Asked  7 Months ago    Answers:  5   Viewed   75 times

How can I make as "perfect" a subclass of dict as possible? The end goal is to have a simple dict in which the keys are lowercase.

It would seem that there should be some tiny set of primitives I can override to make this work, but according to all my research and attempts it seem like this isn't the case:

  • If I override __getitem__/__setitem__, then get/set don't work. How can I make them work? Surely I don't need to implement them individually?

  • Am I preventing pickling from working, and do I need to implement __setstate__ etc?

  • Do I need repr, update and __init__?

  • Should I just use mutablemapping (it seems one shouldn't use UserDict or DictMixin)? If so, how? The docs aren't exactly enlightening.

Here is my first go at it, get() doesn't work and no doubt there are many other minor problems:

class arbitrary_dict(dict):
    """A dictionary that applies an arbitrary key-altering function
       before accessing the keys."""

    def __keytransform__(self, key):
        return key

    # Overridden methods. List from 

    def __init__(self, *args, **kwargs):
        self.update(*args, **kwargs)

    # Note: I'm using dict directly, since super(dict, self) doesn't work.
    # I'm not sure why, perhaps dict is not a new-style class.

    def __getitem__(self, key):
        return dict.__getitem__(self, self.__keytransform__(key))

    def __setitem__(self, key, value):
        return dict.__setitem__(self, self.__keytransform__(key), value)

    def __delitem__(self, key):
        return dict.__delitem__(self, self.__keytransform__(key))

    def __contains__(self, key):
        return dict.__contains__(self, self.__keytransform__(key))

class lcdict(arbitrary_dict):
    def __keytransform__(self, key):
        return str(key).lower()



You can write an object that behaves like a dict quite easily with ABCs (Abstract Base Classes) from the module. It even tells you if you missed a method, so below is the minimal version that shuts the ABC up.

from import MutableMapping

class TransformedDict(MutableMapping):
    """A dictionary that applies an arbitrary key-altering
       function before accessing the keys"""

    def __init__(self, *args, **kwargs): = dict()
        self.update(dict(*args, **kwargs))  # use the free update to set keys

    def __getitem__(self, key):

    def __setitem__(self, key, value):[self._keytransform(key)] = value

    def __delitem__(self, key):

    def __iter__(self):
        return iter(
    def __len__(self):
        return len(

    def _keytransform(self, key):
        return key

You get a few free methods from the ABC:

class MyTransformedDict(TransformedDict):

    def _keytransform(self, key):
        return key.lower()

s = MyTransformedDict([('Test', 'test')])

assert s.get('TEST') is s['test']   # free get
assert 'TeSt' in s                  # free __contains__
                                    # free setdefault, __eq__, and so on

import pickle
# works too since we just use a normal dict
assert pickle.loads(pickle.dumps(s)) == s

I wouldn't subclass dict (or other builtins) directly. It often makes no sense, because what you actually want to do is implement the interface of a dict. And that is exactly what ABCs are for.

Tuesday, June 1, 2021
answered 7 Months ago

Dictionaries are unordered in Python versions up to and including Python 3.6. If you do not care about the order of the entries and want to access the keys or values by index anyway, you can use d.keys()[i] and d.values()[i] or d.items()[i]. (Note that these methods create a list of all keys, values or items in Python 2.x. So if you need them more then once, store the list in a variable to improve performance.)

If you do care about the order of the entries, starting with Python 2.7 you can use collections.OrderedDict. Or use a list of pairs

l = [("blue", "5"), ("red", "6"), ("yellow", "8")]

if you don't need access by key. (Why are your numbers strings by the way?)

In Python 3.7, normal dictionaries are ordered, so you don't need to use OrderedDict anymore (but you still can – it's basically the same type). The CPython implementation of Python 3.6 already included that change, but since it's not part of the language specification, you can't rely on it in Python 3.6.

Tuesday, June 1, 2021
answered 7 Months ago

The @property decorator only works on new style classes. Inherit from object:

class A(object):

With that change your test function passes.

Saturday, July 31, 2021
answered 5 Months ago

You are looking for TypedDict. It is currently only a mypy-only extension, but there are plans to make it an officially sanctioned type in the near-future. I am not sure if PyCharm supports this feature yet, though.

So, in your case, you'd do:

from mypy_extensions import TypedDict

RectangleElements = TypedDict('RectangleElements', {
    'front': Line,
    'left': Line,
    'right': Line,
    'rear': Line,
    'cog': float,
    'area': float,
    'pins': Optional[List[Pin]]

class Rectangle:
    def __init__(self, corners: Tuple[Tuple[float, float]], **kwargs):
        self.x, self.z = corners[0][0], corners[0][1]
        self.elements = {
            'front': Line(corners[0], corners[1]),
            'left': Line(corners[0], corners[2]),
            'right': Line(corners[1], corners[3]),
            'rear': Line(corners[3], corners[2]),
            'cog': calc_cog(corners),
            'area': calc_area(corners),
            'pins': None
        }  # type: RectangleElements

If you are using Python 3.6+, you can type this all more elegantly using the class-based syntax.

In your specific case though, I think most people would just store those pieces of data as regular fields instead of a dict. I'm sure you've already thought through the pros and cons of that approach though, so I'll skip lecturing you about it.

Tuesday, August 3, 2021
answered 4 Months ago

Python already includes support for generalized delegation to a contained class. Just change the definition of MyClass to:

class MyClass:

    def __init__(self, someClass):            
        self.refClass = someClass  # Note: You call this someClass, but it's actually some object, not some class in your example

    def __getattr__(self, name):
        return getattr(self.refClass, name)

When defined, __getattr__ is called on the instance with the name of the accessed attribute any time an attribute is not found on the instance itself. You then delegate to the contained object by calling getattr to look up the attribute on the contained object and return it. This costs a little each time to do the dynamic lookup, so if you want to avoid it, you can lazily cache attributes when they're first requested by __getattr__, so subsequent access is direct:

def __getattr__(self, name):
     attr = getattr(self.refClass, name)
     setattr(self, name, attr)
     return attr
Thursday, October 14, 2021
Matt Bullock
answered 2 Months ago
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