Eryk Sun <eryk...@gmail.com> added the comment:
Please read about augmented assignment [1]. In the REPL, use help("+=").
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.
An augmented assignment expression 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 is performed in-place, meaning that
rather than creating a new object and assigning that to the target,
the old object is modified instead.
Unlike normal assignments, augmented assignments evaluate the left-
hand side before evaluating the right-hand side. For example,
a[i] += f(x) first looks-up a[i], then it evaluates f(x) and
performs the addition, and lastly, it writes the result back to
a[i].
---
[1]
https://docs.python.org/3/reference/simple_stmts.html#augmented-assignment-statements
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nosy: +eryksun
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Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue46550>
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