False positive `no-value-for-parameter` due to inference edge case

[tushar-deepsource]

Bug description

Due to incorrect inference, E1120 can be raised on code that is functional.

Example code:

def get_nums():
    nums = ()
    if input() == '1':
        nums = (1, 2)
    return nums

def add(x, y):
    return x + y

nums = get_nums()
if nums:
    add(*nums)  # issue raised here, pylint thinks `nums` will always be `()`

Command used

pylint asd.py --disable=all --enable=E1120

Pylint output

************* Module asd
asd.py:12:4: E1120: No value for argument 'x' in function call (no-value-for-parameter)
asd.py:12:4: E1120: No value for argument 'y' in function call (no-value-for-parameter)

Expected behavior

No issues raised.

Pylint version

pylint 2.17.2
astroid 2.15.2
Python 3.11.2 (main, Feb 16 2023, 02:55:59) [Clang 14.0.0 (clang-1400.0.29.202)]

OS / Environment

MacOS Ventura, 13.2.1

Additional dependencies

No response

[jacobtylerwalls]

When CallSite._unpack_args infers the values of an unpacked Starred node, it should keep the knowledge that there were multiple inference results so that pylint's TypeCheck.visit_call can bail out early. Would you like to prepare PRs?

[tusharsadhwani]

sure thing, can you point me to the right files? I'm assuming it'll go in the infer() implementation for CallSite in astroid?

[jacobtylerwalls]

Here's where we're dealing with the first inference result only (well, one of two places in this file):

https://github.com/pylint-dev/astroid/blob/5e8c89afe4a12eeab2ab97d95495187990e5df27/astroid/arguments.py#L138

If we change this simply to safe_infer, that should be enough to disregard ambiguous results, but it will affect other messages, too. That's probably what we want, but it would be good to see the pylint primer results with such a change. An alternative would be to just add this knowledge to the CallSite object with a "number of inferred nodes" attribute and then only bail out of this specific pylint check if > 1, but that's probably over-engineering things.