dry-python/returns

Currying support for generic attrs classes

JarnoRFB opened this issue · 2 comments

JarnoRFB commented

Bug report

I tried out returns, because I was looking for typesafe curry implementation, but I hit a problem with my Generic / Protocol implementing attrs classes.

What's wrong

from typing import TypeVar, Generic

from attr import frozen
from returns.curry import curry

T = TypeVar("T")

@curry
@frozen(kw_only=True)
class Pair(Generic[T]):
    _a: T
    _b: T

p = Pair(b=1)(a=2)
print(p)
TypeError: __init__() missing 1 required keyword-only argument: 'a'

How is that should be

The above code should simply print

Pair(_a=2, _b=1)

This seems to be due to python-attrs/attrs#374

A fix I hacked together could be something like this.

def curry(function: Callable[..., _ReturnType]) -> Callable[..., _ReturnType]:
    import inspect
    if inspect.isclass(function):
        init_signature = Signature.from_callable(function.__init__)
        init_params_without_self = tuple(init_signature.parameters.values())[1:]
        argspec = init_signature.replace(parameters=init_params_without_self).bind_partial()
    else:
        argspec = Signature.from_callable(function).bind_partial()

    def decorator(*args, **kwargs):
        return _eager_curry(function, argspec, args, kwargs)
    return wraps(function)(decorator)

If you are interested in support this admittedly special use case, I could submit a PR.

System information

  • python version: 3.7
  • returns version: 0.17.0
  • mypy version: 0.910
sobolevn commented

Thanks!

The same would be needed for our mypy plugin.
Also, what about __new__ method?

JarnoRFB commented

Also, what about new method?

Not exactly sure what you mean, but as far as I understood it, the problem is that by default __new__ get's picked up by Signature.from_callable.

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