String formatting is inferred as empty string

[pylint-bot]

Originally reported by: Florian Bruhin (BitBucket: The-Compiler, GitHub: @The-Compiler?)

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With this code:

def foo(arg):
    pass

foo('foo{}bar'.format(42))
foo('const')

The first call is inferred as an empty string.

I can reproduce this with pylint/astroid from tip and this pylint plugin:

import astroid
from pylint import interfaces, checkers
from pylint.checkers import utils


class TestChecker(checkers.BaseChecker):

    __implements__ = interfaces.IAstroidChecker
    name = 'foo'
    msgs = {
        'E0000': ('Got called with empty string!', 'empty-string', None),
    }
    priority = -1

    @utils.check_messages('bad-config-call')
    def visit_callfunc(self, node):
        if not hasattr(node, 'func'):
            return
        infer = utils.safe_infer(node.func)
        if infer is None or infer.name != 'foo':
            return
        try:
            arg = utils.get_argument_from_call(node, position=0)
        except utils.NoSuchArgumentError:
            return
        arg = utils.safe_infer(arg)
        if not isinstance(arg, astroid.Const):
            return
        if arg.value == '':
            self.add_message('empty-string', node=node)


def register(linter):
    linter.register_checker(TestChecker(linter))

output:

$ PYTHONPATH=$PWD ./venv/bin/pylint foo.py --load-plugins=plugin
No config file found, using default configuration
************* Module foo
[...]
E:  4, 3: Got called with empty string! (empty-string)
[...]

Assigning to PCManticore as discussed in IRC.

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• Bitbucket: https://bitbucket.org/logilab/astroid/issue/104