A python class that acts like dict
Question:
I want to write a custom class that behaves like dict
– so, I am inheriting from dict
.
My question, though, is: Do I need to create a private dict
member in my __init__()
method?. I don’t see the point of this, since I already have the dict
behavior if I simply inherit from dict
.
Can anyone point out why most of the inheritance snippets look like the one below?
class CustomDictOne(dict):
def __init__(self):
self._mydict = {}
# other methods follow
Instead of the simpler…
class CustomDictTwo(dict):
def __init__(self):
# initialize my other stuff here ...
# other methods follow
Actually, I think I suspect the answer to the question is so that users cannot directly access your dictionary (i.e. they have to use the access methods that you have provided).
However, what about the array access operator []
? How would one implement that? So far, I have not seen an example that shows how to override the []
operator.
So if a []
access function is not provided in the custom class, the inherited base methods will be operating on a different dictionary?
I tried the following snippet to test out my understanding of Python inheritance:
class myDict(dict):
def __init__(self):
self._dict = {}
def add(self, id, val):
self._dict[id] = val
md = myDict()
md.add('id', 123)
print md[id]
I got the following error:
KeyError: < built-in function id>
What is wrong with the code above?
How do I correct the class myDict
so that I can write code like this?
md = myDict()
md['id'] = 123
[Edit]
I have edited the code sample above to get rid of the silly error I made before I dashed away from my desk. It was a typo (I should have spotted it from the error message).
Answers:
Check the documentation on emulating container types. In your case, the first parameter to add
should be self
.
The problem with this chunk of code:
class myDict(dict):
def __init__(self):
self._dict = {}
def add(id, val):
self._dict[id] = val
md = myDict()
md.add('id', 123)
…is that your ‘add’ method (…and any method you want to be a member of a class) needs to have an explicit ‘self’ declared as its first argument, like:
def add(self, 'id', 23):
To implement the operator overloading to access items by key, look in the docs for the magic methods __getitem__
and __setitem__
.
Note that because Python uses Duck Typing, there may actually be no reason to derive your custom dict class from the language’s dict class — without knowing more about what you’re trying to do (e.g, if you need to pass an instance of this class into some code someplace that will break unless isinstance(MyDict(), dict) == True
), you may be better off just implementing the API that makes your class sufficiently dict-like and stopping there.
Like this
class CustomDictOne(dict):
def __init__(self,*arg,**kw):
super(CustomDictOne, self).__init__(*arg, **kw)
Now you can use the built-in functions, like dict.get()
as self.get()
.
You do not need to wrap a hidden self._dict
. Your class already is a dict.
class Mapping(dict):
def __setitem__(self, key, item):
self.__dict__[key] = item
def __getitem__(self, key):
return self.__dict__[key]
def __repr__(self):
return repr(self.__dict__)
def __len__(self):
return len(self.__dict__)
def __delitem__(self, key):
del self.__dict__[key]
def clear(self):
return self.__dict__.clear()
def copy(self):
return self.__dict__.copy()
def has_key(self, k):
return k in self.__dict__
def update(self, *args, **kwargs):
return self.__dict__.update(*args, **kwargs)
def keys(self):
return self.__dict__.keys()
def values(self):
return self.__dict__.values()
def items(self):
return self.__dict__.items()
def pop(self, *args):
return self.__dict__.pop(*args)
def __cmp__(self, dict_):
return self.__cmp__(self.__dict__, dict_)
def __contains__(self, item):
return item in self.__dict__
def __iter__(self):
return iter(self.__dict__)
def __unicode__(self):
return unicode(repr(self.__dict__))
o = Mapping()
o.foo = "bar"
o['lumberjack'] = 'foo'
o.update({'a': 'b'}, c=44)
print 'lumberjack' in o
print o
In [187]: run mapping.py
True
{'a': 'b', 'lumberjack': 'foo', 'foo': 'bar', 'c': 44}
Don’t inherit from Python built-in dict, ever! for example update
method woldn’t use __setitem__
, they do a lot for optimization. Use UserDict.
from collections import UserDict
class MyDict(UserDict):
def __delitem__(self, key):
pass
def __setitem__(self, key, value):
pass
Here is an alternative solution:
class AttrDict(dict):
def __init__(self, *args, **kwargs):
super().__init__(*args, **kwargs)
self.__dict__ = self
a = AttrDict()
a.a = 1
a.b = 2
I really don’t see the right answer to this anywhere
class MyClass(dict):
def __init__(self, a_property):
self[a_property] = a_property
All you are really having to do is define your own __init__
– that really is all that there is too it.
Another example (little more complex):
class MyClass(dict):
def __init__(self, planet):
self[planet] = planet
info = self.do_something_that_returns_a_dict()
if info:
for k, v in info.items():
self[k] = v
def do_something_that_returns_a_dict(self):
return {"mercury": "venus", "mars": "jupiter"}
This last example is handy when you want to embed some kind of logic.
Anyway… in short class GiveYourClassAName(dict)
is enough to make your class act like a dict. Any dict operation you do on self
will be just like a regular dict.
