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gh-65557: Clarify 'only evaluated once' in augmented assignment docs #153264

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gh-65557: Clarify 'only evaluated once' in augmented assignment docs #153264

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8 changes: 6 additions & 2 deletions Doc/reference/simple_stmts.rst
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Expand Up @@ -286,11 +286,15 @@ symbols.)
An augmented assignment evaluates the target (which, unlike normal assignment
statements, cannot be an unpacking) and the expression list, performs the binary
operation specific to the type of assignment on the two operands, and assigns
the result to the original target. The target is only evaluated once.
the result to the original target. Subexpressions of the target are only
evaluated once.

An augmented assignment statement like ``x += 1`` can be rewritten as ``x = x +
1`` to achieve a similar, but not exactly equal effect. In the augmented
version, ``x`` is only evaluated once. Also, when possible, the actual operation
version, subexpressions of ``x`` are only evaluated once: for a target of the
form ``y[k]``, ``y`` and ``k`` are each evaluated only once, but the
subscription may be performed twice, once to fetch the old value and once to
store the result. Also, when possible, the actual operation
is performed *in-place*, meaning that rather than creating a new object and
assigning that to the target, the old object is modified instead.

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