On 3/12/21 5:28 PM, Guido van Rossum wrote:
On Fri, Mar 12, 2021 at 1:52 PM Ethan Furman wrote:
A question that comes up quite a bit on Stackoverflow is how to test
to see if a value will result in an Enum member, preferably without
having to go through the whole try/except machinery.
A couple versions ago one could use a containment check:
if 1 in Color:
but than was removed as Enums are considered containers of members,
not containers of the member values.
Maybe you were a bit too quick in deleting it. Was there a serious
bug that led to the removal? Could it be restored?
Part of the reason is that there are really two ways to identify an
enum -- by name, and by value -- which should `__contains__` work with?
At this point I see three options:
1) add a `get(value, default=None)` to EnumMeta (similar to `dict.get()`
But the way to convert a raw value to an enum value is Color(1), not
Color[1], so Color.get(1) seems inconsistent.
Very good point.
Maybe you can just change the constructor so you can spell this as
Color(1, default=None) (and then check whether that's None)?
An interesting idea.
2) add a recipe to the docs
But what would the recipe say? Apparently you're looking for a one-liner,
since you reject the try/except solution.
The recipe would be for a method that could be added to an Enum, such as:
@classmethod
def get_by_value(cls, value, default=None):
for member in cls:
if member.value == value:
return member
return default
3) do nothing
Always a good option. :-)
Yes, but not always a satisfying one. :)
Where's that StackOverflow item? How many upvotes does it have?
93 - How do I test if int value exists in Python Enum without using try/catch?
https://stackoverflow.com/q/43634618/208880
25 - How to test if an Enum member with a certain name exists?
https://stackoverflow.com/q/29795488/208880
3 - Validate value is in Python Enum values
https://stackoverflow.com/q/54126570/208880
2 - How to check if string exists in Enum of strings?
https://stackoverflow.com/q/63335753/208880
I think I like your constructor change idea, with a small twist:
Color(value=<sentinel>, name=<sentinel>, default=<sentinal>)
This would make it possible to search for an enum by value or by name, and also
specify a default return value (raising an exception if the default is not set
and a member cannot be found).
--
~Ethan~
_______________________________________________
Python-ideas mailing list -- python-ideas@python.org
To unsubscribe send an email to python-ideas-le...@python.org
https://mail.python.org/mailman3/lists/python-ideas.python.org/
Message archived at
https://mail.python.org/archives/list/python-ideas@python.org/message/LJXMAORK2JVYTPQQ332NFEV2JPVFVJAX/
Code of Conduct: http://python.org/psf/codeofconduct/
