False positive with operator+ and lists (no-member)

[hemberger]

Bug description

When using operator+ to merge two lists (in the context of a class method?), pylint incorrectly thinks that the objects in the combined list are also of type list.

The simplest example I can construct is the following:

# pylint: disable=C0103,C0114,C0115,C0116,R0903

class A:
    def __init__(self):
        self.Alist = []
        self.data = []

    def run(self):
        for a in [self] + self.Alist:
            for _ in a.data:  # # <-- causes "no-member" warning
                pass

Configuration

No response

Command used

pylint a.py

Pylint output

************* Module a
a.py:12:21: E1101: Instance of 'list' has no 'data' member (no-member)

Expected behavior

I would expect there to be no warning, since both self and self.Alist are (nominally) of type A, which does have a member called data.

If instead of using [self] + self.Alist, any of the following will not result in a warning:

• for a in [self]
• for a in self.Alist

Pylint version

pylint 2.9.6
astroid 2.6.5
Python 3.7.4 (default, Aug 13 2019, 20:35:49) 
[GCC 7.3.0]

OS / Environment

CentOS 8.4

Additional dependencies

No response

[Pierre-Sassoulas]

Thank you for opening this issue. I'm classifying as minor since

class A:
    def __init__(self):
        self.Alist = [A()]
        self.data = []

    def run(self):
        for a in [self] + self.Alist:
            for _ in a.data:  # # <-- causes "no-member" warning
                pass

A().run()

Gives no pylint warning and putting a string instead of A() gives a proper pylint warning.

[DanielNoord]

This is probably an issue in astroid.
I wrote the following code to show different behaviour in similar code:

import astroid

module = astroid.parse(
    """
for a in [1] + [2]:
    print(a)
"""
)

print(list(module.body[0].body[0].value.args[0].infer()))

module = astroid.parse(
    """
class A:
    def __init__(self):
        for a in [self] + []:
            print(a)
"""
)

print(list(module.body[0].body[0].body[0].body[0].value.args[0].infer()))

module = astroid.parse(
    """
class A:
    def __init__(self):
        for a in [self, 1]:
            print(a)
"""
)

print(list(module.body[0].body[0].body[0].body[0].value.args[0].infer()))

❯ python3 test.py
[<Const.int l.2 at 0x1075c9060>, <Const.int l.2 at 0x1075c90f0>]
[<Instance of builtins.list at 0x4418479552>]
[<Instance of .A at 0x4418481136>, <Const.int l.4 at 0x1075c8400>]

With for a in [self] + [] the inference seems to stop at recognising that it is a list and is unable to go any further.
I haven't looked at any astroid internals yet, but this should be a good start to find where this needs to be fixed or handled better.