How to get attributes of a class without knowing their names

Question

I have the following class:

class TestClass(object):
def __init__(self, **kwargs):
    for key, value in kwargs.items(): #items return list of dict
        setattr(self, key, value)

Examplary use:

obj = MessageItem(**{"testkey1":"tval1", "tkey2":"tval2", "tkey3":"tval3"})

How can I iterate on this structure without knowing the names of the attributes? Python provides us with the built-in method __getattribute__, but I still need to know the name of the requested attribute:

print(obj.__getattribute__("testkey1"))

Answer 1

The __dict__ attribute holds what you want. The class has it:

>>> class Foo:
...     def __init__(self, x):
...             self.x = x
...
>>> Foo.__dict__
mappingproxy({'__module__': '__main__', '__init__': <function Foo.__init__ at
0x000001CC1821EEA0>, '__dict__': <attribute '__dict__' of 'Foo' objects>, '__weakref__':
<attribute '__weakref__' of 'Foo' objects>, '__doc__': None})

And any instance has it as well:

>>> f = Foo(2)
>>> f.__dict__
{'x': 2}

You should access this attribute through the vars builtin function. Calling vars(foo) will return foo.__dict__. See this related post: Use __dict__ or vars()?.

Documentation for vars:

vars([object])

Return the __dict__ attribute for a module, class, instance, or any other object with a __dict__ attribute.

Objects such as modules and instances have an updateable __dict__ attribute; however, other objects may have write restrictions on their __dict__ attributes (for example, classes use a types.MappingProxyType to prevent direct dictionary updates).

Without an argument, vars() acts like locals(). Note, the locals dictionary is only useful for reads since updates to the locals dictionary are ignored.

---

In addition, I tried and wrote a decorator that might interest you. This is a class decorator, that adds a initKwargs to the class it decorates. Besides, it wraps the __init__ method of that class as well, so as to have it append the kwargs dictionary it receives to the class' initKwargs attribute.

def class_wrapper(cls):
    cls.initKwargs = []
    f = cls.__init__

    def wrapped_init(instance, **kwargs):
        cls.initKwargs.append(kwargs)
        return f(instance, **kwargs)            
    cls.__init__ = wrapped_init

    return cls

@class_wrapper
class Foo:
    def __init__(self, **kwargs):
        for k, v in kwargs.items():
            setattr(self, k, v)

Demonstration:

>>> f1 = Foo()
>>> f2 = Foo(a=1, b=2)
>>> f3 = Foo(a=1, b=2, c=3, d=4)
>>> Foo.initKwargs
[{}, {'a': 1, 'b': 2}, {'a': 1, 'b': 2, 'c': 3, 'd': 4}]

I find this approach much cleaner that using vars, because you know what you're accessing since you define it yourself. It gives you more control on the class' behaviour.