String-based enum in Python

Question

To encapsulate a list of states I am using enum module:

from enum import Enum

class MyEnum(Enum):
    state1='state1'
    state2 = 'state2'

state = MyEnum.state1
MyEnum['state1'] == state  # here it works
'state1' == state  # here it does not throw but returns False (fail!)

However, the issue is that I need to seamlessly use the values as strings in many contexts in my script, like:

select_query1 = select(...).where(Process.status == str(MyEnum.state1))  # works but ugly

select_query2 = select(...).where(Process.status == MyEnum.state1)  # throws exeption

How to do it avoiding calling additional type conversion (str(state) above) or the underlying value (state.value)?

Answer 1

It seems that it is enough to inherit from str class at the same time as Enum:

from enum import Enum

class MyEnum(str, Enum):
    state1 = 'state1'
    state2 = 'state2'

The tricky part is that the order of classes in the inheritance chain is important as this:

class MyEnum(Enum, str):
    state1 = 'state1'
    state2 = 'state2'

throws:

TypeError: new enumerations should be created as `EnumName([mixin_type, ...] [data_type,] enum_type)`

With the correct class the following operations on MyEnum are fine:

print('This is the state value: ' + state)

---

As a side note, it seems that the special inheritance trick is not needed for formatted strings which work even for Enum inheritance only:

msg = f'This is the state value: {state}'  # works without inheriting from str

Answer 2

By reading the documentation (i.e., I didn't try it because I use an older version of Python, but I trust the docs), since Python 3.11 you can do the following:

from enum import StrEnum

class Directions(StrEnum):
    NORTH = 'north'
    SOUTH = 'south'

print(Directions.NORTH)
>>> north

Note that it looks like when subclassing StrEnum, defining the enum fields as single-value tuples will make no difference at all and would also be treated as strings, like so:

class Directions(StrEnum):
    NORTH = 'north',    # notice the trailing comma
    SOUTH = 'south'

Please refer to the docs and the design discussion for further understanding.

If you're running python 3.6+, execute pip install StrEnum, and then you can do the following (confirmed by me):

from strenum import StrEnum

class URLs(StrEnum):
    GOOGLE = 'www.google.com'
    STACKOVERFLOW = 'www.stackoverflow.com'

print(URLs.STACKOVERFLOW)

>>> www.stackoverflow.com

You can read more about it here.

Also, this was mentioned in the docs - how to create your own enums based on other classes:

While IntEnum is part of the enum module, it would be very simple to implement independently:

class IntEnum(int, Enum): pass This demonstrates how similar derived enumerations can be defined; for example a StrEnum that mixes in str instead of int.

Some rules:

When subclassing Enum, mix-in types must appear before Enum itself in the sequence of bases, as in the IntEnum example above.

While Enum can have members of any type, once you mix in an additional type, all the members must have values of that type, e.g. int above. This restriction does not apply to mix-ins which only add methods and don’t specify another type.

When another data type is mixed in, the value attribute is not the same as the enum member itself, although it is equivalent and will compare equal.

%-style formatting: %s and %r call the Enum class’s str() and repr() respectively; other codes (such as %i or %h for IntEnum) treat the enum member as its mixed-in type.

Formatted string literals, str.format(), and format() will use the mixed-in type’s format() unless str() or format() is overridden in the subclass, in which case the overridden methods or Enum methods will be used. Use the !s and !r format codes to force usage of the Enum class’s str() and repr() methods.

Source: https://docs.python.org/3/library/enum.html#others

Answer 3

what is wrong with using the value?

Imho, unless using Python version 3.11 with StrEnum I just override the __str__(self) method in the proper Enum class:

class MyStrEnum(str, Enum):

    OK     = 'OK'
    FAILED = 'FAILED'

    def __str__(self) -> str:
        return self.value

Best

Answer 4

While a mixin class between str and Enum can solve this problem, you should always also think about getting the right tool for the job.

And sometimes, the right tool could easily just be a MODULE_CONSTANT with a string value. For example, logging has a few constants like DEBUG, INFO, etc with meaningful values - even if they're ints in this case.

Enums are a good tool and I often use them. However, they're intended to be primarily compared against other members of the same Enum, which is why comparing them to, for example, strings requires you to jump through an additional hoop.

Answer 5

If associated string values are valid Python names then you can get names of enum members using .name property like this:

from enum import Enum
class MyEnum(Enum):
    state1=0
    state2=1

print (MyEnum.state1.name)  # 'state1'

a = MyEnum.state1
print(a.name)  # 'state1'

If associated string values are arbitrary strings then you can do this:

class ModelNames(str, Enum):
    gpt2 = 'gpt2'
    distilgpt2 = 'distilgpt2'
    gpt2_xl = 'gpt2-XL'
    gpt2_large = 'gpt2-large'

print(ModelNames.gpt2) # 'ModelNames.gpt2'
print(ModelNames.gpt2 is str) # False
print(ModelNames.gpt2_xl.name) # 'gpt2_xl'
print(ModelNames.gpt2_xl.value) # 'gpt2-XL'

Try this online: https://repl.it/@sytelus/enumstrtest

Answer 6

With auto:

from enum import Enum, auto

class AutoStrEnum(str, Enum):
    """
    StrEnum where auto() returns the field name.
    See https://docs.python.org/3.9/library/enum.html#using-automatic-values
    """
    @staticmethod
    def _generate_next_value_(name: str, start: int, count: int, last_values: list) -> str:
        return name

class MyEnum(AutoStrEnum):
    STATE_1 = auto()
    STATE_2 = auto()

Try it:

MyEnum.STATE_1 == "STATE_1"  # True

Answer 7

If you want to work with strings directly, you could consider using

MyEnum = collections.namedtuple(
    "MyEnum", ["state1", "state2"]
)(
    state1="state1", 
    state2="state2"
)

rather than enum at all. Iterating over this or doing MyEnum.state1 will give the string values directly. Creating the namedtuple within the same statement means there can only be one.

Obviously there are trade offs for not using Enum, so it depends on what you value more.

Answer 8

Simply use .value :

MyEnum.state1.value == 'state1'
# True