I have a situation where I need to enforce and give the user the option of one of a number of select functions, to be passed in as an argument to another function:
I really want to achieve something like the following:
from enum import Enum #Trivial Function 1 def functionA(): pass #Trivial Function 2 def functionB(): pass #This is not allowed (as far as i can tell the values should be integers) #But pseudocode for what I am after class AvailableFunctions(Enum): OptionA = functionA OptionB = functionB
So the following can be executed:
def myUserFunction(theFunction = AvailableFunctions.OptionA): #Type Check assert isinstance(theFunction,AvailableFunctions) #Execute the actual function held as value in the enum or equivalent return theFunction.value()
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Answer
Your assumption is wrong. Values can be arbitrary, they are not limited to integers. From the documentation:
The examples above use integers for enumeration values. Using integers is short and handy (and provided by default by the Functional API), but not strictly enforced. In the vast majority of use-cases, one doesn’t care what the actual value of an enumeration is. But if the value is important, enumerations can have arbitrary values.
However the issue with functions is that they are considered to be method definitions instead of attributes!
In [1]: from enum import Enum In [2]: def f(self, *args): ...: pass ...: In [3]: class MyEnum(Enum): ...: a = f ...: def b(self, *args): ...: print(self, args) ...: In [4]: list(MyEnum) # it has no values Out[4]: [] In [5]: MyEnum.a Out[5]: <function __main__.f> In [6]: MyEnum.b Out[6]: <function __main__.MyEnum.b>
You can work around this by using a wrapper class or just functools.partial
or (only in Python2) staticmethod
:
from functools import partial class MyEnum(Enum): OptionA = partial(functionA) OptionB = staticmethod(functionB)
Sample run:
In [7]: from functools import partial In [8]: class MyEnum2(Enum): ...: a = partial(f) ...: def b(self, *args): ...: print(self, args) ...: In [9]: list(MyEnum2) Out[9]: [<MyEnum2.a: functools.partial(<function f at 0x7f4130f9aae8>)>] In [10]: MyEnum2.a Out[10]: <MyEnum2.a: functools.partial(<function f at 0x7f4130f9aae8>)>
Or using a wrapper class:
In [13]: class Wrapper: ...: def __init__(self, f): ...: self.f = f ...: def __call__(self, *args, **kwargs): ...: return self.f(*args, **kwargs) ...: In [14]: class MyEnum3(Enum): ...: a = Wrapper(f) ...: In [15]: list(MyEnum3) Out[15]: [<MyEnum3.a: <__main__.Wrapper object at 0x7f413075b358>>]
Also note that if you want you can define the __call__
method in your enumeration class to make the values callable:
In [1]: from enum import Enum In [2]: def f(*args): ...: print(args) ...: In [3]: class MyEnum(Enum): ...: a = partial(f) ...: def __call__(self, *args): ...: self.value(*args) ...: In [5]: MyEnum.a(1,2,3) # no need for MyEnum.a.value(1,2,3) (1, 2, 3)