Writing a JSON parser is one of the easiest ways to get familiar with parsing techniques. The format is extremely simple. It's defined recursively so you get a slight challenge compared to, say, parsing Brainfuck; and you probably already use JSON. Aside from that last point, parsing S-expressions for Scheme might be an even simpler task.

If you'd just like to see the code for the library, pj , check it out on Github.

What parsing is and (typically) is not

Parsing is often broken up into two stages: lexical analysis and syntactic analysis. Lexical analysis breaks source input into the simplest decomposable elements of a language called "tokens". Syntactic analysis (often itself called "parsing") receives the list of tokens and tries to find patterns in them to meet the language being parsed.

Parsing does not determine semantic viability of an input source. Semantic viability of an input source might include whether or not a variable is defined before being used, whether a function is called with the correct arguments, or whether a variable can be declared a second time in some scope.

There are, of course, always variations in how people choose to parse and apply semantic rules, but I am assuming a "traditional" approach to explain the core concepts.

The JSON library's interface

Ultimately, there should be a from_string method that accepts a JSON-encoded string and returns the equivalent Python dictionary.

For example:

assert_equal(from_string('{"foo": 1}'), {"foo": 1})

Lexical analysis

Lexical analysis breaks down an input string into tokens. Comments and whitespace are often discarded during lexical analysis so you are left with a simpler input you can search for grammatical matches during the syntactic analysis.

Assuming a simple lexical analyzer, you might iterate over all the characters in an input string (or stream) and break them apart into fundemental, non-recursively defined language constructs such as integers, strings, and boolean literals. In particular, strings must be part of the lexical analysis because you cannot throw away whitespace without knowing that it is not part of a string.

In a helpful lexer you keep track of the whitespace and comments you've skipped, the current line number and file you are in so that you can refer back to it at any stage in errors produced by analysis of the source. The V8 Javascript engine recently became able to do reproduce the exact source code of a function. This, at the very least, would need the help of a lexer to make possible.

Implementing a JSON lexer

The gist of the JSON lexer will be to iterate over the input source and try to find patterns of strings, numbers, booleans, nulls, or JSON syntax like left brackets and left braces, ultimately returning each of these elements as a list.

Here is what the lexer should return for an example input:

assert_equal(lex('{"foo": [1, 2, {"bar": 2}]}'), ['{', 'foo', ':', '[', 1, ',', 2, ',', '{', 'bar', ':', 2, '}', ']', '}'])

Here is what this logic might begin to look like:

def lex(string): tokens = [] while len(string): json_string, string = lex_string(string) if json_string is not None: tokens.append(json_string) continue # TODO: lex booleans, nulls, numbers if string[0] in JSON_WHITESPACE: string = string[1:] elif string[0] in JSON_SYNTAX: tokens.append(string[0]) string = string[1:] else: raise Exception('Unexpected character: {}'.format(string[0])) return tokens

The goal here is to try to match strings, numbers, booleans, and nulls and add them to the list of tokens. If none of these match, check if the character is whitespace and throw it away if so. Otherwise store it as a token if it is part of JSON syntax (like left brackets). Finally throw an exception if the character/string didn't match any of these patterns.

Let's extend the core logic here a little bit to support all the types and add the function stubs.

def lex_string(string): return None, string def lex_number(string): return None, string def lex_bool(string): return None, string def lex_null(string): return None, string def lex(string): tokens = [] while len(string): json_string, string = lex_string(string) if json_string is not None: tokens.append(json_string) continue json_number, string = lex_number(string) if json_number is not None: tokens.append(json_number) continue json_bool, string = lex_bool(string) if json_bool is not None: tokens.append(json_bool) continue json_null, string = lex_null(string) if json_null is not None: tokens.append(None) continue if string[0] in JSON_WHITESPACE: string = string[1:] elif string[0] in JSON_SYNTAX: tokens.append(string[0]) string = string[1:] else: raise Exception('Unexpected character: {}'.format(string[0])) return tokens

Lexing strings

For the lex_string function, the gist will be to check if the first character is a quote. If it is, iterate over the input string until you find an ending quote. If you don't find an initial quote, return None and the original list. If you find an initial quote and an ending quote, return the string within the quotes and the rest of the unchecked input string.

def lex_string(string): json_string = '' if string[0] == JSON_QUOTE: string = string[1:] else: return None, string for c in string: if c == JSON_QUOTE: return json_string, string[len(json_string)+1:] else: json_string += c raise Exception('Expected end-of-string quote')

Lexing numbers

For the lex_number function, the gist will be to iterate over the input until you find a character that cannot be part of a number. (This is, of course, a gross simplification, but being more accurate will be left as an exercise to the reader.) After finding a character that cannot be part of a number, either return a float or int if the characters you've accumulated number more than 0. Otherwise return None and the original string input.

