[Python-Dev] Alternative to more ABCs [was:] Iterable String Redux (aka String ABC) (original) (raw)

Guido van Rossum guido at python.org
Sun Jun 1 00:29:18 CEST 2008

I'm willing to meet you halfway. I really don't want isinstance(x, str) to return True for something that doesn't inherit from the concrete str type; this is bound to lead to too much confusion and breakage. But I'm fine with a String ABC (or any other ABC, e.g. Atomic?) that doesn't define any methods but can be used for type testing. How about that?

--Guido

On Sat, May 31, 2008 at 5:25 AM, Raymond Hettinger <python at rcn.com> wrote:

ISTM, the whole reason people are asking for a String ABC is so you can write isinstance(obj, String) and allow registered string-like objects to be accepted.

The downside is that everytime you want this for a concrete class or type, it is necessary to write a whole new ABC listing all of the required methods. Also, you have to change all of the client code's isinstance tests from concrete to abstract. I propose a simpler approach. Provide an alternative registration function that overrides isinstance() so that objects can explicitly fake any concrete type: s = UserString('whiffleball') print isinstance(s, str) --> False registerlookalike(UserString, str) print isinstance(s, str) --> True Besides saving us from writing tons of new ABCs, the approach works with existing code that already uses isinstance() with concrete types. The ABCs that would remain are ones that are truly abstract, that define a generic interface (like mappings and sequences) and ones that offer some useful mixin behavior. The remaining ABCs are ones where you have a fighting chance of actually being able to implement the interface (unlike String where it would be darned tricky to fully emulate encode(), split(), etc.) This would completely eliminate the need for numbers.Integral for example. AFAICT, its sole use case is to provide a way for numeric's integral types (like int8, int32) to pass themselves off as having the same API as regular ints. Unfortunately, the current approach requires all consumer code to switch from isinstance(x,int) to isinstance(x,Integral). It would be more useful if we could simply write registerlookalike(x,int) and be done with it (no need for numbers.py and its attendant abc machinery). If we don't do this, then String won't be the last request. People will want Datetime for example. Pretty much any concrete type could have a look-a-like that wanted its own ABC and for all client code to switch from testing concrete types.

Raymond

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-- --Guido van Rossum (home page: http://www.python.org/~guido/)

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