Is typed implicit conversion (coercion) in Python 3.x possible?

Question

Is it possible to implement custom automatic/implicit conversion (aka coercion) in Python 3.6+ that won't make mypy and other static analyzers sad? An example would be a def(foo: A), and given def b_to_a(b: B) -> A, is there a way I could potentially write foo(some_b) (where some_b: B ) instead of foo(b_to_a(some_b))?

I think there are definitely some good ways to do this in the dynamics of Python (tacking on members to classes that include converters, for instance), or even tacking on such converters to the function object itself so that it can handle conversion for selected types, but my current understanding of Python types makes me thing it would not satisfy mypy and the like.

For comparison, see Scala's implicit conversions.

Answer 1

Here's an implementation of this feature I came up with. We keep a dictionary of single-dispatch converters for types we know the "implicit" conversions for. We add converters to this using the @implicit decorator.

We then have a @coerce decorator that can inspect the function annotations at runtime, get the appropriate converters and apply the conversions. Below is the framework:

from functools import wraps, singledispatch
from inspect import signature
from collections import OrderedDict

converters = {}

def implicit(func):
    ret = func.__annotations__.get('return', None)
    if not ret or len(func.__annotations__) != 2:
        raise ValueError("Function not annotated properly or too many params")
    if ret not in converters:    
        @singledispatch
        def default(arg):
            raise ValueError("No such converter {} -> {}".format(type(arg).__name__, ret.__name__))    
        converters[ret] = default
    else:
        default = converters[ret]
    t = next(v for k, v in func.__annotations__.items() if k != 'return')
    default.register(t)(func)
    return wraps(func)(default)

def convert(val, t):
    if isinstance(val, t):
        return t
    else:
        return converters[t](val)


def coerce(func):
    @wraps(func)
    def wrapper(*args, **kwargs):
        sig = signature(func)
        bound = sig.bind(*args, **kwargs)
        bound.apply_defaults()
        bound.arguments = OrderedDict(
            (param, convert(val, sig.parameters[param].annotation)) 
            for param, val in bound.arguments.items())
        return func(*bound.args, **bound.kwargs)    
    return wrapper

And an example:

from typing import Tuple, Type


@implicit
def str_to_int(a: str) ->  int:
    return int(a)

@implicit
def float_to_int(a: float) -> int:
    return int(a)

@coerce
def make_ints(a: int, b: int) -> Tuple[Type, Type]:
    return (type(a), type(b))

print(make_ints("20", 5.0))
# (<class 'int'>, <class 'int'>)