[Python-3000] PEP 3133: Introducing Roles
Phillip J. Eby
pje at telecommunity.com
Mon May 14 18:42:27 CEST 2007
At 10:36 PM 5/13/2007 -0700, Collin Winter wrote:
>2. Querying instances ::
> if performs(my_elf, Thieving):
-1 on using any function other than isinstance() for this.
Rationale: isinstance() makes the code smell of inspection more
obvious, where another function name makes it seem like you are doing
something harmless. In reality, performs() testing (or any other
kind of interface testing) using if-then is always harmful in library code.
> The second argument to ``performs()`` may also be anything with a
> ``__contains__()`` method, meaning the following is legal: ::
> if performs(my_elf, set([Thieving, Spying, BoyScout])):
> Like ``isinstance()``, the object needs only to perform a single
> role out of the set in order for the expression to be true.
Right, so let's just use isinstance(). Likewise, issubclass() for
checking whether instances of a class perform a role. (And if
issubclass() works, then roles will also be usable by PEP 3124
generic functions without any additional effort.)
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