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I wonder a lot, "How do I subclass built-in types?"

There's a related question: "How do I figure out if something is sort of like a particular type?"

My goal is to take something, and say, "Does this act pretty much like an integer?" Or "does this act pretty much like a float?" Or "does this act pretty much like a dictionary?"

You could: count up all the behaviors you rely on. Then test if they all exist. If they all exist, consider that item a fit.

But, that seems really hard, if you've got a whole class with a ton of methods, all of which use the item. "Counting up" seems pretty hard in that case, and it seems like it would require a lot of discipline.

Is there, then, a set of tests that we can run over an item, to see if it acts "pretty much like" an int, a float, a dict, a string, blah blah blah...? (Or, at least supports the interface for all those things?)

What do people do about this?

Because I read all these things saying, "Don't use type!" "Don't use isInstance!" But I don't see much in the way of what to use.

-- LionKimbro 2004-09-12 17:10:47

[lwickjr]: I like to use issubclass. I suppose one could define a module in the Standard Library consisting entirely of empty classes with appropriate names, and inherit from them:

Module interfaces:

class dictLike: pass

User module:

import interfaces
class Shelf(interfaces.dictLike,...):

Then other users could write isinstance(Thing, interfaces.dictLike), and be assured that anything that passes the test /promises/ to be "like a dictionary".


For instance, on the int class, there are some gazillion methods:

#!python >>> dir( 42 ) ['abs', 'add', 'and', 'class', 'cmp', 'coerce', 'delattr', 'div', 'divmod', 'doc', 'float', 'floordiv', 'getattribute', 'getnewargs', 'hash', 'hex', 'init', 'int', 'invert', 'long',

'or', 'pos', 'pow', 'radd', 'rand', 'rdiv', 'rdivmod', 'reduce', 'reduce_ex', 'repr', 'rfloordiv', 'rlshift', 'rmod', 'rmul', 'ror', 'rpow', 'rrshift', 'rshift', 'rsub', 'rtruediv', 'rxor', 'setattr', 'str', 'sub', 'truediv', 'xor'] >>> }}}

If you want to make something that's functionally like an int, can you get around having to implement all these methods?

Maybe I have the wrong page title; Maybe this shouldn't be called "SubclassingBuiltInTypes," but rather "SimulatingBuiltInTypes."

-- LionKimbro 2004-09-13 00:00:50

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