Object of custom type as dictionary key

Question

What must I do to use my objects of a custom type as keys in a Python dictionary  where I don t want the  object id  to act as the key    e g    class MyThing      def   init   self name location length               self name   name             self location   location             self length   length   I d want to use MyThing s as keys that are considered the same if name and location are the same  From C  Java I m used to having to override and provide an equals and hashcode method  and promise not to mutate anything the hashcode depends on   What must I do in Python to accomplish this   Should I even      In a simple case  like here  perhaps it d be better to just place a  name location  tuple as key - but consider I d want the key to be an object

User · Answer

You override   hash   if you want special hash-semantics  and   cmp   or   eq   in order to make your class usable as a key  Objects who compare equal need to have the same hash value   Python expects   hash   to return an integer  returning Banana   is not recommended      User defined classes have   hash   by default that calls id self   as you noted   There is some extra tips from the documentation       Classes which inherit a   hash       method from a parent class but change   the meaning of   cmp     or   eq       such that the hash value returned is   no longer appropriate  e g  by   switching to a value-based concept of   equality instead of the default   identity based equality  can   explicitly flag themselves as being   unhashable by setting   hash     None   in the class definition  Doing so   means that not only will instances of   the class raise an appropriate   TypeError when a program attempts to   retrieve their hash value  but they   will also be correctly identified as   unhashable when checking   isinstance obj  collections Hashable     unlike classes which define their own     hash     to explicitly raise TypeError

User · Answer

You need to add 2 methods  note   hash   and    eq     class MyThing      def   init   self name location length           self name   name         self location   location         self length   length      def   hash   self           return hash  self name  self location        def   eq   self  other           return  self name  self location      other name  other location       def   ne   self  other             Not strictly necessary  but to avoid having both x  y and x  y           True at the same time         return not self    other    The Python dict documentation defines these requirements on key objects  i e  they must be hashable

User · Answer

An alternative in Python 2 6 or above is to use collections namedtuple   -- it saves you writing any special methods   from collections import namedtuple MyThingBase   namedtuple  MyThingBase     name    location    class MyThing MyThingBase       def   new   cls  name  location  length           obj   MyThingBase   new   cls  name  location          obj length   length         return obj  a   MyThing  a    here   10  b   MyThing  a    here   20  c   MyThing  c    there   10  a    b   True hash a     hash b    True a    c   False

[python] Object of custom type as dictionary key

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