On 4/4/2018 11:32 AM, Ethan Furman wrote:
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.
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.
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.
I would find 2 in some_int_flags being True surprising.
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