Inference failure when function returns one of two constant values

[fizbin]

Steps to reproduce

1. Run pylint on this code:

"""pylint test module"""

def return_an_int(param):
    """Returns an int"""
    if param == 0:
        return 1
    return 0

def test_one(param):
    """Should complain about var_one[0], but doesn't"""
    var_one = return_an_int(param)
    return var_one[0]

Current behavior

Your code has been rated at 10.00/10 (previous run: 10.00/10, +0.00)

Expected behavior

E: 12,11: Value 'var_one' is unsubscriptable (unsubscriptable-object)

pylint --version output

Originally found on:

No config file found, using default configuration
pylint 1.9.3, 
astroid 1.6.5
Python 2.7.12 (v2.7.12:d33e0cf91556, Jun 26 2016, 12:10:39) 
[GCC 4.2.1 (Apple Inc. build 5666) (dot 3)]

Also see identical behavior on this code with:

pylint 2.1.1
astroid 2.0.4
Python 3.7.0 (v3.7.0:1bf9cc5093, Jun 26 2018, 23:26:24) 
[Clang 6.0 (clang-600.0.57)]

[fizbin]

Note that astroid seems perfectly able to infer that var_one is either the int 1 or the int 0, so the failure is in pylint failing to see that both of those values are unsubscriptable:

In [1]: import astroid

In [2]: src = open("pylinttest_short.py").read()

In [3]: tree = astroid.parse(src)

In [4]: tree.body[1].body[1].value.value
Out[4]: <Name.var_one l.12 at 0x10e2259e8>

In [5]: list(tree.body[1].body[1].value.value.infer())
Out[5]: [<Const.int l.6 at 0x10e2257b8>, <Const.int l.7 at 0x10e2257f0>]

In [6]: list(x.value for x in tree.body[1].body[1].value.value.infer())
Out[6]: [1, 0]

Not too surprisingly given that, the test case can be shortened to:

"""pylint test module"""

def test_one(param):
    """Should complain about var_one[0], but doesn't"""
    var_one = 0
    if param == 0:
        var_one = 1
    return var_one[0]

[PCManticore]

Thanks for reporting a bug! This is currently caused by our use of safe_infer, which returns None on multiple inferred object. I think we should allow safe_infer to return the inferred value when multiple values are sharing the same types, as in this case we can only infer integers from that function.