Instantiate attrs class from dict with superfluous key/value pairs

Question

I decorated a class using attrs.define and added all important attributes/fields.

import attrs

@attrs.define
class MyAttrsClass:
    important1 = attrs.field(type = int)
    important2 = attrs.field(type = str)
    [...]

Now I would like to initialize instances of that class from a dict. So what I do is calling the contructor with a dict-unpacking.

test_dict = {'important1': 100, 'important2': 'foo'}
ci = MyAttrsClass(**test_dict)

This works in that simple example case, because I specify the same fields in the dict as well as in the MyAttrsClass class.

However, in the real scenario the dict is a deserialized response from a web-api (using json.loads) and it returns many key-value-pairs I'm not interested in, despite the important ones. Then, I get the following error TypeError: MyAttrsClass.__init__() got an unexpected keyword argument 'XYZ'.

Is is possible to initialize an instance of MyAttrsClass, though? I would like to discard the superfluous elements. Do I need to manually remove entries from the input dict or is there a better way using attrs.

Answer 1

You can use the documented attrs.fields function:

big_dic = {"important1": 3, "important2": "abc", "spuriouskey": "dontcare"}
fields = {a.name for a in attrs.fields(MyAttrsClass)}
init_kwargs = {k: v for k, v in big_dic.items() if k in fields}

MyAttrsClass(**init_kwargs)
# MyAttrsClass(important1=3, important2='abc')

Answer 2

I'm an attrs maintainer and the author of cattrs. You should probably just use cattrs since it was designed for cases like this.

from cattrs import structure

structure(big_dict, MyAttrsClass)

Otherwise, you'll have to do a little preprocessing yourself, as others have mentioned.