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pythonpython-3.xmypylintpylint

How to catch potentially undefined variables with pylint or mypy?

import time

if time.time() > 42:
    x = 1

print(x)

My IDE (PyCharm) warns me about x potentially being undefined:

enter image description here

But pylint and mypy do not say anything. Is there a way to make one of them complain about this situation too?

Solution

  • mypy has partial* support via the error code [possibly-undefined], which you can enable through one of the following:

    • The CLI when running mypy (mypy <...> --enable-error-code possibly-undefined);

    • A mypy configuration file setting which looks like

      [mypy]
enable_error_code = possibly-undefined

    • The file-level configuration comment line

      # mypy: enable-error-code=possibly-undefined

    See a demonstration of your example in the mypy Playground.

    *Partial, because mypy treats annotated assignment statements as an implicit declaration of existence; it won't catch this example:

    import time

x: int  # Annotated assignment statement with no value set
if time.time() > 42:
    x = 1

print(x)  # No errors caught here

    Note that, for all intents and purposes, this is good enough as full support. It's not possible for a type-checker to determine whether all execution paths can be met for (e.g.) independent conditional blocks; any more inference of [possibly-undefined] would result in a copious amount of false positives.