Equality in Python

== (Same value)

== checks that two variables have the same value. The Python interpreter will look at the content of the variables to evaluate a == b.

For standard objects

a = [1, 2]
b = []
b.append(1)
b.append(2)
a == b

True

123456789 == 123456789

True

a = 123456789
b = 123456788+1
a == b

True

a = (1, 2, 3)
b = [1, 2, 3]
a == b

False

For custom objects

For custom objects, == has to specified via the magic method __eq__. Actually... for all objects!

class User:
    def __init__(self, username: str, email: str):
        self.username = username
        self.email = email

User("francois", "francois@ens-lyon.fr") == User("francois", "francois@ens-lyon.fr")

False

class User:
    def __init__(self, username: str, email: str):
        self.username = username
        self.email = email

    def __eq__(self, other: object):
        if isinstance(other, User):
            return self.username == other.username and self.email == other.email
        return NotImplemented

User("francois", "francois@ens-lyon.fr") == User("francois", "francois@ens-lyon.fr")

True

class User2:
    def __init__(self, username: str, email: str):
        self.username = username
        self.email = email

User2("francois", "francois@ens-lyon.fr") == User2("francois", "francois@ens-lyon.fr")

False

is (Same object)

is means the same object. The Pyther interpreter will just check the memory address. a is b is true iff the memory address of a and b are the same (same pointers, i.e. pointers pointed to the very same object).

class Person:
    pass

a = Person()
b = Person()
a is b

False

class Person:
    pass

a = Person()
b = a
a is b

True

a = 123456789
b = 123456789
a is b

False

a = 123456789
b = a
a is b

True

a = 123456789
b = 123456788+1
a is b

False

a = 2
b= 1+1
a is b

True

a = [1,2]
b = a
a is b

True

a = (1, 2, 3)
b = (1, 2, 3)
a is b

False

Summary

In German:

• is is das selbe
• == is das gleiche