Python metaclass default attributes

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

Martijn Pieters · Accepted Answer · 2012-08-20 11:17:14Z

3

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

edited Aug 20, 2012 at 11:17

answered Aug 20, 2012 at 11:10

Martijn Pieters

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3 Comments

Martijn Pieters Over a year ago

@ZaarHai: If you have any positional arguments, they'd add to the args list, but not to the defaults tuple.

Zaar Hai Over a year ago

I see. Thanks. It would be great if you could add that note to the answer.

Martijn Pieters Over a year ago

@ZaarHai: I linked to the argspec documentation, which details this.

Stefano Borini · Accepted Answer · 2012-08-20 11:09:04Z

Your situation works if you pass the arguments at object creation

>>> man
<test.Human object at 0x10a71e810>
>>> dir(man)
['__class__', '__delattr__', '__dict__', '__doc__', '__format__', '__getattribute__', '__hash__', '__init__', '__metaclass__', '__module__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__']
>>> man=Human(height=10)
>>> dir(man)
['__class__', '__delattr__', '__dict__', '__doc__', '__format__', '__getattribute__', '__hash__', '__init__', '__metaclass__', '__module__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__', 'height']
>>> man.height
10

but it does not work with default arguments. For that, you have to specifically extract them from the __init__ function object.

An alternative is to decorate the __init__ instead.

Ella Rose · Accepted Answer · 2014-09-14 14:53:33Z

I do this in my framework with *args/**kwargs, a class defaults dictionary, and a call to attribute_setter in my base objects init. I feel like this is simpler then decoration and definitely less complicated then metaclasses.

Class Base(object):
    defaults = {"default_attribute" : "default_value"}

    def __init__(self, *args, **kwargs):
        super(Base, self).__init__()
        self.attribute_setter(self.defaults.items(), *args, **kwargs)

    def attribute_setter(self, *args, **kwargs):
        if args: # also allow tuples of name/value pairs
            for attribute_name, attribute_value in args:
                setattr(self, attribute_name, attribute_value)
        [setattr(self, name, value) for name, value in kwargs.items()]
b = Base(testing=True)
# print b.testing
# True
# print b.default_attribute
# "default_value"

this combination allows the assignment of arbitrary attributes through init at runtime by specifying them as keyword arguments (or as positional argument tuples of name/value pairs).

the class defaults dictionary is used to supply default arguments instead of explicitly named keyword arguments in init's argument list. this makes the default attributes that new instances will be created with modifiable at runtime. You can "inherit" class dictionaries via dict.copy + dict.update.

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