get function with *args from function with **kwargs in python

Question

In Python, I want to have a method that builds a new function from an old one.

Input : a function like f(**kwargs) together with a tuple of pairs that takes a dictionary of keywords

Output : a new function that has the pairs from the tuple as arguments and default values.

What I have so far:

def f(**kwargs):
    return 'test',kwargs['a'], kwargs['b'] 

def magic(f,argtuple):
    arglist=",".join([str(ke)+" = "+str(va) for ke,va in argtuple])
    keylist=",".join([str(ke)+" = "+str(ke) for ke,va in argtuple])
    exec("def g("+arglist+"): return f("+keylist+")")
    return g

It does what I want:

In [25]: f(b=4,a=3)
Out[25]: ('test', 3, 4)

In [26]: g=magic(f,(['a',1],['b',2]))

In [27]: g()
Out[27]: ('test', 1, 2)

In [28]: g(a=3,b=5)
Out[28]: ('test', 3, 5)

In [29]: import inspect

In [30]: print  inspect.getargspec(f)
ArgSpec(args=[], varargs=None, keywords='kwargs', defaults=None)

In [31]: print  inspect.getargspec(g)
ArgSpec(args=['a', 'b'], varargs=None, keywords=None, defaults=(1, 2))

However I don't want to build the string and don't want to use the exec function. What are other ways of achieving this?

Answer 1

What you are looking for is functools.partial() .

import functools

def f(**kwargs):
    return 'test',kwargs['a'], kwargs['b']

def magic(f, argtuple):
    return functools.partial(f, **dict(argtuple))

g = magic(f, (['a',1],['b',2]))
print g()  # ('test', 1, 2)

Answer 2

Here is a solution that also constructs the right signature. It requires a reasonably up-to-date Python since it makes use of relativly recent additions to the inspect module and also the __signature__ special attribute.

import inspect

def build_sig(arg_tuple):
    return inspect.Signature(parameters = [inspect.Parameter(
        name=name, kind=inspect.Parameter.POSITIONAL_OR_KEYWORD, 
        default=default) for name, default in arg_tuple])

def wrap(f, arg_tuple):
    sig = build_sig(arg_tuple)
    def g(*args, **kwds):
        bound = sig.bind(*args, **kwds)
        bound.apply_defaults()
        return f(**bound.arguments)
    g.__signature__ = sig
    return g

def f(*args, **kwds):
    return(args, kwds)

g = wrap(f, (['a', 1], ['b', 7]))

print(inspect.getargspec(g))
print(g(3))

# ArgSpec(args=['a', 'b'], varargs=None, keywords=None, defaults=(1, 7))
# ((), {'a': 3, 'b': 7})