Adding static typing to Python

gbreed at cix.compulink.co.uk gbreed at cix.compulink.co.uk
Wed Feb 20 18:12:36 CET 2002


Alexander Jerusalem wrote:

> I just looked up what CLOS does. I don't know if you call that static
> typing or not. In any case, CLOS does have type declarations in the
> source code which is what I want and what enables the features I'd
> like to see in Python. For example in CLOS you can have the following
> method declarations:
> 
>  (defmethod Area ((C Circle)) (* pi (Square (Radius C)))
>            (defmethod Area ((R Rectangle)) (* (Width R) (Height R)))
>            (defmethod Area ((Sq Square)) (Square (Width Sq)))
>            OR (defmethod Height ((Sq Square)) (Width Sq))

But you can do that in Python:

>>> def area(shape):
...   if isinstance(shape, Circle):
...     return math.pi*shape.radius**2
...   if isinstance(shape, Rectangle):
...     return shape.width*shape.height
...   if isinstance(shape, Square):
...     return shape.width**2
...   raise TypeError, "unknown shape"
...

It can't take much imagination to see how it could be sugared to work with 
multiple functions with types resolved at run time.  As for strict, 
strong, static, explicit typing (which may be a straw man) it looks *less* 
suitable for this example.

Say you want to include a triangle in your list of shapes.  So, you add

...   if isinstance(shape, Triangle):
...     return shape.base*shape.height

to the area function.  But then, will all the other functions that call 
area be allowed to accept a triangle?  If they're defined to accept either 
circles, rectangles or squares, you'll have to change each one's interface 
to allow triangles as well.

You could make them all subclasses of Shape.  In which case, you have 
functions defined to use a polymorphic object that depend on function 
overloading by the real type.  Not all statically typed languages will 
allow that.  If they do, you have the problem that you might have added 
triangles for some other reason, and forgotten to cater for them in 
area().  You aren't getting static type checking at all.

Wouldn't it be better to make area a method of the shapes?  And for static 
typing, make it abstract in the Shape class?


                     Graham

         (http://www.microtonal.co.uk/)



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