Data Model

Equality and Identity

== compares values, while is compares object identity.

Equal containers can be different objects. == compares list contents, while is checks whether both names refer to the same list object.

Source

left = [1, 2, 3]
right = [1, 2, 3]
print(left == right)
print(left is right)

Output

True
False

Identity matters when objects are mutable. same is another name for left, so mutating through one name changes the object seen through the other.

Source

same = left
same.append(4)
print(left)
print(same is left)

Output

[1, 2, 3, 4]
True

Use is for singleton identity checks such as None. This asks whether the value is the one special None object.

Source

value = None
print(value is None)

Output

True

is for integers is unreliable because CPython caches small integers (roughly -5 to 256) but not larger ones. Two equal large integers can be different objects. Use == for value comparisons; reserve is for singletons.

Source

small_a = 100
small_b = 100
print(small_a is small_b)

big_a = int("1000")
big_b = int("1000")
print(big_a is big_b)
print(big_a == big_b)

Output

True
False
True

Notes

Use == for ordinary value comparisons.
Use is primarily for identity checks against singletons such as None.
Equal mutable containers can still be independent objects.
Never use is to compare numbers; CPython's small-integer cache makes the result an implementation detail.

Run the complete example

Expected output

True
False
[1, 2, 3, 4]
True
True
True
False
True

Execution time appears here after you run the example.

Equality and Identity

See also

Run the complete example

Example code

Expected output