gc.get_referrers behavior change 3.6 to 3.7

[bryevdv]

BPO	34608
Nosy	@pitrou, @benjaminp, @methane, @bryevdv

^{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 2018-09-09.13:54:32.678>
created_at = <Date 2018-09-07.16:30:04.024>
labels = ['3.7', 'library']
title = 'gc.get_referrers behavior change 3.6 to 3.7'
updated_at = <Date 2018-09-09.13:54:32.676>
user = 'https://github.com/bryevdv'

bugs.python.org fields:

activity = <Date 2018-09-09.13:54:32.676>
actor = 'methane'
assignee = 'none'
closed = True
closed_date = <Date 2018-09-09.13:54:32.678>
closer = 'methane'
components = ['Library (Lib)']
creation = <Date 2018-09-07.16:30:04.024>
creator = 'bryevdv'
dependencies = []
files = []
hgrepos = []
issue_num = 34608
keywords = []
message_count = 3.0
messages = ['324771', '324819', '324886']
nosy_count = 4.0
nosy_names = ['pitrou', 'benjamin.peterson', 'methane', 'bryevdv']
pr_nums = []
priority = 'normal'
resolution = 'wont fix'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue34608'
versions = ['Python 3.7']

[bryevdv]

When called on a local object inside a function, gc.get_referrers no longer returns a Frame as one of the references. I could not find anything in the release notes or changeling that indicated that this is an intentional change.

The following script generates different output when run on Python 3.6 vs Python 3.7 (on linux, OSX, or Windows):

# referrers.py
import gc, sys

class FakeMod(object): pass

extra = []

def test():
    mod = FakeMod()
    extra.append(mod)
    referrers = gc.get_referrers(mod)
    print(".".join(str(x) for x in sys.version_info[:3]), ":", len(referrers), referrers)

test()

Output:

~ master*
(py37) ❯ python test.py
3.7.0 : 1 [[<main.FakeMod object at 0x10b65e320>]]

~ master*
(base) ❯ python test.py
3.6.6 : 2 [[<main.FakeMod object at 0x106f3ea90>], <frame object at 0x10684b648>]

[benjaminp]

Likely fallout from 5a625d0.

get_referrers() doesn't really guarantee anything. It's more of a clever hack based on however the Python GC works at the moment. So, this it probably WONTFIX.

[methane]

Benjamin is right.
This is very implementation detail. We shouldn't rely on such edge case.