def lex_number(string): json_number = '' number_characters = [str(d) for d in range(0, 10)] + ['-', 'e', '.'] for c in string: if c in number_characters: json_number += c else: break rest = string[len(json_number):] if not len(json_number): return None, string if '.' in json_number: return float(json_number), rest return int(json_number), rest

Lexing booleans and nulls

Finding boolean and null values is a very simple string match.

def lex_bool(string): string_len = len(string) if string_len >= TRUE_LEN and \ string[:TRUE_LEN] == 'true': return True, string[TRUE_LEN:] elif string_len >= FALSE_LEN and \ string[:FALSE_LEN] == 'false': return False, string[FALSE_LEN:] return None, string def lex_null(string): string_len = len(string) if string_len >= NULL_LEN and \ string[:NULL_LEN] == 'null': return True, string[NULL_LEN] return None, string

And now the lexer code is done! See the pj/lexer.py for the code as a whole.

Syntactic analysis

The syntax analyzer's (basic) job is to iterate over a one-dimensional list of tokens and match groups of tokens up to pieces of the language according to the definition of the language. If, at any point during syntactic analysis, the parser cannot match the current set of tokens up to a valid grammar of the language, the parser will fail and possibly give you useful information as to what you gave, where, and what it expected from you.

Implementing a JSON parser

The gist of the JSON parser will be to iterate over the tokens received after a call to lex and try to match the tokens to objects, lists, or plain values.

Here is what the parser should return for an example input:

tokens = lex('{"foo": [1, 2, {"bar": 2}]}') assert_equal(tokens, ['{', 'foo', ':', '[', 1, ',', 2, '{', 'bar', ':', 2, '}', ']', '}']) assert_equal(parse(tokens), {'foo': [1, 2, {'bar': 2}]})

Here is what this logic might begin to look like:

def parse_array(tokens): return [], tokens def parse_object(tokens): return {}, tokens def parse(tokens): t = tokens[0] if t == JSON_LEFTBRACKET: return parse_array(tokens[1:]) elif t == JSON_LEFTBRACE: return parse_object(tokens[1:]) else: return t, tokens[1:]

A key structural difference between this lexer and parser is that the lexer returns a one-dimensional array of tokens. Parsers are often defined recursively and returns a recursive, tree-like object. Since JSON is a data serialization format instead of a language, the parser should produce objects in Python rather than a syntax tree on which you could perform more analysis (or code generation in the case of a compiler).

And, again, the benefit of having the lexical analysis happen independent from the parser is that both pieces of code are simpler and concerned with only specific elements.

Parsing arrays

Parsing arrays is a matter of parsing array members and expecting a comma token between them or a right bracket indicating the end of the array.

def parse_array(tokens): json_array = [] t = tokens[0] if t == JSON_RIGHTBRACKET: return json_array, tokens[1:] while True: json, tokens = parse(tokens) json_array.append(json) t = tokens[0] if t == JSON_RIGHTBRACKET: return json_array, tokens[1:] elif t != JSON_COMMA: raise Exception('Expected comma after object in array') else: tokens = tokens[1:] raise Exception('Expected end-of-array bracket')

Parsing objects

Parsing objects is a matter of parsing a key-value pair internally separated by a colon and externally separated by a comma until you reach the end of the object.

def parse_object(tokens): json_object = {} t = tokens[0] if t == JSON_RIGHTBRACE: return json_object, tokens[1:] while True: json_key = tokens[0] if type(json_key) is str: tokens = tokens[1:] else: raise Exception('Expected string key, got: {}'.format(json_key)) if tokens[0] != JSON_COLON: raise Exception('Expected colon after key in object, got: {}'.format(t)) json_value, tokens = parse(tokens[1:]) json_object[json_key] = json_value t = tokens[0] if t == JSON_RIGHTBRACE: return json_object, tokens[1:] elif t != JSON_COMMA: raise Exception('Expected comma after pair in object, got: {}'.format(t)) tokens = tokens[1:] raise Exception('Expected end-of-object brace')

And now the parser code is done! See the pj/parser.py for the code as a whole.

Unifying the library

To provide the ideal interface, create the from_string function wrapping the lex and parse functions.

def from_string(string): tokens = lex(string) return parse(tokens)[0]

And the library is complete! (ish). Check out the project on Github for the full implementation including basic testing setup.

Appendix A: Single-step parsing

Some parsers choose to implement lexical and syntactic analysis in one stage. For some languages this can simplify the parsing stage entirely. Or, in more powerful languages like Common Lisp, it can allow you to dynamically extend the lexer and parser in one step with reader macros.

I wrote this library in Python to make it more accessible to a larger audience. However, many of the techniques used are more amenable to languages with pattern matching and support for monadic operations -- like Standard ML. If you are curious what this same code would look like in Standard ML, check out the JSON code in Ponyo.

Please reply on Twitter with questions or comments.