[Python-ideas] Re: Enum: determining if a value is valid

On Wed, Mar 17, 2021, 5:34 AM Serhiy Storchaka storchaka@gmail.com wrote:

12.03.21 23:48, Ethan Furman пише:

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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.

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Thoughts?

The Enum class is already too overloaded. I sometimes think about adding SimpleEnum class with minimal simple functionality which would allow to use enums in more modules sensitive to import time.

As for solving your problem, try/except looks the best solution to me.

try:
    Color(1)
except ValueError:
    ... # invalid color
else:
    ... # valid color

If you don't like try/except, the second best solution is to add a module level helper in the enum module:

def find_by_value(cls, value, default=None):
    try:
        return cls(value)
    except ValueError:
        return default

You can add also find_all_by_value(), get_aliases(), etc. It is important that they are module-level function, so they do not spoil the namespace of the Enum class.

+1 on a module level helper.

The preponderance of stackoverflow questions seem convincing enough that people want to do this and it seems like a reasonable request. And I have wanted to do something like this myself, so I'd probably use it. I see it as analogous to situations when you want to use dict.get(key) instead of dict[key].

But adding a non-dunder method to the Enum class namespace seems more suboptimal to me compared to a module level helper, because of the namespace spoiling/crowding issue. No matter what method name were to be chosen, someone at some point would want to use it as an Enum member name (unless of course it's a reserved dunder method).