Check for custom objects in Python set.

Fri, Jan 9, 2015 Read as Markdown

Python set data structure is commonly used for removing duplicate entriesand make lookup faster (O(1)). Any hashable object can be stored in set.For example, list and dict can’t be stored.

User defined objects can be stored. Here is how it looks.

class Person(object):
    def __init__(self, name, age):
        self.name, self.age = name, age


In [25]: s = set()
In [26]: s.add(Person('kracekumar', 25))

In [27]: s
Out[27]: set([<__main__.Person at 0x1033c5e10>])

In [29]: Person('kracekumar', 25) in s
Out[29]: False

Implement equality check

Even though Person object with same value is present but check failed.This is because default python __eq__ checks for reference.

class Person(object):
    def __init__(self, name, age):
        self.name, self.age = name, age

    def __eq__(self, other):
        return (isinstance(other, self.__class__) and
            getattr(other, 'name', None) == self.name and
            getattr(other, 'age', None) == self.age)

    def __hash__(self):
        return hash(self.name + str(self.age))

In [38]: s = set()

In [39]: s.add(Person('kracekumar', 25))

In [40]: Person('kracekumar', 25) in s
Out[40]: True

In [41]: s
Out[41]: set([<__main__.Person at 0x1033d0590>])

In [42]: s.add(Person('kracekumar', 25))

In [43]: s
Out[43]: set([<__main__.Person at 0x1033d0590>])

__hash__ is used for calculating hash of the object and __eq__ is used byin during check after hash value match.