This is my best solution. I used this many times.
class DictLikeClass:
...
def __getitem__(self, key):
return getattr(self, key)
def __setitem__(self, key, value):
setattr(self, key, value)
...
You can use like:
>>> d = DictLikeClass()
>>> d["key"] = "value"
>>> print(d["key"])
A python class that acts like dict
What’s wrong with this?
Can anyone point out why most of the inheritance snippets look like the one below?
class CustomDictOne(dict):
def __init__(self):
self._mydict = {}
Presumably there’s a good reason to inherit from dict (maybe you’re already passing one around and you want a more specific kind of dict) and you have a good reason to instantiate another dict to delegate to (because this will instantiate two dicts per instance of this class.) But doesn’t that sound incorrect?
I never run into this use-case myself. I do like the idea of typing dicts where you are using dicts that are type-able. But in that case I like the idea of typed class attributes even moreso – and the whole point of a dict is you can give it keys of any hashable type, and values of any type.
So why do we see snippets like this? I personally think it’s an easily made mistake that went uncorrected and thus perpetuated over time.
I would rather see, in these snippets, this, to demonstrate code reuse through inheritance:
class AlternativeOne(dict):
__slots__ = ()
def __init__(self):
super().__init__()
# other init code here
# new methods implemented here
or, to demonstrate re-implementing the behavior of dicts, this:
from collections.abc import MutableMapping
class AlternativeTwo(MutableMapping):
__slots__ = '_mydict'
def __init__(self):
self._mydict = {}
# other init code here
# dict methods reimplemented and new methods implemented here
By request – adding slots to a dict subclass.
Why add slots? A builtin dict
instance doesn’t have arbitrary attributes:
>>> d = dict()
>>> d.foo = 'bar'
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'dict' object has no attribute 'foo'
If we create a subclass the way most are doing it here on this answer, we see we don’t get the same behavior, because we’ll have a __dict__
attribute, causing our dicts to take up to potentially twice the space:
my_dict(dict):
"""my subclass of dict"""
md = my_dict()
md.foo = 'bar'
Since there’s no error created by the above, the above class doesn’t actually act, "like dict
."
We can make it act like dict by giving it empty slots:
class my_dict(dict):
__slots__ = ()
md = my_dict()
So now attempting to use arbitrary attributes will fail:
>>> md.foo = 'bar'
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'my_dict' object has no attribute 'foo'
And this Python class acts more like a dict
.
For more on how and why to use slots, see this Q&A: Usage of __slots__?
UserDict from the Python standard library is designed for this purpose.
I want to write a custom class that behaves like dict
– so, I am inheriting from dict
.
My question, though, is: Do I need to create a private dict
member in my __init__()
method?. I don’t see the point of this, since I already have the dict
behavior if I simply inherit from dict
.
Can anyone point out why most of the inheritance snippets look like the one below?
class CustomDictOne(dict):
def __init__(self):
self._mydict = {}
# other methods follow
Instead of the simpler…
class CustomDictTwo(dict):
def __init__(self):
# initialize my other stuff here ...
# other methods follow
Actually, I think I suspect the answer to the question is so that users cannot directly access your dictionary (i.e. they have to use the access methods that you have provided).
However, what about the array access operator []
? How would one implement that? So far, I have not seen an example that shows how to override the []
operator.
So if a []
access function is not provided in the custom class, the inherited base methods will be operating on a different dictionary?
I tried the following snippet to test out my understanding of Python inheritance:
class myDict(dict):
def __init__(self):
self._dict = {}
def add(self, id, val):
self._dict[id] = val
md = myDict()
md.add('id', 123)
print md[id]
I got the following error:
KeyError: < built-in function id>
What is wrong with the code above?
How do I correct the class myDict
so that I can write code like this?
md = myDict()
md['id'] = 123
[Edit]
I have edited the code sample above to get rid of the silly error I made before I dashed away from my desk. It was a typo (I should have spotted it from the error message).
Check the documentation on emulating container types. In your case, the first parameter to add
should be self
.
The problem with this chunk of code:
class myDict(dict):
def __init__(self):
self._dict = {}
def add(id, val):
self._dict[id] = val
md = myDict()
md.add('id', 123)
…is that your ‘add’ method (…and any method you want to be a member of a class) needs to have an explicit ‘self’ declared as its first argument, like:
def add(self, 'id', 23):
To implement the operator overloading to access items by key, look in the docs for the magic methods __getitem__
and __setitem__
.
Note that because Python uses Duck Typing, there may actually be no reason to derive your custom dict class from the language’s dict class — without knowing more about what you’re trying to do (e.g, if you need to pass an instance of this class into some code someplace that will break unless isinstance(MyDict(), dict) == True
), you may be better off just implementing the API that makes your class sufficiently dict-like and stopping there.
Like this
class CustomDictOne(dict):
def __init__(self,*arg,**kw):
super(CustomDictOne, self).__init__(*arg, **kw)
Now you can use the built-in functions, like dict.get()
as self.get()
.
