Feature proposal: cast values in Literals

[antonagestam]

I ran into a case where I'm modeling API responses like this, simplified:

@dataclass
class Contact:
    contactType: Literal["Member", "Contact"]


@dataclass
class Member(Contact):
    contactType: Literal["Member"]

dacite enables me to make sure that objects returned from a certain endpoint are Members and not any Contact, by looking at the contactType field. Now, I'm finding myself also wanting to use the "Member" and "Contact" strings in a few other places and so it would be nice to introduce an enum for them. Literals support enum values, so these would be valid models as well:

class ContactType(enum.Enum):
    member = "Member"
    contact = "Contact"


@dataclass
class Contact:
    contactType: ContactType


@dataclass
class Member(Contact):
    contactType: Literal[ContactType.member]

However, dacite doesn't currently recognize that the literal contains an enum value and won't try to cast string values to their enum equivalents. I realize that this might be a pretty complicated thing to ask for, but it would be nice to see this implemented. If you think this is a behavior that makes sense I'd be willing to give a shot at implementing this.

To clarify, the feature would be to try and apply matching classes from cast=[...] in the config.

[konradhalas]

Hi @antonagestam - it sounds like a reasonable feature. I will be more than happy to receive PR :)

[antonagestam]

@konradhalas Awesome, unfortunately it's not likely that I'll have time to spend on this anytime soon, just in case anyone else wants to work on this in the meanwhile.

[konradhalas]

@antonagestam sure, I will add it to my roadmap :)

[miknet]

Hello,

I'd also like to see this feature in some near future. In v1.7.0. this could be potentially implemented with the following change:

index 6971f11..e2f5a2b 100644
--- a/dacite/types.py
+++ b/dacite/types.py
@@ -1,4 +1,5 @@
 from dataclasses import InitVar
+from enum import Enum
 from typing import (
     Type,
     Any,
@@ -12,6 +13,7 @@ from typing import (
     List,
     Tuple,
     cast as typing_cast,
+    get_args,
 )
 
 T = TypeVar("T", bound=Any)
@@ -49,6 +51,11 @@ def transform_value(
             )
         item_cls = extract_generic(target_type, defaults=(Any,))[0]
         return collection_cls(transform_value(type_hooks, cast, item_cls, item) for item in value)
+    if is_literal(target_type):
+        for literal_arg in get_args(target_type):
+            for cast_type in cast:
+                if isinstance(literal_arg, Enum) and isinstance(literal_arg, cast_type):
+                    value = cast_type(value)
     return value

If you think it's fine, I can submit a PR with few tests.

[emosenkis]

I had to implement this as type hooks. It would be great if this could be supported natively and @miknet's proposal looks pretty good.

[OffByOnee]

Has there been any progress on this feature?

[OffByOnee]

For anyone else who needs this in the meantime, here's an example of how to do it with type_hooks like @emosenkis mentioned.

from dataclasses import dataclass
import enum
from typing import Literal

import dacite


class ContactType(enum.Enum):
    member = "Member"
    contact = "Contact"


@dataclass
class Contact:
    contactType: ContactType


LiteralContactMember = Literal[ContactType.member]
@dataclass
class Member(Contact):
    contactType: LiteralContactMember


def transform_literal(v):
    if ContactType(v) != ContactType.member:
        raise ValueError(f'Invalid ContactType "{v}" supplied for {LiteralContactMember}')

    return v


config = dacite.Config(
    check_types=False,
    type_hooks={LiteralContactMember: transform_literal},
)