Cannot use built-in callable as argument to PlainSerializer

[aphedges]

Initial Checks

• I confirm that I'm using Pydantic V2

Description

PlainSerializer does not accept the built-in object str as its func argument. The parameter's type annotation includes Callable[[Any], Any], which str is an instance of. Wrapping str within a lambda expression or a def statement allows the code to work, but that shouldn't be necessary. The problem seems to specifically be with built-ins. The problem isn't classes because a custom class works fine, and the problem isn't restricted to built-in classes because hash() also doesn't work without wrapping.

I have included a stack trace of the thrown exception below:

Stack trace

Traceback (most recent call last):
  File "/Users/ahedges/Downloads/demo.py", line 6, in <module>
    class Demo(BaseModel):
  File "/Users/ahedges/.pyenv/versions/biomed/lib/python3.11/site-packages/pydantic/_internal/_model_construction.py", line 183, in __new__
    complete_model_class(
  File "/Users/ahedges/.pyenv/versions/biomed/lib/python3.11/site-packages/pydantic/_internal/_model_construction.py", line 517, in complete_model_class
    schema = cls.__get_pydantic_core_schema__(cls, handler)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ahedges/.pyenv/versions/biomed/lib/python3.11/site-packages/pydantic/main.py", line 584, in __get_pydantic_core_schema__
    return __handler(__source)
           ^^^^^^^^^^^^^^^^^^^
  File "/Users/ahedges/.pyenv/versions/biomed/lib/python3.11/site-packages/pydantic/_internal/_schema_generation_shared.py", line 82, in __call__
    schema = self._handler(__source_type)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ahedges/.pyenv/versions/biomed/lib/python3.11/site-packages/pydantic/_internal/_generate_schema.py", line 499, in generate_schema
    schema = self._generate_schema(obj)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ahedges/.pyenv/versions/biomed/lib/python3.11/site-packages/pydantic/_internal/_generate_schema.py", line 737, in _generate_schema
    schema = self._post_process_generated_schema(self._generate_schema_inner(obj))
                                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ahedges/.pyenv/versions/biomed/lib/python3.11/site-packages/pydantic/_internal/_generate_schema.py", line 759, in _generate_schema_inner
    return self._model_schema(obj)
           ^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ahedges/.pyenv/versions/biomed/lib/python3.11/site-packages/pydantic/_internal/_generate_schema.py", line 572, in _model_schema
    {k: self._generate_md_field_schema(k, v, decorators) for k, v in fields.items()},
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ahedges/.pyenv/versions/biomed/lib/python3.11/site-packages/pydantic/_internal/_generate_schema.py", line 572, in <dictcomp>
    {k: self._generate_md_field_schema(k, v, decorators) for k, v in fields.items()},
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ahedges/.pyenv/versions/biomed/lib/python3.11/site-packages/pydantic/_internal/_generate_schema.py", line 923, in _generate_md_field_schema
    common_field = self._common_field_schema(name, field_info, decorators)
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ahedges/.pyenv/versions/biomed/lib/python3.11/site-packages/pydantic/_internal/_generate_schema.py", line 1045, in _common_field_schema
    schema = self._apply_annotations(
             ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ahedges/.pyenv/versions/biomed/lib/python3.11/site-packages/pydantic/_internal/_generate_schema.py", line 1747, in _apply_annotations
    schema = get_inner_schema(source_type)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ahedges/.pyenv/versions/biomed/lib/python3.11/site-packages/pydantic/_internal/_schema_generation_shared.py", line 82, in __call__
    schema = self._handler(__source_type)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ahedges/.pyenv/versions/biomed/lib/python3.11/site-packages/pydantic/_internal/_generate_schema.py", line 1829, in new_handler
    schema = metadata_get_schema(source, get_inner_schema)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ahedges/.pyenv/versions/biomed/lib/python3.11/site-packages/pydantic/functional_serializers.py", line 66, in __get_pydantic_core_schema__
    return_type = _decorators.get_function_return_type(
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ahedges/.pyenv/versions/biomed/lib/python3.11/site-packages/pydantic/_internal/_decorators.py", line 763, in get_function_return_type
    hints = get_function_type_hints(
            ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ahedges/.pyenv/versions/biomed/lib/python3.11/site-packages/pydantic/_internal/_typing_extra.py", line 273, in get_function_type_hints
    annotations = function.__annotations__
                  ^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: type object 'str' has no attribute '__annotations__'. Did you mean: '__contains__'?

Example Code

from typing import Annotated

from pydantic import BaseModel, HttpUrl, PlainSerializer


class Demo(BaseModel):
    # Commented-out line below works
    # url: Annotated[HttpUrl, PlainSerializer(lambda x: str(x))]
    url: Annotated[HttpUrl, PlainSerializer(str)]


input_json = {"url": "https://github.com/pydantic/pydantic"}
obj = Demo.parse_obj(input_json)
assert isinstance(obj.model_dump()["url"], str), obj.model_dump()["url"]
print(f'{obj.model_dump()["url"] = }')

Python, Pydantic & OS Version

             pydantic version: 2.6.3
        pydantic-core version: 2.16.3
          pydantic-core build: profile=release pgo=false
                 install path: /Users/ahedges/.pyenv/versions/3.11.8/envs/biomed/lib/python3.11/site-packages/pydantic
               python version: 3.11.8 (main, Feb 14 2024, 14:49:59) [Clang 15.0.0 (clang-1500.1.0.2.5)]
                     platform: macOS-13.6.4-x86_64-i386-64bit
             related packages: typing_extensions-4.10.0 mypy-1.8.0
                       commit: unknown

[sydney-runkle]

@aphedges,

Thanks for the report. This is definitely something we do want to support. Might be a pretty good first issue to pick up if someone is interested.

As shown in the traceback, the issue is that str doesn't have an __annotations__ attribute like almost all other functions that we use. Not sure exactly what workaround we should use for those stdlib callable types, though.