Adding static typing to Python

Jeff Shannon jeff at
Tue Feb 19 20:44:57 CET 2002

Alexander Jerusalem wrote:

> And the final argument I have for static type checking is that it
> enables method dispatching based on parameter types. In a statically
> typed language you can create two methods that have the same name but
> differ on the parameter types. The correct method will be called for
> you depending on the type of the argument you pass in your call. That
> makes for a quite flexible way of extending a program. You can just
> add another method with the same name and another type to handle a
> special case without touching the existing methods.

In Python, you'd simply try to use the object in one way.  If that doesn't
work, catch the exception and try the next.  Then you can clearly see what
happens to different types of objects when you're applying the "same"
operation to them.  You don't need to use multiple methods to have
different behaviors, and at the same time, you don't have to add more
methods when you add a new type that you want to behave the same as an old

> I'm not, however, completely satisfied with static type checking as
> well because I don't see why the compiler forces me to have a
> parameter comply with a static interface that has 10 methods when the
> method that uses the parameter acutally only uses one of those
> methods. I want the compiler to check if the parameter type I'm
> passing has what the method needs but nothing more.

This is exactly what Python does, except that it does it at runtime
instead of compile time; and really, it is *not* any slower to test this
than it is to try a couple of compile cycles.  Plus, as has been
previously noted, PyChecker will scan for most such errors if it's used...

Jeff Shannon
Credit International

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