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All Things Pythonic
Python Optional Typechecking Redux
by Guido van van Rossum
January 18, 2005
After my last blog, many megabytes on the precise definition of adaptation were posted to python-dev (search for "PEP-246"). Overwhelmed by the discussion, I'm going to propose a somewhat Solomonic alternative.


Tony Lownds posted the following in response to my last blog:

There is a property that a typing system for python could have: that removing the type declarations from a program, or ignoring them at run-time, doesn't stop any programs from working.

I like this idea, not so much because it rules out implicit adaptation (the main bone of contention in the python-dev discussions), but because it stresses the optional aspect of the type declarations. But at the same time I still like the adaptation interpretation. So here's a possible way out: make the semantics of type checking configurable at run time.

Specifically, I propose to let module authors set a global variable named __typecheck__. This could be a function defined as follows:

def __typecheck__(x, T):
    if "your condition here":
        raise TypeError("your error message here") # Other errors are okay too
    return x

This is to be used as follows. Consider a function with type declarations:

def foo(a: t1, b: t2) -> t3:
    "your code here"

This would be replaced by something like the following:

def foo__(a, b): # The original function
    "your code here"

def foo(a, b): # Typechecking wrapper
    a = __typecheck__(a, t1)
    b = __typecheck__(b, t2)
    r = foo__(a, b)
    r = __typecheck__(r, t3)
    return r

(However, the expressions for t1, t2, t3 and foo__ should be evaluated only once, at function definition time. Probably, __typecheck__ should also be evaluated at this time.)

There would be a very conservative default __typecheck__ function (a built-in) which would assert isinstance(x, T). The -O option could disable typechecking entirely by not generating wrappers at all, if if the __typecheck__ global is not defined at function definition time.

If you like implicit adaptation, you can put this in your module:

from adaptation import adapt as __typecheck__

You could also define __typecheck__ inside a class, just like for __metaclass__ (although its scope would only be the current class, not its subclasses, unlike for __metaclass__, which is inherited). In either case, -O won't remove the type checking wrapper if __typecheck__ is set. (This can be done dynamically, e.g. by the metaclass checking for __debug__ if __typecheck__ is not set explicitly.)

You can also implement duck typing as defined by adapt(), as follows:

from adaptation import adapt
def __typecheck__(x, T):
    if adapt(x, T) is not x:
        raise TypeError("...")
    return x

This accepts all values for x that conform to T according to the adaptation registry.


After a long conversation with Phillip Eby, and awaiting the new PEP 246, it's likely that this latter version ("duck typing as defined by adapt()") would become the default if PEP 246 is accepted. (Feb 6, 2005.)

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About the Blogger

Guido van Rossum is the creator of Python, one of the major programming languages on and off the web. The Python community refers to him as the BDFL (Benevolent Dictator For Life), a title straight from a Monty Python skit. He moved from the Netherlands to the USA in 1995, where he met his wife. Until July 2003 they lived in the northern Virginia suburbs of Washington, DC with their son Orlijn, who was born in 2001. They then moved to Silicon Valley where Guido now works for Google (spending 50% of his time on Python!).

This weblog entry is Copyright © 2005 Guido van van Rossum. All rights reserved.

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