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bpo-46095: Improve SeqIter documentation. (GH-30316)

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Raymond Hettinger 2022-01-01 10:37:26 -08:00 • committed by GitHub
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2 changed files with 10 additions and 21 deletions
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10
Doc/library/stdtypes.rst
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@ -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)

21
Doc/reference/compound_stmts.rst
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@ -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: