#pragma section-numbers off
== See Also ==

 * SubclassingDictionaries

= Discussion =

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 <<DateTime(2004-09-12T17:10:47Z)>>

----

[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".

Comments?

----
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__',
 '__lshift__', '__mod__', '__mul__', '__neg__', '__new__', '__nonzero__', '__oct__',
'__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 <<DateTime(2004-09-13T00:00:50Z)>>