Raymond Hettinger added the comment:
After looking at this again, I think the entire example should be removed. We
really don't want to encourage people to code like this (it would never make it
through a code review). The example itself is silly (not fully, just weird and
lacking real-world motiviation). The s.insert(0,x) code is an anti-pattern.
And in general, mutating a data structure while iterating over it is a perilous
practice leading to fragile code (many data structures ban the practice
outright: databases, deques, dicts).
Mutating while iterating is only safe if a data structure makes explicit
guarantees about how it iterates. In Python, we have only a handful of such
guarantees (you can safely mutate dict values while iterating over the keys and
lists guarantee that the iterator looks-up consecutive indicies regardless of
changes to the underlying list).
I propose to remove the last two paragraphs and the example, replacing them
with clear practical advice and patterns that would pass a code review.
Something like this:
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.
# Strategy: Iterate over a copy
for user, status in users.copy():
if status == 'inactive':
del users[user]
# Strategy: Create a new collection
active_users = {}
for user, status in users.items():
if status == 'active':
active_users[user] = status
----------
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<https://bugs.python.org/issue30826>
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