[Python-checkins] bpo-46095: Improve SeqIter documentation. (GH-30316)
rhettinger
webhook-mailer at python.org
Sat Jan 1 13:37:50 EST 2022
https://github.com/python/cpython/commit/a09bc3a404befca197b5d9959a9c62110ee61d77
commit: a09bc3a404befca197b5d9959a9c62110ee61d77
branch: main
author: Raymond Hettinger <rhettinger at users.noreply.github.com>
committer: rhettinger <rhettinger at users.noreply.github.com>
date: 2022-01-01T10:37:26-08:00
summary:
bpo-46095: Improve SeqIter documentation. (GH-30316)
files:
M Doc/library/stdtypes.rst
M Doc/reference/compound_stmts.rst
diff --git a/Doc/library/stdtypes.rst b/Doc/library/stdtypes.rst
index 9d80661fdb821..dc423bfbb7f55 100644
--- a/Doc/library/stdtypes.rst
+++ b/Doc/library/stdtypes.rst
@@ -959,6 +959,16 @@ This means that to compare equal, every element must compare equal and the
two sequences must be of the same type and have the same length. (For full
details see :ref:`comparisons` in the language reference.)
+.. index::
+ single: loop; over mutable sequence
+ single: mutable sequence; loop over
+
+Forward and reversed iterators over mutable sequences access values using an
+index. That index will continue to march forward (or backward) even if the
+underlying sequence is mutated. The iterator terminates only when an
+:exc:`IndexError` or a :exc:`StopIteration` is encountered (or when the index
+drops below zero).
+
Notes:
(1)
diff --git a/Doc/reference/compound_stmts.rst b/Doc/reference/compound_stmts.rst
index cf8ad1787b291..03fc2cb962791 100644
--- a/Doc/reference/compound_stmts.rst
+++ b/Doc/reference/compound_stmts.rst
@@ -196,27 +196,6 @@ the built-in function :func:`range` returns an iterator of integers suitable to
emulate the effect of Pascal's ``for i := a to b do``; e.g., ``list(range(3))``
returns the list ``[0, 1, 2]``.
-.. note::
-
- .. index::
- single: loop; over mutable sequence
- single: mutable sequence; loop over
-
- There is a subtlety when the sequence is being modified by the loop (this can
- only occur for mutable sequences, e.g. lists). An internal counter is used
- to keep track of which item is used next, and this is incremented on each
- iteration. When this counter has reached the length of the sequence the loop
- terminates. This means that if the suite deletes the current (or a previous)
- item from the sequence, the next item will be skipped (since it gets the
- index of the current item which has already been treated). Likewise, if the
- suite inserts an item in the sequence before the current item, the current
- item will be treated again the next time through the loop. This can lead to
- nasty bugs that can be avoided by making a temporary copy using a slice of
- the whole sequence, e.g., ::
-
- for x in a[:]:
- if x < 0: a.remove(x)
-
.. _try:
.. _except:
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