classification
Title: More details in reference 'Looping through a list in Python and modifying it'
Type: enhancement Stage:
Components: Documentation Versions: Python 3.6
process
Status: open Resolution:
Dependencies: Superseder:
Assigned To: rhettinger Nosy List: docs@python, r.david.murray, rhettinger, terry.reedy, the_darklord
Priority: normal Keywords:

Created on 2017-07-02 04:19 by the_darklord, last changed 2017-07-12 13:20 by r.david.murray.

Messages (6)
msg297507 - (view) Author: Anmol Gupta (the_darklord) Date: 2017-07-02 04:22
Documentation section: https://docs.python.org/3/reference/compound_stmts.html#for

The documentation does not explain at all why is there an infinite loop when not using a copy of the list.

It leaves the reader in a confused state.

Even there are questions concerning the same on stackoverlflow: https://stackoverflow.com/questions/44633798/loop-through-a-list-in-python-and-modify-it
msg297509 - (view) Author: Anmol Gupta (the_darklord) Date: 2017-07-02 06:29
Wrong documentaion section linked.

Correct seciton: Section 4.2 on https://docs.python.org/3/tutorial/controlflow.html

The last line needs more explanation.
msg297514 - (view) Author: Raymond Hettinger (rhettinger) * (Python committer) Date: 2017-07-02 12:37
The example would be more clear if we replaced the opaque idiom "words[:]" with the more explicit alternative "words.copy()".
msg297556 - (view) Author: Anmol Gupta (the_darklord) Date: 2017-07-03 10:44
And also a small explanation for why there would be an infinite loop without creating a copy.
msg297925 - (view) Author: Terry J. Reedy (terry.reedy) * (Python committer) Date: 2017-07-08 02:47
Are you looking for something like:

Let it = iter(words).  When next(it) returns 'defenestrate', insertion at the beginning moves the original 'defenestrate' over so that next(words) returns 'defenestrate' again.
?
msg298214 - (view) Author: R. David Murray (r.david.murray) * (Python committer) Date: 2017-07-12 13:20
I don't think that helps.  The issue here is that *sequences* are iterated over by incrementing an integer index.  If you change the size of the list, you are potentially changing which value any given index points to.  Presumably the tutorial writer thought this was intuitive, and indeed after years of Python programming I find it so.  I can see how a beginner might not, though :)

What if we replaced:

  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:

With:

  Sequence iteration is preformed by incrementing an implicit integer index until there are no more items in the sequence.  The sequence is *not* copied before iteration, so if you modify the sequence during iteration the value that is affected by the next iteration of the loop may not be the one you are expecting.  You can avoid this problem by iterating over a copy of the sequence, and the slice notation makes this especially convenient:

However, this section has a deeper problem.  It is introducing the 'for' statement, but explains what the for statement does in terms of sequences, when in fact the for statement now operates on any iterable, not just sequences.  (Many Python programmers probably do not remember the time before the iteration protocol was added to the language :)

Fixing that problem not only requires rewriting the section, but also figuring out the best place to introduce the concept of the iteration protocol (which *might* be in this section, but it's been so long since I've looked over the tutorial that I can't say).
History
Date User Action Args
2017-07-12 13:20:33r.david.murraysetnosy: + r.david.murray
messages: + msg298214
2017-07-08 02:47:20terry.reedysetnosy: + terry.reedy
messages: + msg297925
2017-07-03 10:44:14the_darklordsetmessages: + msg297556
2017-07-02 12:37:06rhettingersetassignee: docs@python -> rhettinger

messages: + msg297514
nosy: + rhettinger
2017-07-02 06:29:34the_darklordsetnosy: docs@python, the_darklord
messages: + msg297509
2017-07-02 04:22:08the_darklordsetmessages: + msg297507
2017-07-02 04:19:59the_darklordcreate