Mutable vs Immutable Types in Python: Why a += b Differs from a = a + b

Mutable and Immutable Types

To understand whether a += b and a = a + b produce the same result, you must first know what mutable and immutable variables are.

Immutable Types in Python

Immutable: numbers, strings, tuples

Mutable: lists, dictionaries

Immutable Type Operation Examples

1. Immutable +=

2. Immutable =+

For immutable types, the results of += and =+ are identical.

Mutable Type Operation Examples

1. Mutable +=

2. Mutable =+

Comparing the images shows that for mutable types the variables behave differently: after a += b both references change, but after a = a + b only the left‑hand variable points to a new object while the other reference stays unchanged.

Key Takeaway

Remember the conclusion above: the difference lies in the underlying methods __add__ and __iadd__.

Difference Between __add__ and __iadd__

__add__

receives two arguments and returns their sum; both arguments remain unchanged. __iadd__ receives two arguments, but the first argument is modified in place.

When a mutable type uses the plus operator as a = a + b, __add__ creates a new object, leaving the original unchanged. When using a += b, __iadd__ modifies the existing object, so all references to it see the change.

Illustrative diagrams:

After a = a + b, a1 points to a new result while a2 remains unchanged.

After a += b, a1 's contents change in place, so a2 also reflects the change.