Reimplementing Python's enum.auto() for compatibility

Question

In Python >= 3.6 there is an enum.auto() function.

The auto() function can be used like this:

import enum
from enum import auto

class Fruits(enum.Enum):
  APPLE = auto()
  ORANGE = auto()
  BANANA = auto()

I want to use it, but i cannot be sure if i am on Python >= 3.6 so I implemented it like this:

import enum

try:
  from enum import auto
except ImportError: 
  __my_enum_auto_id = 0
  def auto() -> int:
    global __my_enum_auto_id
    i = __my_enum_auto_id
    __my_enum_auto_id += 1
    return i

class Fruits(enum.Enum):
  APPLE = auto()
  ORANGE = auto()
  BANANA = auto()

It works afaik, but it seems ugly to me. Any suggestions on how to improove it/make it better? or is it ok?

Is there some side effect I may have overlooked?

Answer 1

The easiest solution would be to use the aenum¹ library, which is drop-in compatible with the stdlib and also has some advanced features not found in the stdlib. It also works in Python versions 2.7 and 3.3+.

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¹ Disclosure: I am the author of the Python stdlib Enum, the enum34 backport, and the Advanced Enumeration (aenum) library.

Answer 2

Honestly, you won't be able to get anything as good as the built-in code for auto. This is as they've added a clause to _EnumDict.__setitem__ to handle the class.

Since we can't make it so that the values from auto don't start at one for each enum. Such as the following being equivalent:

class A(Enum):
    A = auto()
    B = auto()

class B(Enum):
    A = auto()

class A(Enum):
    A = 1
    B = 2

class B(Enum):
    A = 1

And so the simplest would be to stick with your code, and have the following be equal. (As you done)

class A(Enum):
    A = auto()
    B = auto()

class B(Enum):
    A = auto()

class A(Enum):
    A = 1
    B = 2

class B(Enum):
    A = 3

And so IMO your code is good. The only thing I'd change is make your code not pollute the global namespace with __my_enum_auto_id. And indent with four spaces. This is as easy as using a closure.

def auto_builder():
    number = 0
    def auto():
        nonlocal number
        number += 1
        return number
    return auto

try:
    from enum import auto
except ImportError: 
    auto = auto_builder()
del auto_builder

Answer 3

My only suggestion would be to user itertools.count() instead of implementing your own counter. Something like:

from itertools import count

_global_counter = count()

def auto():
    return next(_global_counter)