Adding an attribute to a Python dictionary from the standard library

Question

I was wondering if you could add an attribute to a Python dictionary?

class myclass():
    def __init__(): 
     self.mydict = {}  # initialize a regular dict
     self.mydict.newattribute = "A description of what this dictionary will hold"
     >>> AttributeError: 'dict' object has no attribute 'newattribute'
     setattr(self.mydict, "attribute", "A description of what this dictionary will hold"
     >>> AttributeError: 'dict' object has no attribute 'newattribute'

Is there anyway to quickly add my description attribute without having to copy the dict class and overloading the constructor. I thought it would be simple, but I guess I was wrong.

Sven Marnach · Accepted Answer · 2021-06-23 08:08:50Z

37

Just derive from dict:

class MyDict(dict):
    pass

Instances of MyDict behave like a dict, but can have custom attributes:

>>> d = MyDict()
>>> d.my_attr = "whatever"
>>> d.my_attr
'whatever'

edited Jun 23, 2021 at 8:08

answered Jul 17, 2012 at 23:06

Sven Marnach

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moooeeeep · Accepted Answer · 2022-06-20 10:02:07Z

4

If you feel like creating your own class would be overkill, you can also drop-in replace the dict with a collections.UserDict.

Untested example:

from collections import UserDict

d = UserDict({"foo": "bar"})
d.description = "my dict with attributes"
d["baz"] = 42

For reference:

https://docs.python.org/3/library/collections.html#collections.UserDict

answered Jun 20, 2022 at 10:02

moooeeeep

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