From 4fab8f0e067772c4c13c86056c2b659cd8e5a812 Mon Sep 17 00:00:00 2001 From: Chris Jerdonek Date: Mon, 15 Oct 2012 19:44:47 -0700 Subject: Issue #16225: Add additional note to tutorial about changing sequence while looping. --- Doc/tutorial/controlflow.rst | 22 +++++++++++----------- Doc/tutorial/datastructures.rst | 13 +++++++++++++ 2 files changed, 24 insertions(+), 11 deletions(-) diff --git a/Doc/tutorial/controlflow.rst b/Doc/tutorial/controlflow.rst index 1564e96..574f0d0 100644 --- a/Doc/tutorial/controlflow.rst +++ b/Doc/tutorial/controlflow.rst @@ -58,24 +58,24 @@ they appear in the sequence. For example (no pun intended): :: >>> # Measure some strings: - ... a = ['cat', 'window', 'defenestrate'] - >>> for x in a: - ... print(x, len(x)) + ... words = ['cat', 'window', 'defenestrate'] + >>> for w in words: + ... print(w, len(w)) ... cat 3 window 6 defenestrate 12 -It is not safe to modify the sequence being iterated over in the loop (this can -only happen for mutable sequence types, such as lists). If you need to modify -the list you are iterating over (for example, to duplicate selected items) you -must iterate over a copy. The slice notation makes this particularly -convenient:: +If you need to modify the sequence you are iterating over while inside the loop +(for example to duplicate selected items), it is recommended that you first +make a copy. Iterating over a sequence does not implicitly make a copy. The +slice notation makes this especially convenient:: - >>> for x in a[:]: # make a slice copy of the entire list - ... if len(x) > 6: a.insert(0, x) + >>> for w in words[:]: # Loop over a slice copy of the entire list. + ... if len(w) > 6: + ... words.insert(0, w) ... - >>> a + >>> words ['defenestrate', 'cat', 'window', 'defenestrate'] diff --git a/Doc/tutorial/datastructures.rst b/Doc/tutorial/datastructures.rst index 12b5c57..e008dd8 100644 --- a/Doc/tutorial/datastructures.rst +++ b/Doc/tutorial/datastructures.rst @@ -584,6 +584,19 @@ returns a new sorted list while leaving the source unaltered. :: orange pear +To change a sequence you are iterating over while inside the loop (for +example to duplicate certain items), it is recommended that you first make +a copy. Looping over a sequence does not implicitly make a copy. The slice +notation makes this especially convenient:: + + >>> words = ['cat', 'window', 'defenestrate'] + >>> for w in words[:]: # Loop over a slice copy of the entire list. + ... if len(w) > 6: + ... words.insert(0, w) + ... + >>> words + ['defenestrate', 'cat', 'window', 'defenestrate'] + .. _tut-conditions: -- cgit v0.12