Namedtuple annotation for Python -- derive namedtuple definition from the signature of __init__()!

namedtuple.py

"""Namedtuple annotation.

Creates a namedtuple out of a class, based on the signature of that class's
__init__ function. Defaults are respected. After namedtuple's initializer is
run, the original __init__ is run as well, allowing one to assign synthetic
parameters and internal book-keeping variables. (Note that the original
__init__ may not assign to the variables in its signature, because these
variables are made immutable by namedtuple).

The class must not have varargs or keyword args.

Thanks to @mjpieters: http://stackoverflow.com/a/32171046/48251
"""
import collections
import inspect


def namedtuple(cls):
    argspec = inspect.getargspec(cls.__init__)
    assert argspec.varargs is None
    assert argspec.keywords is None
    non_self_args = argspec.args[1:]

    # Now we can create the new class definition, based on a namedtuple.
    bases = (collections.namedtuple(cls.__name__, non_self_args), cls)
    namespace = {'__doc__': cls.__doc__}
    newcls = type(cls.__name__, bases, namespace)

    # Here we set up the new class's __new__, which hands off to namedtuple's
    # after setting defaults.
    @staticmethod
    def new(*args, **kwargs):
        kls = args[0]
        # Resolve default assignments with this utility from inspect.
        values = inspect.getcallargs(cls.__init__, None, *args[1:], **kwargs)
        values = [values[_] for _ in non_self_args]
        obj = super(newcls, kls).__new__(kls, *values)
        cls.__init__(obj, *values)              # Allow initialization to occur
        return obj

    newcls.__new__ = new

    return newcls