How to modify the Python 'default' dictionary so that it always returns a default value

Question

I'm using all of them to print the names of assigned IANA values in a packet. So all of the dictionaries have the same default value "RESERVED".

I don't want to use d.get(key,default) but access dictionaries by d[key] so that if the key is not in d, it returns the default (that is same for all dictionaries).

I do not necessarily need to use dictionaries, but they were the intuitive choice... Also, a dictionary where I could do this

d = {
   1..16 = "RESERVED",
   17 : "Foo",
   18 : "Bar,
   19..255: "INVALID"
  }

Would be the preferred solution

Tuples could be another alternative, but then I'm prone to offset errors assigning the values... (and my code would not be "human readable")

Oh yeah, Python 2.4

Answer 1

If you can, use Mario's solution.

If you can't, you just have to subclass a dictionary-like object. Now, you can do that by inheriting directly from "dict", it will be fast and efficient. For old Python versions, with which you can't inherit from "dict", there is "UserDict", a pure Python dictionary implementation.

With it, you would do it this way :

#!/usr/bin/env python
# -*- coding= UTF-8 -*-

import UserDict

class DefaultDict(UserDict.UserDict) :

    default_value = 'RESERVED'

    def __getitem__(self, key) :
        return self.data.get(key, DefaultDict.default_value)


d = DefaultDict()
d["yes"] = True;
print d["yes"]
print d["no"]

Keep in mind that "UserDict" is slower than "dict".

Answer 2

If you can migrate to Python 2.5, there is the defaultdict class, as shown here. You can pass it an initializer that returns what you want. Otherwise, you'll have to roll your own implementation of it, I fear.

Answer 3

You can use the following class (tested in Python 2.7) Just change zero to any default value you like.

class cDefaultDict(dict):
    # dictionary that returns zero for missing keys
    # keys with zero values are not stored

    def __missing__(self,key):
        return 0

    def __setitem__(self, key, value):
        if value==0:
            if key in self:  # returns zero anyway, so no need to store it
                del self[key]
        else:
            dict.__setitem__(self, key, value)