Python 3.6 change in dict iteration when inserting keys #73606
Description
| BPO | 29420 |
|---|---|
| Nosy | @bitdancer |
| Superseder |
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 2017-02-02.14:38:17.926>
created_at = <Date 2017-02-02.14:10:45.449>
labels = ['type-bug']
title = 'Python 3.6 change in dict iteration when inserting keys'
updated_at = <Date 2017-02-02.15:23:38.378>
user = 'https://bugs.python.org/matthewbrett'bugs.python.org fields:
activity = <Date 2017-02-02.15:23:38.378>
actor = 'matthew.brett'
assignee = 'none'
closed = True
closed_date = <Date 2017-02-02.14:38:17.926>
closer = 'r.david.murray'
components = []
creation = <Date 2017-02-02.14:10:45.449>
creator = 'matthew.brett'
dependencies = []
files = []
hgrepos = []
issue_num = 29420
keywords = []
message_count = 3.0
messages = ['286794', '286797', '286804']
nosy_count = 2.0
nosy_names = ['r.david.murray', 'matthew.brett']
pr_nums = []
priority = 'normal'
resolution = 'duplicate'
stage = 'resolved'
status = 'closed'
superseder = '19332'
type = 'behavior'
url = 'https://bugs.python.org/issue29420'
versions = ['Python 3.6']
Activity
MatthewBrett commentedon Feb 2, 2017
The behavior of dict iteration has changed in Python 3.6, in that inserting keys during iteration has a different and unpredictable affect. For this code:
Python 3.5 prints a single 'foo' (one pass through the loop). Python 3.6 generates five lines of 'foo' (five passes through the loop). Of course this code is pathological, but I found this behavior from a bug in code where the pathology was a lot less obvious - see nipy/nipy#420
bitdancer commentedon Feb 2, 2017
This "bug" has always existed, its just that its manifestation has changed. See bpo-19332 for background.
MatthewBrett commentedon Feb 2, 2017
To clarify from comments on bpo-19332:
"""
"""
The change in Python 3.6 is just a different undefined behavior.