False reports on W0644 unbalanced-dict-unpacking in pylint 2.16.0

[Spindel]

Bug description

When unpacking while iterating over dict.values() we seem to get a spurious warning about unbalanced dict unpacking.

Here's a test-case, result vs. result1 may require a few changes to the types, but the original code was doing a lot more dynamic things with callbacks and more.

from typing import Dict, Tuple, Callable

Inner = Callable[[int], Dict[str, int]]

def returns_dict(a:int, b:int) -> Dict[str, Tuple[int, int, int]]:
    result1 = {
        "abcdef": returns_things(a, b, "abcdef"),
        "cdef": returns_other_things(a,b, "cdef"),
    }
    result = {
        "abc": (1, 2, 3),
        "cde": (1, 2, 3),
    }
    return result

def returns_things(a:int, b:int, c:str) -> Tuple[str, str, Inner]:
    def inner(other: int) -> Dict[str, int]:
        return {"called": other}
    return f"{a}{b}", c, inner


def returns_other_things(a:int, b:int, c:str)  -> Tuple[str, str, Inner]:
    def inner(other: int) -> Dict[str, int]:
        return {"called": other}
    return f"{1}{c}", c, inner


def thing(a:int, b:int) -> None:
    bar = returns_dict(a, b)
    [print(x) for x in bar.values()]
    for x, y, _z in bar.values():
        assert x
        assert y

if __name__ == "__main__":
    thing(1, 2)

Configuration

No response

Command used

pylint pylint_testcase.py

Pylint output

************* Module pylint_testcase
pylint_testcase.py:32:4: W0644: Possible unbalanced dict unpacking with bar.values(): left side has 3 labels, right side has 2 values (unbalanced-dict-unpacking)

Expected behavior

No warning should happen

Pylint version

pylint 2.16.0
astroid 2.14.1
Python 3.11.1 (main, Jan  6 2023, 00:00:00) [GCC 12.2.1 20221121 (Red Hat 12.2.1-4)]

OS / Environment

Fedora 37, x86_64

Additional dependencies

No response

[mbyrnepr2]

Thank you @Spindel. Indeed it is falling over when values() is used and there are 3 or more items to unpack:

for x, y, z in {1: ("a", "b", "c"), 2: ("d", "e", "f")}.values():
    print(x)

[Spindel]

Thank you @Spindel. Indeed it is falling over when values() is used and there are 3 or more items to unpack:

for x, y, z in {1: ("a", "b", "c"), 2: ("d", "e", "f")}.values():
    print(x)

That's a much simpler reproducer, indeed.

[Neowizard]

The following code shows the false-positive isn't caused by how many values are in the tuple, but by how many elements are in the dictionary:

# The unpacking error in this for-loop is intentional to show the warning doen't point to the actual error
for value1, value2 in {1: 'a', 2: 'b', 3: 'c'}.values():
    pass
# lint_warn.py:2:0: W0644: Possible unbalanced dict unpacking with {1: 'a', 2: 'b', 3: 'c'}.values(): left side has 2 labels, right side has 3 values (unbalanced-dict-unpacking)

The warning tells me I should have 3 values unpacked, instead of 1, despite this raising a runtime error.

for value1, value2 in {1: ('a', 'b'), 2: ('c', 'd'), 3: ('e', 'f')}.values():
    pass
#lint_warn.py:6:0: W0644: Possible unbalanced dict unpacking with {1: ('a', 'b'), 2: ('c', 'd'), 3: ('e', 'f')}.values(): left side has 2 labels, right side has 3 values (unbalanced-dict-unpacking)

The warning tells me to unpack 3 elements instead of 2.

# This unpacking error here is intentional to show there's no warning in this case
for value1, value2 in {1: ('a', 'b', 'c'), 2: ('d', 'e', 'f')}.values():
    pass

Pylint raises no warning here, despite the unpacking error.

for value1, value2 in {1: ('a', 'b'), 2: ('c', 'd'), 3: ('e', 'f'), 4: ('g', 'h')}.values():
    pass
#lint_warn.py:15:0: W0644: Possible unbalanced dict unpacking with {1: ('a', 'b'), 2: ('c', 'd'), 3: ('e', 'f'), 4: ('g', 'h')}.values(): left side has 2 labels, right side has 4 values (unbalanced-dict-unpacking)

The warning tells me to unpack 4 elements instead of 2.

It appears that pylint expects dict.values() to return a list of elements each of length equal the dict length (instead of the element's length).

To drive the point home, here's pylint warning me about W0644 when the dictionary value can't exist:

for value1, value2 in {1: does_not_exist(1), 2: does_not_exist(2), 3: does_not_exist(3)}.values():
    pass
#lint_warn.py:19:0: W0644: Possible unbalanced dict unpacking with {1: does_not_exist(1), 2: does_not_exist(2), 3: does_not_exist(3)}.values(): left side has 2 labels, right side has 3 values (unbalanced-dict-unpacking)
#lint_warn.py:19:26: E0602: Undefined variable 'does_not_exist' (undefined-variable)
#lint_warn.py:19:48: E0602: Undefined variable 'does_not_exist' (undefined-variable)
#lint_warn.py:19:70: E0602: Undefined variable 'does_not_exist' (undefined-variable)

[mbyrnepr2]

Thanks for that @Neowizard. Essentially the tuple values of the dict are treated as if they were non iterable values. It just isn’t handled at all at the moment.
Reading your comment I realise my previous comment applies to the ‘.items()’ case, not ‘values()’; I think I scanned the source too quickly at the time.

[Neowizard]

@mbyrnepr2 no worries. I created PR #8892 that I believe fixes the issue