Dynamically add instance variables via decorator to a class in Python

Question

I want to instrument a class via a decorator, to add some instance variables that are specified by the author of the class.

I started with the following code, but this just adds class variables and I want instance variables (those that are normally 'declared' in __ init __)

What is a pythonic way to do this, while allowing the class author control over what they put in __ init __?

def add_list_attributes(klass):
    for attribute in klass.list_attributes:
        setattr(klass, attribute, [])

    return klass

@add_list_attributes
class Person(object):

    list_attributes = [
        'phone_numbers'
    ]

    def __init__(self):
        pass

p1 = Person()
p1.phone_numbers.append('01234')

print p1.phone_numbers

Philipp · Accepted Answer · 2010-07-16 14:34:42Z

2

You can create a custom __new__ method:

def add_list_attributes(klass):
    def new(cls, *args, **kwargs):
        result = super(cls, cls).__new__(cls)
        for attribute in klass.list_attributes:
            setattr(result, attribute, [])
        return result
    klass.__new__ = staticmethod(new)
    return klass

@add_list_attributes
class Person(object):
    list_attributes = [
        'phone_numbers'
    ]
    def __init__(self):
        pass

p1 = Person()
p2 = Person()
p1.phone_numbers.append('01234')

print p1.phone_numbers, p2.phone_numbers

answered Jul 16, 2010 at 14:34

Philipp

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4 Comments

Mike Boers Over a year ago

super is the way to go here; +1

Philipp Over a year ago

But I'm not sure whether this works if the superclass __init__ function takes more than one argument.

Mike Boers Over a year ago

then why not pass on the args/kwargs to the __new__ call?: super(cls, cls).__new__(cls, *args, **kwargs)

harsha kumar Reddy Over a year ago

where can i find definition for setattr and staticmethod

Ignacio Vazquez-Abrams · Accepted Answer · 2010-07-16 14:23:35Z

2

You would have to wrap __init__() in a separate function that would call the original method, then add its own attributes to the first argument.

answered Jul 16, 2010 at 14:23

Ignacio Vazquez-Abrams

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Comments

Mike Boers · Accepted Answer · 2010-07-16 14:23:53Z

2

You can do this by wrapping the __init__ method to do your bidding, and then call the original __init__:

def add_list_attributes(klass):
    old_init = klass.__init__
    def new_init(self, *args, **kwargs):
        for attribute in klass.list_attributes:
            setattr(self, attribute, [])
        old_init(self, *args, **kwargs)
    klass.__init__ = new_init
    return klass

@add_list_attributes
class Person(object):

    list_attributes = [
        'phone_numbers'
    ]

    def __init__(self):
        pass

p1 = Person()
p1.phone_numbers.append('01234')
p2 = Person()
p2.phone_numbers.append('56789')

print p1.phone_numbers
print p2.phone_numbers

answered Jul 16, 2010 at 14:23

Mike Boers

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1 Comment

Philipp Over a year ago

You might use the functools.wraps decorator to carry over the name and docstring of the original __init__.

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