traverseda
/
cpython
1
0
Fork 0
Code Issues Pull Requests Releases Wiki Activity

Issue #16225: Backport from 3.2: Add additional note to tutorial about looping.

This commit is contained in:
Chris Jerdonek 2012-10-15 20:01:38 -07:00
parent fd6d3b149f
commit 0cffd6be56
2 changed files with 24 additions and 11 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

22
Doc/tutorial/controlflow.rst
View File

@ -59,24 +59,24 @@ they appear in the sequence. For example (no pun intended):
:: ::
>>> # Measure some strings: >>> # Measure some strings:
... a = ['cat', 'window', 'defenestrate'] ... words = ['cat', 'window', 'defenestrate']
>>> for x in a: >>> for w in words:
... print x, len(x) ... print w, len(w)
... ...
cat 3 cat 3
window 6 window 6
defenestrate 12 defenestrate 12
It is not safe to modify the sequence being iterated over in the loop (this can If you need to modify the sequence you are iterating over while inside the loop
only happen for mutable sequence types, such as lists). If you need to modify (for example to duplicate selected items), it is recommended that you first
the list you are iterating over (for example, to duplicate selected items) you make a copy. Iterating over a sequence does not implicitly make a copy. The
must iterate over a copy. The slice notation makes this particularly slice notation makes this especially convenient::
convenient::
>>> for x in a[:]: # make a slice copy of the entire list >>> for w in words[:]: # Loop over a slice copy of the entire list.
... if len(x) > 6: a.insert(0, x) ... if len(w) > 6:
... words.insert(0, w)
... ...
>>> a >>> words
['defenestrate', 'cat', 'window', 'defenestrate'] ['defenestrate', 'cat', 'window', 'defenestrate']

13
Doc/tutorial/datastructures.rst
View File

@ -645,6 +645,19 @@ retrieved at the same time using the :meth:`iteritems` method. ::
gallahad the pure gallahad the pure
robin the brave robin the brave
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: .. _tut-conditions: