Dirty tricks to automatically inject keyword arguments from the caller's scope

autoinject.py

"""
Dirty tricks to automatically inject keyword arguments from the caller's scope.
"""

import functools
import inspect
import sys


def auto_inject_kwargs(f):
    """
    Decorator to automatically inject missing keyword arguments.

    The values are looked up by inspecting the caller's local variables of the
    same name.
    """
    signature = inspect.getfullargspec(f)
    injectable_kwargs = set(signature.kwonlyargs)
    if signature.kwonlydefaults is not None:
        injectable_kwargs.difference_update(signature.kwonlydefaults.keys())

    @functools.wraps(f)
    def wrapper(*args, **kwargs):
        missing_kwarg_names = injectable_kwargs.difference(kwargs.keys())
        frame = sys._getframe(1)
        for name in missing_kwarg_names:
            if name in frame.f_locals:
                kwargs[name] = frame.f_locals[name]
        return f(*args, **kwargs)

    return wrapper


def test_auto_inject_kwargs():
    @auto_inject_kwargs
    def foo(a, *, b, c, d=4):
        return (a, b, c, d)

    c = 3  # Should be used b/c not present in call below.
    d = 5  # Should not be used b/c default value.
    actual = foo(1, b=2)
    expected = (1, 2, 3, 4)
    assert actual == expected