Cannot use str 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. I previously reported this as #8967, and it was reported fixed in #9031 / 5c8afe6. However, the newly added test uses float instead of str as the argument to PlainSerializer, but float does not trigger this bug.

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

Stack trace

Traceback (most recent call last):
  File "/Users/ahedges/Downloads/test.py", line 6, in <module>
    class Demo(BaseModel):
  File "/Users/ahedges/.pyenv/versions/test/lib/python3.12/site-packages/pydantic/_internal/_model_construction.py", line 202, in __new__
    complete_model_class(
  File "/Users/ahedges/.pyenv/versions/test/lib/python3.12/site-packages/pydantic/_internal/_model_construction.py", line 539, in complete_model_class
    schema = cls.__get_pydantic_core_schema__(cls, handler)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ahedges/.pyenv/versions/test/lib/python3.12/site-packages/pydantic/main.py", line 626, in __get_pydantic_core_schema__
    return handler(source)
           ^^^^^^^^^^^^^^^
  File "/Users/ahedges/.pyenv/versions/test/lib/python3.12/site-packages/pydantic/_internal/_schema_generation_shared.py", line 82, in __call__
    schema = self._handler(source_type)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ahedges/.pyenv/versions/test/lib/python3.12/site-packages/pydantic/_internal/_generate_schema.py", line 502, in generate_schema
    schema = self._generate_schema_inner(obj)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ahedges/.pyenv/versions/test/lib/python3.12/site-packages/pydantic/_internal/_generate_schema.py", line 753, in _generate_schema_inner
    return self._model_schema(obj)
           ^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ahedges/.pyenv/versions/test/lib/python3.12/site-packages/pydantic/_internal/_generate_schema.py", line 580, in _model_schema
    {k: self._generate_md_field_schema(k, v, decorators) for k, v in fields.items()},
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ahedges/.pyenv/versions/test/lib/python3.12/site-packages/pydantic/_internal/_generate_schema.py", line 916, in _generate_md_field_schema
    common_field = self._common_field_schema(name, field_info, decorators)
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ahedges/.pyenv/versions/test/lib/python3.12/site-packages/pydantic/_internal/_generate_schema.py", line 1081, in _common_field_schema
    schema = self._apply_annotations(
             ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ahedges/.pyenv/versions/test/lib/python3.12/site-packages/pydantic/_internal/_generate_schema.py", line 1820, in _apply_annotations
    schema = get_inner_schema(source_type)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ahedges/.pyenv/versions/test/lib/python3.12/site-packages/pydantic/_internal/_schema_generation_shared.py", line 82, in __call__
    schema = self._handler(source_type)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ahedges/.pyenv/versions/test/lib/python3.12/site-packages/pydantic/_internal/_generate_schema.py", line 1902, in new_handler
    schema = metadata_get_schema(source, get_inner_schema)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ahedges/.pyenv/versions/test/lib/python3.12/site-packages/pydantic/functional_serializers.py", line 74, in __get_pydantic_core_schema__
    info_arg=_decorators.inspect_annotated_serializer(self.func, 'plain'),
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ahedges/.pyenv/versions/test/lib/python3.12/site-packages/pydantic/_internal/_decorators.py", line 596, in inspect_annotated_serializer
    sig = signature(serializer)
          ^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ahedges/.pyenv/versions/3.12.3/lib/python3.12/inspect.py", line 3310, in signature
    return Signature.from_callable(obj, follow_wrapped=follow_wrapped,
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ahedges/.pyenv/versions/3.12.3/lib/python3.12/inspect.py", line 3054, in from_callable
    return _signature_from_callable(obj, sigcls=cls,
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ahedges/.pyenv/versions/3.12.3/lib/python3.12/inspect.py", line 2635, in _signature_from_callable
    raise ValueError(
ValueError: no signature found for builtin type <class 'str'>

Example Code

from typing import Annotated

from pydantic import BaseModel, PlainSerializer


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

Python, Pydantic & OS Version

             pydantic version: 2.7.1
        pydantic-core version: 2.18.2
          pydantic-core build: profile=release pgo=false
                 install path: /Users/ahedges/.pyenv/versions/3.12.3/envs/test/lib/python3.12/site-packages/pydantic
               python version: 3.12.3 (main, May  9 2024, 15:37:42) [Clang 15.0.0 (clang-1500.1.0.2.5)]
                     platform: macOS-13.6.6-x86_64-i386-64bit
             related packages: typing_extensions-4.11.0
                       commit: unknown

[Viicos]

Unlike float, str (and a bunch of builtin types) doesn't have a __text_signature__ attribute. I'll look into this issue soon

[sydney-runkle]

@aphedges,

Oh no! Thanks for bringing this to our attention. Sorry about that faulty fix! Thanks @Viicos, ping me when you want a review :).