, [step]) imap starmap() tee() takewhile() izip() izip_longest() product() permutations() combinations() combinations_with_replacement()

ruby:

Ruby generators are called fibers.

A decorator replaces an invocation of one function with another in a way that that is imperceptible to the client.

Normally a decorator will add a small amount of functionality to the original function which it invokes. A decorator can modify the arguments before passing them to the original function or modify the return value before returning it to the client. Or it can leave the arguments and return value unmodified but perform a side effect such as logging the call.

How to call an operator using the function invocation syntax.

This is useful when dealing with an API which accepts a function as an argument.

python:

The operator module provides functions which perform the same operations as the various operators. Using these functions is more efficient than wrapping the operators in lambdas.

ruby:

All operators can be invoked with method invocation syntax. The binary operator invocation syntax can be regarded as syntactic sugar.

The conditional branch statement.

php:

PHP has the following alternate syntax for if statements:

if ($n == 0): echo "no hits

"; elseif ($n == 1): echo "one hit

"; else: echo "$n hits

"; endif;

ruby:

If an if statement is the last statement executed in a function, the return value is the value of the branch that executed.

Ruby if statements are expressions. They can be used on the right hand side of assignments:

m = if n 1 else 0 end

A statement which branches based on the value of an expression.

How to loop over a block while a condition is true.

php:

PHP provides a do-while loop. The body of such a loop is guaranteed to execute at least once.

$i = 0; do { echo $i; } while ($i > 0);

ruby:

Ruby provides a loop with no exit condition:

def yes(expletive="y") loop do puts expletive end end

Ruby also provides the until loop.

Ruby loops can be used in expression contexts but they always evaluate to nil .

How to write a C-style for loop.

A break statement exits a while or for loop immediately.

A continue statement skips ahead to the next iteration of a while or for loop.

ruby:

There is also a redo statement, which restarts the current iteration of a loop.

Clauses added to the end of a statement to control execution.

Ruby has conditional statement modifiers. Ruby also has looping statement modifiers.

ruby:

Ruby has the looping statement modifiers while and until :

i = 0 i += 1 while i < 10 j = 10 j -= 1 until j < 0

The base exception type or class that can be used to catch all exceptions.

A list of the more commonly encountered exceptions.

python:

Code for inspecting the descendants of a base class:

def print_class_hierarchy(cls, indent=0): print(' ' * indent, cls.__name__) for subclass in cls.__subclasses__(): print_class_hierarchy(subclass, indent + 2)

The complete Python 3.5 exception hierarchy:

BaseException Exception TypeError ImportError ZipImportError AssertionError error ArithmeticError FloatingPointError OverflowError ZeroDivisionError SyntaxError IndentationError TabError OSError BlockingIOError TimeoutError PermissionError FileExistsError ConnectionError BrokenPipeError ConnectionAbortedError ConnectionResetError ConnectionRefusedError NotADirectoryError UnsupportedOperation ChildProcessError IsADirectoryError ItimerError InterruptedError FileNotFoundError ProcessLookupError BufferError ReferenceError MemoryError StopIteration StopAsyncIteration Error SystemError CodecRegistryError ValueError UnicodeError UnicodeEncodeError UnicodeDecodeError UnicodeTranslateError UnsupportedOperation NameError UnboundLocalError AttributeError Warning DeprecationWarning SyntaxWarning FutureWarning RuntimeWarning UserWarning UnicodeWarning BytesWarning PendingDeprecationWarning ResourceWarning ImportWarning RuntimeError RecursionError NotImplementedError _DeadlockError LookupError IndexError KeyError CodecRegistryError EOFError GeneratorExit KeyboardInterrupt SystemExit

How to raise exceptions.

ruby:

Ruby has a throw keyword in addition to raise. throw can have a symbol as an argument, and will not convert a string to a RuntimeError exception.

How to catch exceptions.

php:

PHP code must specify a variable name for the caught exception. Exception is the top of the exception hierarchy and will catch all exceptions.

Internal PHP functions usually do not throw exceptions. They can be converted to exceptions with this signal handler:

function exception_error_handler($errno, $errstr, $errfile, $errline ) { throw new ErrorException($errstr, 0, $errno, $errfile, $errline); } set_error_handler("exception_error_handler");

ruby:

A rescue Exception clause will catch any exception. A rescue clause with no exception type specified will catch exceptions that are subclasses of StandardError. Exceptions outside StandardError are usually unrecoverable and hence not handled in code.

In a rescue clause, the retry keyword will cause the begin clause to be re-executed.

In addition to begin and rescue, ruby has catch:

catch (:done) do loop do retval = work throw :done if retval < 10 end end

How to re-raise an exception preserving the original stack trace.

python:

If the exception is assigned to a variable in the except clause and the variable is used as the argument to raise , then a new stack trace is created.

ruby:

If the exception is assigned to a variable in the rescue clause and the variable is used as the argument to raise , then the original stack trace is preserved.

The global variable name for the last exception raised.

How to define a new variable class.

How to catch exceptions of a specific type and assign the exception a name.

php:

PHP exceptions when caught must always be assigned a variable name.

A block of statements that is guaranteed to be executed even if an exception is thrown or caught.

ruby:

Ruby MRI threads are operating system threads, but a global interpreter lock prevents more than one thread from executing Ruby code at a time.

How to make a thread wait for another thread to finish.