Tools for static type checking in Python

Question

I'm working with a large existing Python codebase and would like to start adding in type annotations so I can get some level of static checking. I'm imagining something like Erlang, Strongtalk, or Typed Scheme/Racket.

I've seen quick-and-dirty decorators that insert dynamic checks based on function parameter and return type annotations, but I'm looking for something that is more robust and that performs checks at compile-time.

What tools are available right now for this kind of thing? I'm familiar with compilers and type checking and am definitely willing to improve an incomplete tool if it has a good foundation.

(Note: I'm not interested in a discussion of the pros/cons of static typing.)

EDIT: An example:

def put(d, k, v):
   d[k] = v

I'd like to be able to annotate the put function as having type put<K,V>(dict<K,V>, K, V) -> None.

UPDATE: The new PEP 484 (Sep 2014) defines a standard for static typing and type annotations in Python 3.5+. There's a type-checking tool called mypy that is compatible with PEP 484.

Answer 1

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.

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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:

• pylint
• PyFlakes
• PyChecker

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.