Initialize a Literal enum in a pydantic model

Question

pydantic supports regular enums just fine, and one can initialize an enum-typed field using both an enum instance and an enum value:

from enum import Enum
from pydantic import BaseModel

class MyEnum(Enum):
    FOO = 'foo'
    BAR = 'bar'

class MyModel(BaseModel):
    x: MyEnum

MyModel(x=MyEnum.BAR)  # Enum instance, works
MyModel(x='foo')       # Enum value, works
MyModel(x='?')         # Fails, as expected

pydantic also supports typing.Literal:

from typing import Literal
from pydantic import BaseModel

class MyModel(BaseModel):
    x: Literal['foo']

MyModel(x='foo')  # Works
MyModel(x='bar')  # Fails, as expected

Now I want to combine enums and literals, i.e. force a field value to equal one particular enum instance. However, I still want to be able to pass the correct enum value (i.e. not only the correct enum instance), and that doesn't seem to work:

from enum import Enum
from typing import Literal
from pydantic import BaseModel

class MyEnum(Enum):
    FOO = 'foo'
    BAR = 'bar'

class MyModel(BaseModel):
    x: Literal[MyEnum.FOO]

MyModel(x=MyEnum.FOO)  # Enum instance, works
MyModel(x=MyEnum.BAR)  # Fails, as expected
MyModel(x='foo')       # Enum value, fails but I'd like it to work

There is now an open ticket for this (at the time of writing): https://github.com/samuelcolvin/pydantic/issues/2257 — Scott Stevens, Mar 23 '22 at 00:59

alex_noname · Answer 1 · 2022-01-29T19:27:30.317

14

Try to use string based Enum. Like so:

from enum import Enum
from typing import Literal
from pydantic import BaseModel


class MyEnum(str, Enum):
    FOO = 'foo'
    BAR = 'bar'


class MyModel(BaseModel):
    x: Literal[MyEnum.FOO]


MyModel(x=MyEnum.FOO)  # Enum instance, works
# MyModel(x=MyEnum.BAR)  # Fails, as expected
MyModel(x='foo')       # String value, works

edited Jan 29 '22 at 19:27

answered Mar 11 '21 at 11:50

alex_noname

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That's clever, thanks! I'm not sure whether subclassing `str` will work in all use cases, but it should in mine. – Florian Brucker Mar 11 '21 at 12:02
This can be found in the docs: https://docs.python.org/3/library/enum.html#others – rocksteady Jul 18 '22 at 12:51
In Python 3.11+ there is even a pre-built class for this: [enum.StrEnum](https://docs.python.org/3/library/enum.html#enum.StrEnum) – Roméo Després Jul 26 '23 at 09:16

score 2 · Answer 2 · answered Jul 13 '22 at 14:44

You might have noticed that an Enum based class attribute is not of type str. Accessing the actual value of the attribute can be achieved using .value.

Please refer to: https://docs.python.org/3/library/enum.html

Example:

>>> from enum import Enum

>>> class MyEnum(Enum):
...    FOO = 'foo'
...    BAR = 'bar'

>>> MyEnum.FOO
<MyEnum.FOO: 'foo'>
>>> type(MyEnum.FOO)
<enum 'MyENum'>

You need to retrieve the actual value of it like this:

>>> MyEnum.FOO.value
'foo'
>>> type(MyEnum.FOO.value)
str
>>> MyEnum.FOO.name
'FOO'

Initialize a Literal enum in a pydantic model

2 Answers2