Show clear error message on circular import

[cool-RR]

BPO	20490
Nosy	@brettcannon, @bitdancer, @cool-RR, @1st1, @zooba, @asottile, @pablogsal
PRs	bpo-20490: Improve circular import error messaging #15308 [3.8] bpo-20490: Improve circular import error message (GH-15308) #15791

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

Show more details

GitHub fields:

assignee = None
closed_at = <Date 2019-09-09.15:48:07.477>
created_at = <Date 2014-02-02.19:04:48.278>
labels = ['interpreter-core', 'type-feature', '3.8', '3.9']
title = 'Show clear error message on circular import'
updated_at = <Date 2019-09-10.13:06:52.667>
user = 'https://github.com/cool-RR'

bugs.python.org fields:

activity = <Date 2019-09-10.13:06:52.667>
actor = 'pablogsal'
assignee = 'none'
closed = True
closed_date = <Date 2019-09-09.15:48:07.477>
closer = 'steve.dower'
components = ['Interpreter Core']
creation = <Date 2014-02-02.19:04:48.278>
creator = 'cool-RR'
dependencies = []
files = []
hgrepos = []
issue_num = 20490
keywords = ['patch']
message_count = 6.0
messages = ['210006', '210153', '351496', '351505', '351529', '351642']
nosy_count = 8.0
nosy_names = ['brett.cannon', 'Arfrever', 'r.david.murray', 'cool-RR', 'yselivanov', 'steve.dower', 'Anthony Sottile', 'pablogsal']
pr_nums = ['15308', '15791']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue20490'
versions = ['Python 3.8', 'Python 3.9']

[cool-RR]

If there's a circular import in my program, I don't want to see an error message, "Cannot import name 'foo'" and then say in my mind, "ah, I'm an experienced Python developer, I know that when Python says that it often means that there's a circular import problem." I want the error message to **say there's a circular import problem**, so (a) I won't have to remember this piece of trivia and (b) less experienced developers won't have to bang their heads on their desks trying to figure out why the import isn't working.

Bonus points if the error message actually shows which module is trying to import a module that imports itself, so we'll know where to fix.

[brettcannon]

It's not as simple as it sounds because failures from "from ... import ..." are triggered in the eval loop and not in import and without thinking it through I don't know if you could use the fine-grained import locks to inspect if something is in the middle of being imported or not that way.

[zooba]

New changeset 65366bc by Steve Dower (Anthony Sottile) in branch 'master':
bpo-20490: Improve circular import error message (GH-15308)
65366bc

[zooba]

Thanks for the patch, Anthony!

[zooba]

New changeset 2d5594f by Steve Dower in branch '3.8':
bpo-20490: Improve circular import error message (GH-15308)
2d5594f

[pablogsal]

This PR introduced a reference leak tracked in:

https://bugs.python.org/issue38090