Support for Callable type

[dmfigol]

Feature Request

It would be great if pydantic supported callables as types.
Use-case: a user can specify a path to the class to be used dynamically or to provide a callable object directly (instead of the path as a string). We wanted to use pre validator to transform a string to the callable object.

from typing import Type, Callable, Any
from pydantic import BaseModel

def useless(x: Any) -> Any:
    return x

class DemoModel(BaseModel):
    t: Callable[..., Any]


print(DemoModel(t=useless))
...

-> % python main.py
Traceback (most recent call last):
  File "main.py", line 8, in <module>
    class DemoModel(BaseModel):
  File ".../.venv/lib/python3.7/site-packages/pydantic/main.py", line 124, in __new__
    config=config,
  File ".../.venv/lib/python3.7/site-packages/pydantic/fields.py", line 107, in infer
    schema=schema,
  File ".../.venv/lib/python3.7/site-packages/pydantic/fields.py", line 87, in __init__
    self.prepare()
  File ".../.venv/lib/python3.7/site-packages/pydantic/fields.py", line 135, in prepare
    self._populate_sub_fields()
  File ".../.venv/lib/python3.7/site-packages/pydantic/fields.py", line 240, in _populate_sub_fields
    assert issubclass(origin, Mapping)
AssertionError

[samuelcolvin]

entirely possible as far as I'm concerned.

There might be some complex logic required using inspect to check that the callable arguments match the declaration, but maybe that's not required at this stage?

PR welcome.