This commit is contained in:
Raymond Hettinger 2015-09-01 02:33:20 -07:00
commit 799b05b052
1 changed files with 10 additions and 10 deletions

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@ -612,18 +612,18 @@ 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::
It is sometimes tempting to change a list while you are looping over it;
however, it is often simpler and safer to create a new list instead. ::
>>> 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)
>>> import math
>>> raw_data = [56.2, float('NaN'), 51.7, 55.3, 52.5, float('NaN'), 47.8]
>>> filtered_data = []
>>> for value in raw_data:
... if not math.isnan(value):
... filtered_data.append(value)
...
>>> words
['defenestrate', 'cat', 'window', 'defenestrate']
>>> filtered_data
[56.2, 51.7, 55.3, 52.5, 47.8]
.. _tut-conditions: