Set function signature in Python

Edit 1: Answering new question:

You ask how you can create a function with this signature:

def fun(a, b, opt=None):
    pass

The correct way to do that in Python is thus:

def fun(a, b, opt=None):
    pass

Edit 2: Answering explanation:

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

def f(*args, **kw):
    pass

OK, then f2 looks like so:

def f2(a, b, opt=None):
    f(a, b, opt=opt)

Again, the answer to your question is so trivial, that you obviously want to know something different that what you are asking. You really do need to stop asking abstract questions, and explain your concrete problem.

You can’t do this with live code.

That is, you seem to be wanting to take an actual, live function that looks like this:

def f(*args, **kwargs):
    print args[0]

and change it to one like this:

def f(a):
    print a

The reason this can’t be done–at least without modifying actual Python bytecode–is because these compile differently.

The former results in a function that receives two parameters: a list and a dict, and the code you’re writing operates on that list and dict. The second results in a function that receives one parameter, and which is accessed as a local variable directly. If you changed the function “signature”, so to speak, it’d result in a function like this:

def f(a):
    print a[0]

which obviously wouldn’t work.

If you want more detail (though it doesn’t really help you), a function that takes an *args or *kwargs has one or two bits set in f.func_code.co_flags; you can examine this yourself. The function that takes a regular parameter has f.func_code.co_argcount set to 1; the *args version is 0. This is what Python uses to figure out how to set up the function’s stack frame when it’s called, to check parameters, etc.

If you want to play around with modifying the function directly–if only to convince yourself that it won’t work–see this answer for how to create a code object and live function from an existing one to modify bits of it. (This stuff is documented somewhere, but I can’t find it; it’s nowhere in the types module docs…)

That said, you can dynamically change the docstring of a function. Just assign to func.__doc__. Be sure to only do this at load time (from the global context or–most likely–a decorator); if you do it later on, tools that load the module to examine docstrings will never see it.

We want create_initiation_function to change the signature

Please don’t do this.

We want to use this function in a number of classes

Please use ordinary inheritance.

There’s no value in having the signature “changed” at run time.

You’re creating a maintenance nightmare. No one else will ever bother to figure out what you’re doing. They’ll simply rip it out and replace it with inheritance.

Do this instead. It’s simple and obvious and makes your generic init available in all subclasses in an obvious, simple, Pythonic way.

class Super( object ):
    def __init__( self, *args, **kwargs ):
        # the generic __init__ that we want every subclass to use

class SomeSubClass( Super ):
    def __init__( self, this, that, **kwdefaults ):
        super( SomeSubClass, self ).__init__( this, that, **kwdefaults )

class AnotherSubClass( Super ):
    def __init__( self, x, y, **kwdefaults ):
        super( AnotherSubClass, self ).__init__( x, y, **kwdefaults )

Maybe I didn’t understand the problem well, but if it’s about keeping the same behavior while changing the function signature, then you can do something like :

# define a function
def my_func(name, age) :
    print "I am %s and I am %s" % (name, age)

# label the function with a backup name
save_func = my_func

# rewrite the function with a different signature
def my_func(age, name) :
    # use the backup name to use the old function and keep the old behavior
    save_func(name, age)

# you can use the new signature
my_func(35, "Bob")

This outputs :

I am Bob and I am 35

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.

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})

Have a look at makefun, it was made for that (exposing variants of functions with more or less parameters and accurate signature), and works in python 2 and 3.

Your example would be written like this:

try:  # python 3.3+
    from inspect import signature, Signature, Parameter
except ImportError:
    from funcsigs import signature, Signature, Parameter

from makefun import create_function

def create_initiation_function(cls, gen_init):
    # (1) check which signature we want to create
    params = [Parameter('self', kind=Parameter.POSITIONAL_OR_KEYWORD)]
    for mandatory_arg_name in cls.__init_args__:
        params.append(Parameter(mandatory_arg_name, kind=Parameter.POSITIONAL_OR_KEYWORD))
    for default_arg_name, default_arg_val in cls.__opt_init_args__.items():
        params.append(Parameter(default_arg_name, kind=Parameter.POSITIONAL_OR_KEYWORD, default=default_arg_val))
    sig = Signature(params)

    # (2) create the init function dynamically
    return create_function(sig, generic_init)

# ----- let's use it

def generic_init(self, *args, **kwargs):
    """Function to initiate a generic object"""
    assert len(args) == 0
    for name, val in kwargs.items():
        setattr(self, name, val)

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

my_class.__init__ = create_initiation_function(my_class, generic_init)

and works as expected:

# check 
o1 = my_class(1, 2)
assert vars(o1) == {'y': 2, 'x': 1, 'my_opt': None}

o2 = my_class(1, 2, 3)
assert vars(o2) == {'y': 2, 'x': 1, 'my_opt': 3}

o3 = my_class(my_opt='hello', y=3, x=2)
assert vars(o3) == {'y': 3, 'x': 2, 'my_opt': 'hello'}