LGTM.com - false positive: py/unreachable-statement after context manager with while True loop

[takluyver]

Description of the false positive

The code looks like this:

with trio.CancelScope() as cscope:
    task_status.started(cscope)

    while True:
        # ...
        await trio.sleep(0.5)

do_more_things()  # This code is marked as unreachable

A trio Cancelled exception is expected to break out of the loop, and then be caught by the CancelScope being used as a context manager. Trio's CancelScope is also used to build other context managers like with trio.move_on_after(5):, which implements a timeout by raising a Cancelled error and then catching it.

See also the discussion on #2351 - it's not only contextlib.suppress that can catch exceptions.

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

https://lgtm.com/projects/g/European-XFEL/sfollow/snapshot/2b50fa3bd622a5ed2dd5bf12948cee0174d55faa/files/sfollow/sfollow.py?sort=name&dir=ASC&mode=heatmap#x7fd2565924064ca:1

[aeisenberg]

Indeed, this looks like a false positive. Thank you for reporting it!

Our current focus is on improving our security analysis. Because your report does not relate to a security query, we will put this on our backlog and prioritize it if we get enough reports of the same underlying issue in other projects. If you think that your report is related to our security analysis, please clarify that in a comment. Either way, we'll let you know here as soon as it's fixed!

As you might know, all of our queries are open source. If you do have an idea for a code change, we encourage you to open a pull request. GitHub Code Scanning and lgtm.com have facilities for suppressing individual alerts or disabling a query.