How Does Python Efficiently Swap Tuple Members Using (a, b) = (b, a)?

How Does Swapping of Members in Tuples (a, b) = (b, a) Work Internally?

When swapping the values of a and b using the tuple assignment (a, b) = (b, a), Python follows a specific internal mechanism that eliminates the need for temporary variables.

Stack Evaluation and Assignment

Python evaluates the right-hand side of the assignment first. The result is stored on the stack, which is a last-in-first-out (LIFO) data structure. Subsequently, the values are assigned to the left-hand side names using opcodes.

For tuple assignments with up to three items, Python utilizes the stack directly.

2-3 Item Assignments

For two or three-name assignments, Python utilizes the ROT_TWO and ROT_THREE opcodes, respectively. These opcodes rotate the top elements on the stack, effectively swapping their positions. This rotation ensures that the values are assigned from left to right on the left-hand side.

Longer Assignments

For tuple assignments with more than three items, Python creates an explicit tuple. It builds the tuple from the stack in reverse order and then unpacks the tuple, pushing its elements onto the stack again. This allows the STORE_FAST operations to assign the values to the appropriate variables.

Optimization

Although the creation and unpacking of tuples may seem inefficient, Python employs an optimization step. For assignments with two or three items, it replaces the BUILD_TUPLE / UNPACK_SEQUENCE combination with ROT_TWO and ROT_THREE, respectively, improving performance.

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