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[Q] Static type checker for Python

Neel Krishnaswami Neel.Krishnaswami at p98.f112.n480.z2.fidonet.org
Thu Jul 1 12:17:48 EDT 1999 
From: neelk at brick.cswv.com (Neel Krishnaswami)

In article <377ADAD7.EA105C50 at mojam.com>,
Skip Montanaro  <skip at mojam.com> wrote:
>> Can I make static type checker for Python?
>> or Is there any previous work about this?
>
>In theory, you could do some type inference and generate some warning
>messages, but since you don't declare variables to be of a particular
>type in Python, it's unlikely it would be very useful.  The same
>variable can point to many different types at runtime.

I'm about to go off on a tangent, so first I want to warn the original
poster that there aren't too many actual problems with dynamic typing
-- people have written huge and complex apps with no problems using
Smalltalk and Lisp, for example. 

Now, it is possible to teach Python to check the argument types of
functions at runtime, and I've written a little module called
TypeChecker to do just that. It's a wrapper class with callable
instances, and the instance checks the declaration you make in a
docstring against the actual types and classes of the arguments. 
For example:

def fact(n):
    """foo(n :: LongType) <-- here's the type declaration; it can be
			      either a type from the types module or a
			      class name.
    A factorial that only takes longs as arguments."""
    f = 1L
    for i in range(1L, n+1):
        f = f * i
    return f

f = TypeChecker(fact) 

>>> fact(3)
6L

>>> f(3L)
6L

>>>f(3)
Traceback (innermost last):
  File "<stdin>", line 1, in ?
  File "/usr/tmp/python-6-237", line 25, in __call__
  File "/usr/tmp/python-6-237", line 118, in check_args
TypeError: Argument 1 of fact wrong type, <type 'int'>, declared <type 'long
int'>

What it's doing is parsing the type declaration in the doc string to
generate a list of types and classes, and then the __call__ method of
the instance is comparing the declared types against the runtime
arguments it receives before running the function. I've always wanted
to abuse docstrings, and now I've gotten my chance. :) Though this is
a fairly mild abuse, considering that it doesn't block you from
putting other things into the docstring.

Obviously, this is not terribly useful. (It doesn't handle subclasses
correctly, f'ex, and can't yet be used on class methods.) But it is
interesting to see what Python static types might look like. (The
answer to my eyes is "fairly ugly;" real lexical scoping and
type/class unification seem to me to be prerequisites before static
typing will behave gracefully in Python.)

The actual code for TypeChecker is a little too long to put in the
post, so here's a URL:

    http://www.sff.net/people/neelk/free-software/TypeChecker.py

Neel
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