Set function signature in Python

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From PEP-0362, there actually does appear to be a way to set the signature in py3.3+, using the fn.__signature__ attribute:

from inspect import signature
from functools import wraps

def shared_vars(*shared_args):
    """Decorator factory that defines shared variables that are
       passed to every invocation of the function"""

    def decorator(f):
        @wraps(f)
        def wrapper(*args, **kwargs):
            full_args = shared_args + args
            return f(*full_args, **kwargs)

        # Override signature
        sig = signature(f)
        sig = sig.replace(parameters=tuple(sig.parameters.values())[1:])
        wrapper.__signature__ = sig

        return wrapper
    return decorator

Then:

>>> @shared_vars({"myvar": "myval"})
>>> def example(_state, a, b, c):
>>>     return _state, a, b, c
>>> example(1,2,3)
({'myvar': 'myval'}, 1, 2, 3)
>>> str(signature(example))
'(a, b, c)'

Note: the PEP is not exactly right; Signature.replace moved the params from a positional arg to a kw-only arg.

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