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

[Ethan Furman]

API question.

Background:
----------

When doing checks such as

--> 3 in [4, 5, 6]
--> 'test' in {'test':True, 'live':False}

the result is True or False.

When doing checks such as

--> 3 in 'hello world'
--> [4, 5, 6] in {'test':True, 'live':False}

the result is a TypeError.

The general rule seems to be that if it is impossible for the in-question object to be in the container object then a 
TypeError is raised, otherwise True or False is returned.


Question 1:
----------

(A) A standard Enum class is a container of Enum members.  It cannot hold anything else.  However, it has been returning 
False both in cases where the in-question object was an Enum of a different class (a Fruit in a Color, for example) and 
when the in-question object was not even an Enum ('apple' in Fruit, for example).

(B) The waters get even more muddied when Fruit has a str mixin, so `Fruit.APPLE == 'apple' is True` -- in that case, 
should `'orange' in Fruit` return True, False, or raise TypeError?


Question 2:
----------

(A) The new Flag type allows `in` tests on the members themselves; so, for example:

--> SomeFlag.ONE in SomeFlag.One|SomeFlag.TWO
True

The question, of course, is what to do when a non-Flag member is tested for:

--> 'apple' in SomeFlag.ONE
# False or TypeError?

--> 2 in SomeFlag.TWO
# True or TypeError?

(B) And, of course, the same muddier question arises with IntFlag, where SomeFlag.TWO == 2 is True.


My thoughts:
-----------

For question 1A (pure Enum):  I'm thinking a TypeError should be raised when the in-question object is not an Enum 
member of any kind -- it simply is not possible for that object to ever be in an Enum, and is surely a bug.

For question 1B (mixed Enum):  if 1A is TypeError, then 1B should also be TypeError at least for non-mixin in-question 
types (so checking for 1 in StrEnum would be a TypeError), but I'm torn between TypeError and True/False for cases where 
the in-question type matches the mixin type ('apple' in StrEnum)....  On the one hand, even though an Enum member might 
be equal to some other type, that other type will not have the Enum attributes, etc, and a True answer would lead one to 
believe you could access `.name` and `.value`, etc., while a False answer would lead one to believe there was no match 
even though equality tests pass; on the other hand, how strong is the "container" aspect of a mixed Enum?  How often is 
the test `'apple' in Fruit` meant to discover if you have a Fruit member vs whether you have something that could be a 
Fruit member?  Also, how important is it to be consistent with IntFlag, which I definitely think should return 
True/False for int checks?

For question 2A (pure Flag):  I'm comfortable sticking with a TypeError (assuming we switch to TypeError for 1A).

For question 2B (int Flag):  I think a TypeError if the in-question object is not an int is appropriate (assuming 
TypeError for 1A and 2A), but if it is an int, True/False seems the better path.  My reasoning being that Flag and 
IntFlag are more similar to sets than lists, and IntFlag is specifically meant to work with ints, and a test of `2 in 
some_int_flags` is more concerned with a flag being set than with .name or .value attributes that may or may not exist 
on the in-question object.

Any and all thoughts appreciated.

--
~Ethan~