Update – 2020
Since this question was asked almost ten years ago, quite a bit has changed in Python itself since then.
While the approach in my original answer is still valid for some cases, (e.g. legacy projects stuck to older versions of Python and cases where you really need to handle dictionaries with very dynamic string keys), I think that in general the dataclasses introduced in Python 3.7 are the obvious/correct solution to vast majority of the use cases of AttrDict
.
Original answer
The best way to do this is:
class AttrDict(dict):
def __init__(self, *args, **kwargs):
super(AttrDict, self).__init__(*args, **kwargs)
self.__dict__ = self
Some pros:
- It actually works!
- No dictionary class methods are shadowed (e.g.
.keys()
work just fine. Unless – of course – you assign some value to them, see below) - Attributes and items are always in sync
- Trying to access non-existent key as an attribute correctly raises
AttributeError
instead ofKeyError
- Supports [Tab] autocompletion (e.g. in jupyter & ipython)
Cons:
- Methods like
.keys()
will not work just fine if they get overwritten by incoming data - Causes a memory leak in Python < 2.7.4 / Python3 < 3.2.3
- Pylint goes bananas with
E1123(unexpected-keyword-arg)
andE1103(maybe-no-member)
- For the uninitiated it seems like pure magic.
A short explanation on how this works
- All python objects internally store their attributes in a dictionary that is named
__dict__
. - There is no requirement that the internal dictionary
__dict__
would need to be “just a plain dict”, so we can assign any subclass ofdict()
to the internal dictionary. - In our case we simply assign the
AttrDict()
instance we are instantiating (as we are in__init__
). - By calling
super()
‘s__init__()
method we made sure that it (already) behaves exactly like a dictionary, since that function calls all the dictionary instantiation code.
One reason why Python doesn’t provide this functionality out of the box
As noted in the “cons” list, this combines the namespace of stored keys (which may come from arbitrary and/or untrusted data!) with the namespace of builtin dict method attributes. For example:
d = AttrDict()
d.update({'items':["jacket", "necktie", "trousers"]})
for k, v in d.items(): # TypeError: 'list' object is not callable
print "Never reached!"