[] = (), () = (), and {} = () 'assignments'

Question

I was surprised to find the following, in Python 3, the first two raise nothing:

>>> [] = () >>> () = () >>> {} = ()   File "<stdin>", line 1 SyntaxError: can't assign to literal 

In Python 2.7, only the first one raises nothing:

>>> [] = () >>> () = ()   File "<stdin>", line 1 SyntaxError: can't assign to () >>> {} = ()   File "<stdin>", line 1 SyntaxError: can't assign to literal 

What is going on here? Why are any of then not raising errors? And why was the () = () presumably added to be valid in Python 3?

*Note, you can replace the right hand side with any empty iterable (e.g. [] = set()), I just choose an empty tuple for the illustration

Answer 1

According to Issue23275, these are basically quirks causing no real harm but also no utility. Note that [] = () does not alter the list literal:

>>> [] = () >>> type([]) <class 'list'> 

[] = x statements basically assert that x is iterable and that x is empty (although no-one would recommend using them this way), e.g.

>>> [] = (1) Traceback (most recent call last):   File "<stdin>", line 1, in <module> TypeError: 'int' object is not iterable >>> [] = (1,) Traceback (most recent call last):   File "<stdin>", line 1, in <module> ValueError: too many values to unpack 

As John Y comments it is best to think of [] = () as not an assignment but a way of being consistent with Python's iterable unpacking syntax.

As ArrowCase comments, this syntax also extends to multiple assignments:

>>> a = [] = () >>> a () 

Looking at the CPython bytecode of the multiple assignment illustrates that this operations are similar to the normal iterable unpacking syntax, using the UNPACK_SEQUENCE instruction:

>>> dis.dis('a = [] = ()')   1           0 BUILD_TUPLE              0               2 DUP_TOP               4 STORE_NAME               0 (a)               6 UNPACK_SEQUENCE          0               8 LOAD_CONST               0 (None)              10 RETURN_VALUE >>> dis.dis('[a, b] = (1, 2)')   1           0 LOAD_CONST               3 ((1, 2))               2 UNPACK_SEQUENCE          2               4 STORE_NAME               0 (a)               6 STORE_NAME               1 (b)               8 LOAD_CONST               2 (None)              10 RETURN_VALUE 

The same Issue23275 states that () = () was added as valid syntax to Python 3 for concordance. It was decided that removing [] = () would break code needlessly, since it causes no harm and fits with iterable unpacking logic. {} = () is still invalid because the unpacking syntax does not make sense in this context with braces.

In case anyone is wondering, syntax like list() = () is simply syntactically invalid, because you can never assign to function call.