New submission from Dominik V. <[email protected]>:
https://docs.python.org/3/faq/programming.html#how-do-i-iterate-over-a-sequence-in-reverse-order
It contains the following example:
for x in reversed(sequence):
... # do something with x ...
With the note:
> This won’t touch your original sequence, but build a new copy with reversed
> order to iterate over.
The part about "build a new copy" is not correct in a sense that `reversed`
just returns an iterator over the original sequence. This has mainly two
consequences:
1. It can't be indexed, i.e. `reversed(sequence)[0]` doesn't work.
2. Changing the original sequence after `r = reversed(sequence)` has been
constructed, is reflected in `r` when iterating over it.
So the sentence should be changed into something like:
> This creates an iterator object that can be used to iterate over the original
> sequence in reverse order.
Then for the second example about `sequence[::-1]` it would be good to mention
the difference to `reversed`, namely that this version *does* create a copy of
the original list (in reverse order). It could also be used as an opportunity
to show how to reverse a string, since that is a very popular question on
StackOverflow.
Also the various mentions of Python versions 2.3 and 2.4 seem strange since
this is documentation about Python 3 and those version are anyway very old. So
they should be left out as well.
----------
assignee: docs@python
components: Documentation
messages: 366889
nosy: Dominik V., docs@python
priority: normal
severity: normal
status: open
title: Programming FAQ about "How do I iterate over a sequence in reverse
order?" should be more precise about `reversed`
type: enhancement
versions: Python 3.8, Python 3.9
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Python tracker <[email protected]>
<https://bugs.python.org/issue40345>
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