Tools for static type checking in Python

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Edit 2016-11-11: Just use mypy. Type hints can be added gradually. In Python 3 source code, it verifies standard PEP 484 type hints. Types can still be expressed in Python 2 using special comments. Guido likes it.

This post was originally written a long time ago before mypy was a thing. I’ve preserved the post’s original content below, even though it isn’t quite accurate.

Original post:

You might want to check out some of the projects mentioned in this related StackOverflow post on static analysis for Python.

In summary:

Since Python uses duck typing extensively, things that might be called “type errors” in other languages might end up being “object X doesn’t support method Y” in Python.

Edit 2011-05-17:

I agree with delnan that static typing is not possible for Python [apparently wrong]. But since our skepticism doesn’t seem to deter you, I can only give you more information on the subject. I present:

  • A discussion of type inference for Python. (Other links are from here.)
  • Guido van van Rossum’s articles on adding optional static typing: part 1 and part 2.
  • RPython, a subset of Python that might stand a chance of being statically analyzed enough to do some form of type checking.

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