How Does Python Swap Tuple Values Without a Temporary Variable?

How Swapping Tuples Works Internally: Swapping Variables Without a Temporary

In Python, swapping the values of two variables often involves using a temporary variable. However, the swap operation can be performed without it through tuple assignments.

a = 5
b = 6
(a, b) = (b, a)

Python separates the assignment operation into two steps:

1. Evaluating the right-hand side expression: The values stored in b and a are pushed onto the stack as [b, a].

2. Assigning to left-hand side variables:

   • The ROT_TWO opcode swaps the top two items on the stack, resulting in [a, b].
   • The top two values are then assigned to a and b using STORE_FAST. The first STORE_FAST assigns a the value popped from the stack, while the second assigns b the remaining value.

This process eliminates the need for a temporary third variable.

For assignments involving more than two items, the same principle applies. For three-name assignments, ROT_THREE is used to reverse the top three stack elements. For longer assignments, an explicit tuple is constructed and then unpacked using BUILD_TUPLE and UNPACK_SEQUENCE.

In summary, tuple assignments allow for efficient swapping by directly manipulating values on the stack without the overhead of creating and using a temporary variable.

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