Preconfigured bson converter errors when destructuring class with dictionary field that has a Literal key

[jrpat]

Using cattrs 24.1.2 and attrs 25.1.0

Repro

from typing import Literal

import attrs
import cattrs.preconf.bson

ThingType = Literal["a", "b"]

@attrs.define
class UserDoc:
    things: dict[ThingType, bool | None]


raw_user = {"things": {"a": True}}

converter = cattrs.preconf.bson.make_converter()
demo = converter.structure(raw_user, UserDoc)

Error

Traceback (most recent call last):
  File "/Users/jrpat/.vscode/extensions/ms-python.python-2025.1.2025022102-darwin-arm64/python_files/python_server.py", line 133, in exec_user_input
    retval = callable_(user_input, user_globals)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<string>", line 1, in <module>
  File "/Users/jrpat/example/.venv/lib/python3.12/site-packages/cattrs/converters.py", line 558, in structure
    return self._structure_func.dispatch(cl)(obj, cl)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jrpat/example/.venv/lib/python3.12/site-packages/cattrs/dispatch.py", line 134, in dispatch_without_caching
    res = self._function_dispatch.dispatch(typ)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jrpat/example/.venv/lib/python3.12/site-packages/cattrs/dispatch.py", line 76, in dispatch
    return handler(typ)
           ^^^^^^^^^^^^
  File "/Users/jrpat/example/.venv/lib/python3.12/site-packages/cattrs/converters.py", line 1290, in gen_structure_attrs_fromdict
    return make_dict_structure_fn(
           ^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jrpat/example/.venv/lib/python3.12/site-packages/cattrs/gen/__init__.py", line 772, in make_dict_structure_fn
    return make_dict_structure_fn_from_attrs(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jrpat/example/.venv/lib/python3.12/site-packages/cattrs/gen/__init__.py", line 406, in make_dict_structure_fn_from_attrs
    handler = find_structure_handler(
              ^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jrpat/example/.venv/lib/python3.12/site-packages/cattrs/gen/_shared.py", line 52, in find_structure_handler
    handler = c.get_structure_hook(type, cache_result=False)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jrpat/example/.venv/lib/python3.12/site-packages/cattrs/converters.py", line 576, in get_structure_hook
    else self._structure_func.dispatch_without_caching(type)
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jrpat/example/.venv/lib/python3.12/site-packages/cattrs/dispatch.py", line 134, in dispatch_without_caching
    res = self._function_dispatch.dispatch(typ)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jrpat/example/.venv/lib/python3.12/site-packages/cattrs/dispatch.py", line 76, in dispatch
    return handler(typ)
           ^^^^^^^^^^^^
  File "/Users/jrpat/example/.venv/lib/python3.12/site-packages/cattrs/converters.py", line 697, in _find_type_alias_structure_hook
    res = self.get_structure_hook(base)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jrpat/example/.venv/lib/python3.12/site-packages/cattrs/converters.py", line 574, in get_structure_hook
    self._structure_func.dispatch(type)
  File "/Users/jrpat/example/.venv/lib/python3.12/site-packages/cattrs/dispatch.py", line 134, in dispatch_without_caching
    res = self._function_dispatch.dispatch(typ)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jrpat/example/.venv/lib/python3.12/site-packages/cattrs/dispatch.py", line 76, in dispatch
    return handler(typ)
           ^^^^^^^^^^^^
  File "/Users/jrpat/example/.venv/lib/python3.12/site-packages/cattrs/preconf/bson.py", line 74, in gen_structure_mapping
    if args and issubclass(args[0], bytes):
                ^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: issubclass() arg 1 must be a class

Notes

• This also happens if ThingType is a type alias, eg. type ThingType = … or if inline like things: dict[Literal[…], …]

Thanks!

[Tinche]

Howdy,

I noticed this too and this was fixed already in #599. I haven't had the chance to release it yet since it's blocked by another thing. I'm doing to close it as a duplicate to keep the issue tracker clean.

One advice I can give you in the meantime is to maybe model this data as a typed dict - that's what I did in the codebase I ran into this. Something like this:

from typing import NotRequired, TypedDict

import attrs

import cattrs.preconf.bson


class ThingsDict(TypedDict):
    a: NotRequired[bool | None]
    b: NotRequired[bool | None]


@attrs.define
class UserDoc:
    things: ThingsDict = attrs.field(factory=dict)


raw_user = {"things": {"a": True}}

converter = cattrs.preconf.bson.make_converter()
demo = converter.structure(raw_user, UserDoc)
print(demo)

It's a little more typing but (IMO) more idiomatic, and will give you more flexibility in the future.

If this doesn't work for you, I can give you a piece of code that will solve the issue until the new version comes along.

[Tinche]

(Oh, another solution would be to use cattrs directly from git until the next release - I'm careful to ensure main is very stable.)

[jrpat]

Awesome - thanks! My project is still very much in development, so I can happily depend on the git version for now.

Although you're totally right about the TypedDict approach being more idiomatic. Not sure why I went the Literal route - probably it was a gradual evolution and I never rethought it.

Thanks for all your hard work on cattrs!