Can assign [] = (), but not () = []

[ssbr]

BPO	23275
Nosy	@rhettinger, @amauryfa, @mdickinson, @ncoghlan, @scoder, @ssbr, @ezio-melotti, @bitdancer, @flying-sheep, @berkerpeksag, @vadmium, @eryksun
Files	issue23275.diff issue23275_v2.diff issue23275_v3.diff issue23275_v4.diff

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/vadmium'
closed_at = <Date 2016-05-18.05:48:24.849>
created_at = <Date 2015-01-19.19:05:47.809>
labels = ['interpreter-core', 'type-feature']
title = 'Can assign [] = (), but not () = []'
updated_at = <Date 2016-06-08.13:36:22.613>
user = 'https://github.com/ssbr'

bugs.python.org fields:

activity = <Date 2016-06-08.13:36:22.613>
actor = 'python-dev'
assignee = 'martin.panter'
closed = True
closed_date = <Date 2016-05-18.05:48:24.849>
closer = 'berker.peksag'
components = ['Interpreter Core']
creation = <Date 2015-01-19.19:05:47.809>
creator = 'Devin Jeanpierre'
dependencies = []
files = ['39165', '39703', '42715', '42878']
hgrepos = []
issue_num = 23275
keywords = ['patch']
message_count = 31.0
messages = ['234324', '234325', '234326', '234361', '234364', '234365', '241473', '241474', '241523', '241673', '241790', '241792', '244157', '244159', '244161', '244208', '244215', '244227', '245308', '246382', '264223', '264226', '264249', '264811', '265108', '265764', '265809', '265811', '265816', '265817', '267847']
nosy_count = 17.0
nosy_names = ['rhettinger', 'amaury.forgeotdarc', 'mark.dickinson', 'ncoghlan', 'scoder', 'Devin Jeanpierre', 'ezio.melotti', 'ionelmc', 'r.david.murray', 'flying sheep', 'python-dev', 'berker.peksag', 'martin.panter', 'eryksun', 'Cesar.Kawakami', 'Kyle.Buzsaki', 'Rahul Gupta']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue23275'
versions = ['Python 3.6']

[ssbr]

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

This contradicts the assignment grammar, which would make both illegal: https://docs.python.org/3/reference/simple_stmts.html#assignment-statements

[bitdancer]

My guess is that it is not worth complicating the parser in order to make these two cases consistent, and it should be treated as a doc error. We'll see what other developers think.

[rhettinger]

The starting point is recognizing that this has been around for very long time and is harmless.

[KyleBuzsaki]

It seems that assigning to [] is the odd one out in this case. Why is this even possible?

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

[vadmium]

But () is the odd one out if you consider

>>> [a, b] = range(2)
>>> [] = range(0)
>>> (a, b) = range(2)
>>> () = range(0)
  File "<stdin>", line 1
SyntaxError: can't assign to ()

[eryksun]

In ast.c, set_context checks for assignment to an empty tuple, but not an empty list.

        case List_kind:
            e->v.List.ctx = ctx;
            s = e->v.List.elts;
            break;
        case Tuple_kind:
            if (asdl_seq_LEN(e->v.Tuple.elts))  {
                e->v.Tuple.ctx = ctx;
                s = e->v.Tuple.elts;
            }
            else {
                expr_name = "()";
            }
            break;

https://hg.python.org/cpython/file/ab2c023a9432/Python/ast.c#l912

[ncoghlan]

As Raymond notes, this is a fairly harmless quirk - it changes a SyntaxError to an iterable length dependent ValueError:

>>> () = []
  File "<stdin>", line 1
SyntaxError: can't assign to ()
>>> [] = ()
>>> [] = [1]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: too many values to unpack (expected 0)

I only found out about this after being puzzled when a typo in a live demo at PyCon 2015 failed to fail as I expected after seeing the presenter type a "[]" into the LHS of an assignment instead of the intended "_".

Removing the data dependence to make the assignment fail immediately (even if never tested against a non-empty iterable) would involve making the handling of List_kind match that of Tuple_kind in the switch statement that eryksun quoted.

I don't think it's an urgent fix, but if someone wanted to fix it (including a new test), I think it would be a reasonable contribution to accept.

[vadmium]

I would prefer this be fixed in the opposite direction, to allow “unpacking” an empty iterable using round brackets. I have used this syntax on purpose as a concise way to ensure that a generator is exhaused with no more yields:

>>> def gen():
...     yield "partial computation"
...     print("computation allowed to complete")
... 
>>> g = gen()
>>> next(g)
'partial computation'
>>> [] = g
computation allowed to complete

[bitdancer]

There is also no reason to break currently working code, which turning this into a syntax error would od.

[mdickinson]

There is also no reason to break currently working code

Agreed. To take one example, David Beazley's PyCon 2015 talk would have been broken by the suggested change! (See https://www.youtube.com/watch?v=MCs5OvhV9S4, at around the 42:17 mark.)

If there's any code change resulting from this issue, I also think it should be to make assignment to () legal.

[berkerpeksag]

I don't have a strong opinion on this, but here is a patch to make () = [] a valid assignment.

[amauryfa]

About the patch: I'm sure there are other tests to change,
in test_syntax.py for example::

It's a syntax error to assign to the empty tuple.  Why isn't it an
error to assign to the empty list?  It will always raise some error  at
runtime.

    >>> () = 1
    Traceback (most recent call last):
      File "<doctest test.test_syntax[3]>", line 1
    SyntaxError: can't assign to ()

[RahulGupta]

isn't it logical?

[] is a mutable data structure
while () is a immutable data structure

(b, a) = [1, 2] is fine because a and b are mutable