Anže's Blog

Python, Django, and the Web

21 Dec 2022

Enum with `str` or `int` Mixin Breaking Change in Python 3.11

Python 3.11 was released almost two months ago and brought some great improvements to the Enum class. Unfortunately, there is also a breaking change. You might be impacted if you are using Enum classes with a str/int mixin:

from enum import Enum

class Foo(str, Enum):
    BAR = "bar"

Foo.BAR in Python 3.11 will no longer return the member value "bar" when used in the format() function or f-strings the way that prior Python versions used to. Instead, it will return the Foo.BAR member class.

# Python 3.10
f"{Foo.BAR}"  # > bar
# Python 3.11
f"{Foo.BAR}"  # > Foo.BAR

If you have f"{Foo.BAR}" in your code, it will likely break when you migrate to 3.11, so upgrade with caution.

The easiest way to fix it is to replace the str mixin with the newly added StrEnum class, but let’s take one step back and take a look at Enum classes and why my project ended up having more than 50 of them with the str mixin.


Enum classes were introduced in Python 3.4 with PEP-435 to help developers define symbolic names for constant values. This is useful because having Foo.BAR is much more readable than having the "bar" constant spread across the codebase.

Enum classes do have some surprising behaviors. Let’s look at the example:

class Foo(Enum):
    BAR = "bar"

If you try to use the BAR value in the code like this:

assert Foo.BAR == "bar"

You will receive an assertion error! This is because Foo.BAR is not a str instance as it might seem from the class definition. To get to the str value you’d need to write:

assert Foo.BAR.value == "bar"

But knowing when to use .value is inconvenient. Engineers will try to find a workaround. On my project the workaround was to add a str mixin:

class Foo(str, Enum):
    BAR = "bar"

This makes it seem like the problem is solved since assert Foo.BAR == "bar" no longer raises an error (in Python 3.10 or older versions), but…

There be dragons!

This only works for f-strings and format functions. The old % based str formatting syntax will still be broken. The str function will also not return "bar". More examples in Python 3.10:

print(Foo.BAR)                 # > Foo.BAR
print(str(Foo.BAR))            # > Foo.BAR
print("%s" % Foo.BAR)          # > Foo.BAR
print(f"{Foo.BAR}")            # > bar
print("{}".format(Foo.BAR))    # > bar

This inconsistency was fixed in Python 3.11 and the output is now the same everywhere:

print(Foo.BAR)                 # > Foo.BAR
print(str(Foo.BAR))            # > Foo.BAR
print("%s" % Foo.BAR)          # > Foo.BAR
print(f"{Foo.BAR}")            # > Foo.BAR
print("{}".format(Foo.BAR))    # > Foo.BAR

Consistency is always good! But this fix was what broke our code.

We were expecting "bar" but were now getting Foo.BAR! So now we are back on square one. In the cases above Foo(str, Enum) behaves the same as Foo(Enum). So how can we avoid writing .value everywhere?

StrEnum to the rescue

Luckily, there is a solution for this in Python 3.11. The newly added StrEnum class! Now we can write:

from enum import StrEnum

class Foo(StrEnum):
    BAR = "bar"

And the results are going to be exactly what we originally wanted with the str mixin, except that it’s going to work consistenlty for all cases that I could come up with:

print(Foo.BAR)                 # > bar
print(str(Foo.BAR))            # > bar
print("%s" % Foo.BAR)          # > bar
print(f"{Foo.BAR}")            # > bar
print("{}".format(Foo.BAR))    # > bar

Python 3.10 or older

If you want to stop using the str mixin and aren’t quite ready to upgrade to Python 3.11 yet, I suggest you check out the StrEnum PyPi package. For the examples above, it behaves the same as Python 3.11, you just need to import StrEnum from strenum instead of enum.

from strenum import StrEnum

class Foo(StrEnum):
    BAR = "bar"

You can keep using the StrEnum package even after you upgrade to Python 3.11, it even has some extra features that the standard library StrEnum version doesn’t have.

“What’s New In Python 3.11” Page

The “What’s New In Python 3.11” page seems to have a small mistake in the changelog that describes this change:

Changed Enum.format() (the default for format(), str.format() and f-strings) of enums with mixed-in types (e.g. int, str) to also include the class name in the output, not just the member’s key. This matches the existing behavior of enum.Enum.str(), returning e.g. ‘AnEnum.MEMBER’ for an enum AnEnum(str, Enum) instead of just ‘MEMBER’.

Unless I’m misunderstanding something, the documentation should state that the format function and f-strings were returning the MEMBER value and not just the MEMBER without the class. I’ve opened a PR in the CPython project to see if we can clear this up a little bit.


I was avoiding the use of Enums because of the gotchas outlined in this post, but I do like how the new StrEnum (and IntEnum) classes work and I think I’ll be using them a lot more going forward 🎉

Do you have a better way of dealing with Enum classes? Send a toot, tweet, email, or open a PR and I’ll update the blog post with your idea.

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