[Python-Dev] operator.is*Type

Raymond Hettinger python at rcn.com
Wed Feb 22 22:18:58 CET 2006

> But  given :
> True True Instance w getitem <type 'instance'>
> True True NewStyle Instance w getitem <class '__main__.cng'>
> True True [] <class UserList.UserList at 0x00F11B70>
> True True {} <type 'instance'>
> (Last one is UserDict)
> I can't conceive of circumstances where this is useful without duck
> typing *as well*.

Yawn.  Give it up.  For user defined instances, these functions can only 
discriminate between the presence or absence of __getitem__.  If you're trying 
to distinguish between sequences and mappings for instances, you're own your own 
with duck-typing.  Since there is no mandatory mapping or sequence API, the 
operator module functions cannot add more checks without getting some false 
negatives (your original example is a case in point).

Use the function as-is and add your own isinstance checks for your own personal 
definition of what makes a mapping a mapping and what makes a sequence a 
sequence.  Or better yet, stop designing APIs that require you to differentiate 
things that aren't really different ;-)


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