Compare object instances for equality by their attributes in Python

Question

What is the best way to compare two instances of some object for equality in Python? I'd like to be able to do something like

Example:

doc1 = ErrorDocument(path='/folder',title='Page')
doc2 = ErrorDocument(path='/folder',title='Page')

if doc1 == doc2: # this should be True
    #do something

EDIT:

To further clarify the question. I'd like to compare by attribute values, and to make a more generic solution than

def __eq__(self, other):
    return self.path == other.path and self.title == other.title

Should the __eq__() method look something like this?

def __eq__(self, other):
    # Is the other instance of the same object
    # Loop through __dict__ and compare values to attributes of other

Answer 1

When comparing instances of objects, the cmp function is called.

If the == operator is not working for you by default, you can always redefine the cmp function for the object.

Edit:

As has been pointed out, the cmp function is deprecated since 3.0. Instead you should use the “rich comparison” methods.

Answer 2

Implement the __eq__ method in your class; something like this:

def __eq__(self, other):
    return self.path == other.path and self.title == other.title

Edit: if you want your objects to compare equal if and only if they have equal instance dictionaries:

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

Answer 3

You override the rich comparison operators in your object.

class MyClass:
 def __lt__(self, other):
      # return comparison
 def __le__(self, other)
      # return comparison
 def __eq__(self, other)
      # return comparison
 def __ne__(self, other)
      # return comparison
 def __gt__(self, other)
      # return comparison
 def __ge__(self, other)
      # return comparison

Answer 4

Instance of a class when compared with == comes to non-equal. The best way is to ass the cmp function to u r class which will do the stuff.

If u want to do comparison by the content u can simply use cmp(obj1,obj2)

In u r case cmp(doc1,doc2) It will return -1 if the content wise thy are same.

Answer 5

As usual with Python, it's kiss :

class Test(object) :

    def __init__(self, attr1, attr2) :
        self.attr1 = attr1
        self.attr2 = attr2

    def __str__(self) :
        return str(self.__dict__)

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

t1 = Test("foo", 42)
t2 = Test("foo", 42)
t3 = Test("bar", 42)

print t1, t2, t3
print t1 == t2
print t2 == t3

It outputs :

{'attr2': 42, 'attr1': 'foo'} {'attr2': 42, 'attr1': 'foo'} {'attr2': 42, 'attr1': 'bar'}
True
False

N.B : be aware that before Python 3.0, you are more likely to use __cmp__ instead of __eq__, working the same way.

Answer 6

You can speed things little by using a Hash

class ErrorDocument(object):
    def __init__(self, path, name):
        self.path = path
        self.name = name
        self.uid = hash(path + name)

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

This of course will only work to compare ErrorDocument objects.