Question

Is there any way to make a list of classes behave like a set in python?

Basically, I’m working on a piece of software that does some involved string comparison, and I have a custom class for handling the strings. Therefore, there is an instance of the class for each string.

As a result, I have a large list containing all these classes. I would like to be able to access them like list[key], where in this case, the key is a string the class is based off of (note: the string will never change once the class is instantiated, so it should be hashable).

It seems to me that I should be able to do this somewhat easily, by adding something like __cmp__ to the class, but either I’m being obtuse (likely), or I’m missing something in the docs.

Basically, I want to be able to do something like this (Python prompt example):

>>class a:
... def __init__(self, x):
...  self.var = x
...
>>> from test import a
>>> cl = set([a("Hello"), a("World"), a("Pie")])
>>> print cl
set([<test.a instance at 0x00C866C0>, <test.a instance at 0x00C866E8>, <test.a instance at 0x00C86710>])
>>> cl["World"]
<test.a instance at 0x00C866E8>

Thanks!

Edit Some additional Tweaks:

class a:
... def __init__(self, x):
...     self.var = x
... def __hash__(self):
...     return hash(self.var)
...
>>> v = a("Hello")
>>> x = {}
>>> x[v]=v
>>> x["Hello"]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 'Hello'
>>> x["Hello"]

Answer 1

Just write a class that behaves a bit like a mapping:

class ClassDict(object):
  def __init__(self):
    self.classes = {}

  def add(self, cls):
    self.classes[cls.__name__] = cls

  def remove(self, cls):
    if self.classes[cls.__name__] == cls:
      del self.classes[cls.__name__]
    else:
      raise KeyError('%r' % cls)

  def __getitem__(self, key):
    return self.classes[key]

  def __repr__(self):
    return 'ClassDict(%s)' % (', '.join(self.classes),)

class C(object):
  pass

class D(object):
  pass

cd = ClassDict()
cd.add(C)
cd.add(D)

print cd

print cd['C']