New submission from Terry J. Reedy:
https://docs.python.org/3/reference/simple_stmts.html#augmented-assignment-statements
"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."
As discussed and demonstrated as part of #21101 (see msg 215560) and following,
this is not exactly true in current CPython. If the expression 'x' is 'y[k]',
then the subscription is performed twice, once to get and once to set. Both y
and k are still evaluated just once.
def ob():
print('ob fetched')
return d
def key():
print('key called')
return 0
d = [[]]
ob()[key()] += [1]
print(d)
# prints
ob fetched
key called
[[1]]
I suggest changing "x is only evaluated once." to something like
"x is usually only evaluated once. However, if x has the form y[k], y and k are
evaluated once but the subscription may be done twice, once to get and once to
set."
I intentionally said 'subscription may be' rather than 'subscription is'
because implementations should remain free to do the subscription (and the
whole expression x) just once -- by directly replacing the reference to the old
value in the internals of the mapping structure with a reference to the new
value.
#16701 discusses possible ambiguity in the next sentence, about 'in place'.
----------
assignee: docs@python
components: Documentation
messages: 217212
nosy: docs@python, terry.reedy
priority: normal
severity: normal
stage: needs patch
status: open
title: Augmented assignment doc: clarify 'only evaluated once'
type: enhancement
versions: Python 2.7, Python 3.4, Python 3.5
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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue21358>
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