[Python-ideas] __iter__ implies __contains__?

[Steven D'Aprano]

Antoine Pitrou wrote:
> On Sat, 1 Oct 2011 22:30:21 -0400
> Nick Coghlan <ncoghlan at gmail.com> wrote:
>> On Sat, Oct 1, 2011 at 9:21 PM, Chris Rebert <pyideas at rebertia.com> wrote:
>>> Actually, my suggestion was just that Sequence is one possible (but
>>> clean) way to obtain the behavior; one could *of course* reimplement
>>> the functionality without recourse to Sequence if they desired.
>> But why would that would forcing everyone implementation standard
>> sequences to reimplement the wheel be an improvement over the status
>> quo?
> 
> You don't reinvent the wheel if you accept to inherit from abc.Sequence.

You shouldn't be forced to inherit from abc.Sequence to implement the 
sequence protocols. The whole point of protocols is that they you don't 
need inheritance to make them work, let alone buy into the ABC mindset.

If you have a class that you don't want to be iterable but otherwise 
obeys the iteration protocol, that is easy to fix: have __iter__ raise 
TypeError. An easy fix for unusual and trivial problem.

 >>> class Test:
...     def __getitem__(self, i):
...             return i
...     def __iter__(self):
...             raise TypeError
...
 >>> for i in Test():
...     print(i)
...
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
   File "<stdin>", line 5, in __iter__
TypeError




-- 
Steven