What's the Difference Between Python's '==' and 'is' Operators for Equality Testing?

Understanding the Difference Between "==" and "is" Equality Tests in Python

In Python, distinguishing between "==" and "is" equality tests is crucial when performing comparisons. While both operators evaluate equality, they differ in their underlying principles.

"is" Test:

The "is" operator checks if two variables refer to the same object in memory. This means that it evaluates if both variables point to the exact same location in the computer's RAM.

"==" Test:

The "==" operator, on the other hand, compares the values of two objects. It determines if the values stored by the variables are identical.

Application to Lists:

For immutable objects like strings and integers, both "is" and "==" return the same result. However, the behavior differs for mutable objects like lists. Consider the following example:

L = []
L.append(1)

if L == [1]:
    # True

if L is [1]:
    # False

In this example, the "==" test returns True because the values of the list L and [1] are equal. However, the "is" test returns False because the two variables do not refer to the same object in memory. This is because L is a reference to the original list, while [1] is a newly created list object.

Larger Objects:

For larger objects like lists, "is" will only return True if both variables point to the exact same object. For instance:

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

if b is a:
    # True

b[:] = [4, 5, 6]

if b is a:
    # False

Cachings Considerations:

It's worth noting that Python caches small integer objects and string literals. As a result, in certain cases, "==" and "is" may return the same result. However, this is an implementation detail and should not be relied upon.

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