Dynamic generate function with ordered default kwargs in python

init.py

import types


def init_generator(spec):
    """Generate `__init__` function based on spec
    """
    varnames, defaults = zip(*spec)
    varnames = ('self', ) + varnames

    def init(self):
        kwargs = locals()
        kwargs.pop('self')
        self.__dict__.update(kwargs)

    code = init.__code__
    new_code = types.CodeType(len(spec) + 1,
                              0,
                              len(spec) + 2,
                              code.co_stacksize,
                              code.co_flags,
                              code.co_code,
                              code.co_consts,
                              code.co_names,
                              varnames,
                              code.co_filename,
                              "__init__",
                              code.co_firstlineno,
                              code.co_lnotab,
                              code.co_freevars,
                              code.co_cellvars)
    return types.FunctionType(new_code,
                              {"__builtins__": __builtins__},
                              argdefs=defaults)


class A(object):
    pass


spec = [('a', 1), ('b', 'foo'), ('c', None)]
A.__init__ = init_generator(spec)
a = A(2, c='wow!')
# {'a': 2, 'b': 'foo', 'c': 'wow!'}
print(a.__dict__)

import inspect
# ArgSpec(args=['self', 'a', 'b', 'c'], varargs=None, keywords=None, defaults=(1, 'foo', None))
print(inspect.getargspec(A.__init__))