Equality vs Identity in Python

Zettel#127

zettelkasten
Published

2023-10-05

Equality (==) and Identity (is)

  • Equality (==): Checks if two objects represent the same data.

    x = [2, 1, 3, 4]
y = [2, 1, 3, 4]
x == y  # True

  • Identity (is): Checks if two objects are the same object in memory.

    x is y # False
z = x
x is z  # True

Default Behavior in Custom Classes

  • By default, equality checks in custom classes behave like identity checks.

  • This can be customized using the __eq__ method.

    class Point:
    def __init__(self, x, y):
        self.x = x
        self.y = y
p = Point(1, 2)
q = Point(1, 2)
p == q  # False

Integer Identity Quirks

  • Python optimizes integer objects for values between -5 and 256.

  • This optimization can sometimes extend to other integers within the same scope.

    x, y = 300, 300
x is y  # True

    Quirkiness Explained

  • Assigning the same integer value to different variables doesn’t guarantee identity.

a = 300
b = 300
a is b  # False
a == b  # True

This behavior is quirky because it defies the intuition that two variables with the same value should be identical. However, Python’s memory optimization rules make this untrue for integers outside the -5 to 256 range.

String Optimization

  • Python also optimizes short strings.

    word1 = "Hello"
word2 = "Hello"
word1 is word2  # True

When to Use is

  • Rarely used except for checking sentinel values like None.

    x = None
x is None  # True

Summary

  • Use == for equality checks.
  • Use is mainly for None checks, as recommended by PEP 8.

References