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

View File

@ -59,24 +59,24 @@ 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']

View File

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