[issue21358] Augmented assignment doc: clarify 'only evaluated once'

[Irit Katriel]

Change by Irit Katriel :


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versions: +Python 3.10, Python 3.11, Python 3.9 -Python 2.7, Python 3.4, Python 
3.5

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[issue21358] Augmented assignment doc: clarify 'only evaluated once'

[Ezio Melotti]

Changes by Ezio Melotti ezio.melo...@gmail.com:


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nosy: +ezio.melotti

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[issue21358] Augmented assignment doc: clarify 'only evaluated once'

[Terry J. Reedy]

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'.

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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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[issue21358] Augmented assignment doc: clarify 'only evaluated once'

[Martin v . Löwis]

Martin v. Löwis added the comment:

This is not limited to dictionaries. Augmented assignment *always* involves a 
read operation and a write operation. 
So Antoine's remark in msg215573 is more general;

a.x += 1

has a get and a set, and even

x += 1

has a get and a set.

I still agree that the original statement is confusing. It (implicitly) claims 
that 

  x = x + 1

evaluates x twice, which it does not. Instead, x is only *evaluated* once, and 
then written to. Only if x has subexpressions, they get evaluated only once 
(evaluation being the thing that produces a value).

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