Is there any way to get class instance attributes without creating class instance?

Question

Here is the link to my first question: Creating class instance from dictionary?
So I am trying to create class instance from dictionary which holds keys that class has not. For example:

class MyClass(object):
    def __init__(self, value1, value2):
        self.attr1 = value1
        self.attr2 = value2

dict = {'attr1': 'value1', 'attr2': 'value2', 'redundant_key': 'value3'}

Before creating class instance I have to delete this redundant_key from dict

dict.pop('redundant_key', None)
new_instance = MyClass(**dict)

The problem is that I have several classes and several dicts with a lot of keys (I have several responses in json format which are represented as dicts and I want to create objects from this json responses). I already found an interim solution from previous question - how to delete redundant keys. I can create new dict from old dict only with keys that I need:

new_dict = {key: old_dict[key] for key in allowed_keys}

So here is the code:

class MyClass(object):
    def __init__(self, value1, value2):
        self.attr1 = value1
        self.attr2 = value2

dict = {'attr1': 'value1', 'attr2': 'value2', 'redundant_key': 'value3'}
new_instance = MyClass(**{key: dict[key] for key in allowed_keys})

All I need now is to get allowed_keys. So the question - Is there any way to get class instance attributes without creating class instance?

Answer 1

If you insist on using an overly general dictionary to initialize your object, just define __init__ to accept, but ignore, the extra keys.

class MyClass(object):
    def __init__(self, attr1, attr2, **kwargs):
        self.attr1 = attr1
        self.attr2 = attr2

d = {'attr1': 'value1', 'attr2': 'value2', 'extra_key': 'value3'}
new_instance = MyClass(**d)

If you can't modify __init__ (as appears to be the case if it inherits from a SQLAlchemy declarative base), add an alternate constructor to accept all the keyword arguments but pick out the ones you need.

class MyClass(Base):
    @classmethod
    def from_dict(cls, **kwargs):
        # Let any KeyErrors propagate back to the caller.
        # They're the one who forgot to make sure **kwargs
        # contained the right keys.
        value1 = kwargs['attr1']
        value2 = kwargs['attr2']
        return cls(value1, value2)

new_instance = MyClass.from_dict(**d)

Answer 2

Disclaimer: This answers what OP was asking about (getting attributes of an instance), not what they needed. Which seems to be constructor's parameter list.

You cannot do what you want. In python attributes are added to an instance of a class dynamically. Two instances of the same class can have different attributes. Well... to be precise, there are things called instance attributes, and class attributes.

Instance attributes are the ones associated with the instance of a class. Class attributes are associated with its definition i.e. if you wrote (MyClass.foo)

If you look at the example, the attributes are added during the __init__ on self so they are instance attributes.

What you could do is to create a valid instance and inspect it (look at below example), provide a static list of allowed_keys (e.g. as a class attribute), or in some way require allowed_keys as constructor parameter. All of these are kind of workarounds as what you really need is impossible. They have their pros and cons.

example:

class MyClass(object):
    def __init__(self, value1, value2):
        self.attr1 = value1
        self.attr2 = value2

instance = MyClass(1,2)                        # create a valid object
instance_attributes = instance.__dict__.keys() # inspect it
print(instance_attributes)