[Python] [LGTM.com] False-positive: Unreachable statement: #2206 sequel

[webknjaz]

Description of the false positive

Same as #2206. Except the reproducer contains several subsequent suppress()es:

import contextlib


def some_func():
    with contextlib.suppress(KeyError):
        return dict()['a_key']

    with contextlib.suppress(KeyError):  # LGTM complains that it's unreachable
        return dict()['b_key']

    with contextlib.suppress(KeyError):
        return dict()['c_key']

    return 'fallback'

So I guess #2078 needs more test cases maybe...

URL to the alert on the project page on LGTM.com

https://lgtm.com/projects/g/cherrypy/cheroot/snapshot/0c678bab518ca17151540b7d744faca49b941763/files/cheroot/ssl/builtin.py#x46e331ef60ba4bf1:1

[tausbn]

This was an interesting case. The problem here is that we are not identifying the calls to suppress as uses of contextlib.suppress. If we follow the definition of suppress, we see that it comes from a compatibility library that -- in the absence of contextlib.suppress (e.g. if the version of Python is too old) -- defines it itself as a fallback.

As it turns out, we are analysing this project as a Python 2.7 project, so this explains why the fallback version of suppress is used.

In this case, I think there's a more general solution that can be applied, without resorting to looking for specific well-known context handlers (which can fail, as the present example shows). Hopefully we should be able to roll out a fix soon.

Thanks for the report!

[webknjaz]

In this case, I think there's a more general solution that can be applied, without resorting to looking for specific well-known context handlers (which can fail, as the present example shows).

Right, any context manager can choose to suppress exceptions via its __exit__() method.

[webknjaz]

Got another instance of this: https://lgtm.com/projects/g/ansible/ansible-lint/snapshot/4741210b9feefe9ed54c59a731c1636fd648400e/files/lib/ansiblelint/utils.py?sort=name&dir=ASC&mode=heatmap#x92b14f87ccfa33b7:1

Basically, the same except it uses continue in place of return:

for ...:
    contextlib.suppress(Exc):
        ...
        continue

    some_code  #  <-- marked as unreachable but in fact, is totally reachable