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    lt  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

User · Answer

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

User · Answer

Check the documentation on emulating container types  In your case  the first parameter to add should be self

User · Answer

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    gt  gt  gt  d   DictLikeClass    gt  gt  gt  d  key      value   gt  gt  gt  print d  key

User · Answer

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

User · Answer

UserDict from the Python standard library is designed for this purpose

User · Answer

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

User · Answer

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

User · Answer

A python class that acts like dict  By request - adding slots to a dict subclass  Why add slots  A builtin dict instance doesn t have arbitrary attributes   gt  gt  gt  d   dict    gt  gt  gt  d foo    bar  Traceback  most recent call last     File  quot  lt stdin gt  quot   line 1  in  lt module gt  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        quot  quot  quot my subclass of dict quot  quot  quot    md   my dict   md foo    bar   Since there s no error created by the above  the above class doesn t actually act   quot like dict  quot  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   gt  gt  gt  md foo    bar  Traceback  most recent call last     File  quot  lt stdin gt  quot   line 1  in  lt module gt  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 amp A  Usage of   slots

User · Answer

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

User · Answer

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   quot mercury quot    quot venus quot    quot mars quot    quot jupiter quot    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

[python] A python class that acts like dict

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