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bpo-30826: Improve control flow examples (GH-15407)

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Raymond Hettinger 2019-08-22 23:44:19 -07:00 committed by GitHub
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Doc/tutorial/controlflow.rst
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@ -66,20 +66,20 @@ they appear in the sequence. For example (no pun intended):
window 6
defenestrate 12
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::
Code that modifies a collection while iterating over that same collection can
be tricky to get right. Instead, it is usually more straight-forward to loop
over a copy of the collection or to create a new collection::
>>> 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']
# Strategy: Iterate over a copy
for user, status in users.copy().items():
if status == 'inactive':
del users[user]
With ``for w in words:``, the example would attempt to create an infinite list,
inserting ``defenestrate`` over and over again.
# Strategy: Create a new collection
active_users = {}
for user, status in users.items():
if status == 'active':
active_users[user] = status
.. _tut-range: