step bug in turtle's for loop

[Yehuda]

BPO	43289
Nosy	@terryjreedy, @sweeneyde

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2021-02-27.02:41:41.829>
created_at = <Date 2021-02-21.21:29:59.509>
labels = ['type-bug', 'invalid', '3.9']
title = "step bug in turtle's for loop"
updated_at = <Date 2021-02-27.07:55:45.656>
user = 'https://bugs.python.org/Yehuda'

bugs.python.org fields:

activity = <Date 2021-02-27.07:55:45.656>
actor = 'Yehuda'
assignee = 'none'
closed = True
closed_date = <Date 2021-02-27.02:41:41.829>
closer = 'terry.reedy'
components = []
creation = <Date 2021-02-21.21:29:59.509>
creator = 'Yehuda'
dependencies = []
files = []
hgrepos = []
issue_num = 43289
keywords = []
message_count = 6.0
messages = ['387472', '387474', '387483', '387508', '387761', '387771']
nosy_count = 3.0
nosy_names = ['terry.reedy', 'Dennis Sweeney', 'Yehuda']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue43289'
versions = ['Python 3.9']

[Yehuda]

from turtle import *

col = ["black", "white"]

for rad in range(40, 0, -1): 
    dot(5*rad, col[rad % 2])
    
done()

======================
Any other step than -1 crashes the code.

Thank you.

[Yehuda]

Correction to my msg387472: Every *even* step crashes the code.

[sweeneyde]

If I understand correctly, changing the -1 to a -2 does not actually make the program "crash" -- you just only see one black circle.

The reason is that range(40, 0, -2) produces 40, 38, 36, etc., all of which are even numbers, so rad % 2 is always 0, so col[rad % 2] is always "black". You could try:

    for index, rad in enumerate(range(40, 0, -2)): 
        dot(5*rad, col[index % 2])

In the future, I would suggest asking questions like this on StackOverflow, since this is not a bug in Python itself.

[Yehuda]

Thank you Dennis for taking the time to answer.
Yehuda

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On Mon, Feb 22, 2021 at 3:21 AM Dennis Sweeney <report@bugs.python.org>
wrote:

Dennis Sweeney <sweeney.dennis650@gmail.com> added the comment:

If I understand correctly, changing the -1 to a -2 does not actually make
the program "crash" -- you just only see one black circle.

The reason is that range(40, 0, -2) produces 40, 38, 36, etc., all of
which are even numbers, so rad % 2 is always 0, so col[rad % 2] is always
"black". You could try:

for index, rad in enumerate(range(40, 0, -2)):
    dot(5\*rad, col[index % 2])

In the future, I would suggest asking questions like this on
StackOverflow, since this is not a bug in Python itself.

----------
nosy: +Dennis Sweeney

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Python tracker <report@bugs.python.org>
<https://bugs.python.org/issue43289\>

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[terryjreedy]

Yehuda, one can also ask questions like this on python-list.

In the future, when responding by email, please delete the message being responded to, so it does not get repeated when posted to the web page.

[Yehuda]

Thank you Terry, and take care.

Yehuda

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On Sat, Feb 27, 2021 at 4:41 AM Terry J. Reedy <report@bugs.python.org>
wrote:

Terry J. Reedy <tjreedy@udel.edu> added the comment:

Yehuda, one can also ask questions like this on python-list.

In the future, when responding by email, please delete the message being
responded to, so it does not get repeated when posted to the web page.

----------
nosy: +terry.reedy
resolution: -> not a bug
stage: -> resolved
status: open -> closed

---

Python tracker <report@bugs.python.org>
<https://bugs.python.org/issue43289\>

---