I wrote the other day about two things I think are weird about Python's += operator. In the comments, famed Twisted hacker Jean-Paul Calderone showed me something far, far weirder. This post is a record of me playing around and trying to understand it.

To begin let's review what we know. Tuples are immutable in Python, so you can't increment a member of a tuple:

>>> x = ( 0 ,) >>> x (0,) >>> x[ 0 ] += 1 Traceback (most recent call last): File "<stdin>" , line 1 , in <module> TypeError : 'tuple' object does not support item assignment >>> x (0,)

That's fine. But here's the bizarre behavior Jean-Paul showed me: if you put a list in a tuple and use the += operator to extend the list, the increment succeeds and you get a TypeError !:

>>> x = ([],) >>> x ([],) >>> x[ 0 ] += [ 1 ] Traceback (most recent call last): File "<stdin>" , line 1 , in <module> TypeError : 'tuple' object does not support item assignment >>> x ([1],)

The equivalent statement using extend succeeds without the TypeError :

>>> x = ([],) >>> x[ 0 ] . extend([ 1 ]) >>> x ([1],)

So what's going on with += ? As always, looking at the bytecode is a good step toward understanding. I'll compile and disassemble the statement x[0] += [1] , and add some annotations:

>>> import dis >>> dis . dis( compile ( 'x[0] += [1]' , '<string>' , 'exec' )) 1 0 LOAD_NAME 0 (x) 3 LOAD_CONST 0 (0) 6 DUP_TOPX 2 -- put x[0] on the stack -- 9 BINARY_SUBSCR 10 LOAD_CONST 1 (1) 13 BUILD_LIST 1 -- do the "+=" -- 16 INPLACE_ADD 17 ROT_THREE -- store new value in x[0] -- 18 STORE_SUBSCR 19 LOAD_CONST 2 (None) 22 RETURN_VALUE

(See Dan Crosta's Exploring Python Code Objects for more on this technique).

Looks like the statement puts a reference to x[0] on the stack, makes the list [1] and uses it to successfully extend the list in x[0] . But then the statement executes STORE_SUBSCR , which calls the C function PyObject_SetItem , which checks if the object supports item assignment. In our case the object is a tuple, so PyObject_SetItem throws the TypeError . Mystery solved.

Is this a Python bug or just very surprising?