You do not need to wrap a hidden self._dict
. Your class already is a dict.
class Mapping(dict):
def __setitem__(self, key, item):
self.__dict__[key] = item
def __getitem__(self, key):
return self.__dict__[key]
def __repr__(self):
return repr(self.__dict__)
def __len__(self):
return len(self.__dict__)
def __delitem__(self, key):
del self.__dict__[key]
def clear(self):
return self.__dict__.clear()
def copy(self):
return self.__dict__.copy()
def has_key(self, k):
return k in self.__dict__
def update(self, *args, **kwargs):
return self.__dict__.update(*args, **kwargs)
def keys(self):
return self.__dict__.keys()
def values(self):
return self.__dict__.values()
def items(self):
return self.__dict__.items()
def pop(self, *args):
return self.__dict__.pop(*args)
def __cmp__(self, dict_):
return self.__cmp__(self.__dict__, dict_)
def __contains__(self, item):
return item in self.__dict__
def __iter__(self):
return iter(self.__dict__)
def __unicode__(self):
return unicode(repr(self.__dict__))
o = Mapping()
o.foo = "bar"
o['lumberjack'] = 'foo'
o.update({'a': 'b'}, c=44)
print 'lumberjack' in o
print o
In [187]: run mapping.py
True
{'a': 'b', 'lumberjack': 'foo', 'foo': 'bar', 'c': 44}
Don’t inherit from Python built-in dict, ever! for example update
method woldn’t use __setitem__
, they do a lot for optimization. Use UserDict.
from collections import UserDict
class MyDict(UserDict):
def __delitem__(self, key):
pass
def __setitem__(self, key, value):
pass
Here is an alternative solution:
class AttrDict(dict):
def __init__(self, *args, **kwargs):
super().__init__(*args, **kwargs)
self.__dict__ = self
a = AttrDict()
a.a = 1
a.b = 2
I really don’t see the right answer to this anywhere
class MyClass(dict):
def __init__(self, a_property):
self[a_property] = a_property
All you are really having to do is define your own __init__
– that really is all that there is too it.
Another example (little more complex):
class MyClass(dict):
def __init__(self, planet):
self[planet] = planet
info = self.do_something_that_returns_a_dict()
if info:
for k, v in info.items():
self[k] = v
def do_something_that_returns_a_dict(self):
return {"mercury": "venus", "mars": "jupiter"}
This last example is handy when you want to embed some kind of logic.
Anyway… in short class GiveYourClassAName(dict)
is enough to make your class act like a dict. Any dict operation you do on self
will be just like a regular dict.
This is my best solution. I used this many times.
class DictLikeClass:
...
def __getitem__(self, key):
return getattr(self, key)
def __setitem__(self, key, value):
setattr(self, key, value)
...
You can use like:
>>> d = DictLikeClass()
>>> d["key"] = "value"
>>> print(d["key"])
A python class that acts like
dict
What’s wrong with this?
Can anyone point out why most of the inheritance snippets look like the one below?
class CustomDictOne(dict):
def __init__(self):
self._mydict = {}
Presumably there’s a good reason to inherit from dict (maybe you’re already passing one around and you want a more specific kind of dict) and you have a good reason to instantiate another dict to delegate to (because this will instantiate two dicts per instance of this class.) But doesn’t that sound incorrect?
I never run into this use-case myself. I do like the idea of typing dicts where you are using dicts that are type-able. But in that case I like the idea of typed class attributes even moreso – and the whole point of a dict is you can give it keys of any hashable type, and values of any type.
So why do we see snippets like this? I personally think it’s an easily made mistake that went uncorrected and thus perpetuated over time.
I would rather see, in these snippets, this, to demonstrate code reuse through inheritance:
class AlternativeOne(dict):
__slots__ = ()
def __init__(self):
super().__init__()
# other init code here
# new methods implemented here
or, to demonstrate re-implementing the behavior of dicts, this:
from collections.abc import MutableMapping
class AlternativeTwo(MutableMapping):
__slots__ = '_mydict'
def __init__(self):
self._mydict = {}
# other init code here
# dict methods reimplemented and new methods implemented here
By request – adding slots to a dict subclass.
Why add slots? A builtin dict
instance doesn’t have arbitrary attributes:
>>> d = dict()
>>> d.foo = 'bar'
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'dict' object has no attribute 'foo'
If we create a subclass the way most are doing it here on this answer, we see we don’t get the same behavior, because we’ll have a __dict__
attribute, causing our dicts to take up to potentially twice the space:
my_dict(dict):
"""my subclass of dict"""
md = my_dict()
md.foo = 'bar'
Since there’s no error created by the above, the above class doesn’t actually act, "like dict
."
We can make it act like dict by giving it empty slots:
class my_dict(dict):
__slots__ = ()
md = my_dict()
So now attempting to use arbitrary attributes will fail:
>>> md.foo = 'bar'
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'my_dict' object has no attribute 'foo'
And this Python class acts more like a dict
.
For more on how and why to use slots, see this Q&A: Usage of __slots__?
UserDict from the Python standard library is designed for this purpose.