[Python-Dev] Enum, Flag, __contains__, and False vs TypeError

[Guido van Rossum]

I don't think of "in" as an equality test -- it's special just like <, <=,
>, >= are special, and those can certainly raise TypeError.

On Wed, Apr 4, 2018 at 2:07 PM, Ethan Furman <ethan at stoneleaf.us> wrote:

> On 04/04/2018 01:24 PM, Glenn Linderman wrote:
>
> I think the "in" test should raise TypeError if tested against _anything_
>> that is not an Enum member.
>>
>> Why? I see a parallel between Enum and mappings.
>>
>> x = {'test':True, 'live':False}
>>
>> So it is True that  'test' in x   and  'live' in x  and False that True
>> in x and False in x.
>>
>> It is False that  'foo' in x  and  3 in x
>>
>> It is TypeError that [4,5,6] in x, or {'foo': 'bar'} in x.
>>
>> Note that it is False that (4,5,6) in x  which is a little surprising
>> given the above two, but not when you realize the
>> differences between this and the above two.
>>
>> So with mappings, you can have any hashable type as a key: with Enum, you
>> can only have Enum members as keys.
>>
>
> Technically, an Enum has strings as keys (the member names), while the
> members themselves are the values; but list()ing an Enum returns the values
> (members) while a mapping would return the names (keys);  Enums are
> definitely a bit strange!
>
> So I have no idea why you would want to return False, rather than
>> TypeError, other than (1) the distinction probably
>> doesn't matter to most people (2) backward compatibility.
>>
>
> To me, `in` is an equality test, and equality tests should never, ever
> raise exceptions -- they should return True or False.  Which is why I was
> surprised that `1 in "hello"` raised instead of returning False.  However,
> I have learned that in certain situations, such as str or dict or set, if
> it is impossible to contain the item being searched for then raising an
> exception is appropriate.
>
> Hence my questions now on deciding what the behavior /should/ be, and
> whether practicality has a log to stand on in this case.  ;)
>
>
> --
> ~Ethan~
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-- 
--Guido van Rossum (python.org/~guido)
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