Question

I’m trying to create metaclass in Python (2.7) that will set arguments passed to object’s __init__ as object attributes.

class AttributeInitType(type):        
    def __call__(self, *args, **kwargs):
        obj = super(AttributeInitType, self).__call__(*args, **kwargs)
        for k, v in kwargs.items():
            setattr(obj, k, v)
        return obj

Usage:

class Human(object):
    __metaclass__ = AttributeInitType

    def __init__(self, height=160, age=0, eyes="brown", sex="male"):
        pass

man = Human()

Question: I want man instance to have defaults attributes set as in class’s __init__. How can I do it?

Update: I’ve came to even better solution that:

• inspects __init__ method only once during class creation
• does not override attributes that where (possibly) set by class’s real __init__

Here is the code:

import inspect
import copy

class AttributeInitType(type):
    """Converts keyword attributes of the init to object attributes"""
    def __new__(mcs, name, bases, d):
        # Cache __init__ defaults on a class-level
        argspec = inspect.getargspec(d["__init__"])
        init_defaults = dict(zip(argspec.args[-len(argspec.defaults):], argspec.defaults))
        cls = super(AttributeInitType, mcs).__new__(mcs, name, bases, d)
        cls.__init_defaults = init_defaults
        return cls

    def __call__(mcs, *args, **kwargs):
        obj = super(AttributeInitType, mcs).__call__(*args, **kwargs)
        the_kwargs = copy.copy(obj.__class__.__init_defaults)
        the_kwargs.update(kwargs)
        for k, v in the_kwargs.items():
            # Don't override attributes set by real __init__
            if not hasattr(obj, k):
                setattr(obj, k, v)
        return obj

Answer 1

You would need to introspect the __init__ method and extract any default values from there. The getargspec function would be helpful there.

The getargspec function returns (among others) a list of argument names, and a list of default values. You can combine these to find the default argument specification of a given function, then use that information to set attributes on the object:

import inspect

class AttributeInitType(type):        
    def __call__(self, *args, **kwargs):
        obj = super(AttributeInitType, self).__call__(*args, **kwargs)
        argspec = inspect.getargspec(obj.__init__)
        defaults = dict(zip(argspec.args[-len(argspec.defaults):], argspec.defaults))
        defaults.update(kwargs)
        for key, val in defaults.items():
            setattr(obj, key, val)
        return obj

With the above metaclass you can omit any of the arguments and they’ll be set on the new instance, or you can override them by passing them in explicitly:

>>> man = Human()
>>> man.age
0
>>> man.height
160
>>> Human(height=180).height
180