Set function signature in Python

Suppose I have a generic function f. I want to programmatically create a function f2 that behaves the same as f, but has a customized signature.

More detail

Given a list l and and dictionary d I want to be able to:

Set the non-keyword arguments of f2 to the strings in l
Set the keyword arguments of f2 to the keys in d and the default values to the values of d

ie. Suppose we have

l = ["x", "y"]
d = {"opt": None}

def f(*args, **kwargs):
    # My code

Then I would want a function with signature:

def f2(x, y, opt=None):
    # My code

A specific use case

This is just a simplified version of my specific use case. I am giving this as an example only.

My actual use case (simplified) is as follows. We have a generic initiation function:

def generic_init(self, *args, **kwargs):
    """Function to initiate a generic object"""
    for name, arg in zip(self.__init_args__, args):
        setattr(self, name, arg)
    for name, default in self.__init_kw_args__.items():
        if name in kwargs:
            setattr(self, name, kwargs[name])
        else:
            setattr(self, name, default)

We want to use this function in a number of classes. In particular, we want to create a function __init__ that behaves like generic_init, but has the signature defined by some class variables at creation time:

class my_class:
    __init_args__ = ["x", "y"]
    __kw_init_args__ = {"my_opt": None}

__init__ = create_initiation_function(my_class, generic_init)
setattr(myclass, "__init__", __init__)

We want create_initiation_function to create a new function with the signature defined using __init_args__ and __kw_init_args__. Is it possible to write create_initiation_function?

Please note:

If I just wanted to improve the help, I could set __doc__.
We want to set the function signature on creation. After that, it doesn't need to be changed.
Instead of creating a function like generic_init, but with a different signature we could create a new function with the desired signature that just calls generic_init
We want to define create_initiation_function. We don't want to manually specify the new function!

Preserving signatures of decorated functions: This is how to preserve a signature when decorating a function. We need to be able to set the signature to an arbitrary value

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.

For your usecase, having a docstring in the class/function should work -- that will show up in help() okay, and can be set programmatically (func.__doc__ = "stuff").

I can't see any way of setting the actual signature. I would have thought the functools module would have done it if it was doable, but it doesn't, at least in py2.5 and py2.6.

You can also raise a TypeError exception if you get bad input.

Hmm, if you don't mind being truly vile, you can use compile()/eval() to do it. If your desired signature is specified by arglist=["foo","bar","baz"], and your actual function is f(*args, **kwargs), you can manage:

argstr = ", ".join(arglist)
fakefunc = "def func(%s):\n    return real_func(%s)\n" % (argstr, argstr)
fakefunc_code = compile(fakefunc, "fakesource", "exec")
fakeglobals = {}
eval(fakefunc_code, {"real_func": f}, fakeglobals)
f_with_good_sig = fakeglobals["func"]

help(f)               # f(*args, **kwargs)
help(f_with_good_sig) # func(foo, bar, baz)

Changing the docstring and func_name should get you a complete solution. But, uh, eww...

I wrote a package named forge that solves this exact problem for Python 3.5+:

With your current code looking like this:

l=["x", "y"]
d={"opt":None}

def f(*args, **kwargs):
    #My code

And your desired code looking like this:

def f2(x, y, opt=None):
    #My code

Here is how you would solve that using forge:

f2 = forge.sign(
    forge.arg('x'),
    forge.arg('y'),
    forge.arg('opt', default=None),
)(f)

As forge.sign is a wrapper, you could also use it directly:

@forge.sign(
    forge.arg('x'),
    forge.arg('y'),
    forge.arg('opt', default=None),
)
def func(*args, **kwargs):
    # signature becomes: func(x, y, opt=None)
    return (args, kwargs)

assert func(1, 2) == ((), {'x': 1, 'y': 2, 'opt': None})

Set function signature in Python

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