Thursday, December 4, 2014

Using user classes with Python's set operations

Python has an efficient, and built in, implementation of set operations. With a tiny bit of work you can make your user defined classes work nicely with set operations (as well as dictionaries).

Say you have a class:

class A:
  def __init__(self, x, s):
    self.x, self.s = x, s

Python actually doesn't have a way of knowing if the objects are the same or not:

a1 = A(1, 'Hi')
a2 = A(1, 'Hi')
a = set([a1, a2])
-> a1, a2

and by default assumes they are different.

All you have have to do, for things to work properly, is to define a hash function (__hash__()) and an equality (__eq__()) operator for your class. The two functions need to be congruent i.e. if the hash value for two objects is the same, they should also register as equal. It is possible to define the functions differently but this will lead to funny things happening with set operations.

The hash value is an integer. If your class has several attributes that you consider important for distinguishing between objects you can hash each attribute separately and then XOR the individual hashes.

class A:
  def __init__(self, x, s):
    self.x, self.s = x, s

  def __hash__(self):
    return self.x ^ self.s.__hash__()


  def __eq__(self, other):
    return self.__hash__() == other.__hash__()

And now

a1 = A(1, 'Hi')
a2 = A(1, 'Hi')
a = set([a1, a2])
-> just a1 as expected

No comments:

Post a Comment