[2.7] Add PYTHONSHOWREFCOUNT environment variable to Python 2.7

[vstinner]

BPO	31733
Nosy	@vstinner, @benjaminp, @ezio-melotti, @serhiy-storchaka, @stratakis
PRs	[2.7] bpo-31733: Add PYTHONSHOWREFCOUNT env var #3932 [2.7] bpo-31733, bpo-31692: Document 2 new env vars in What's New in Python 2.7 #4019

^{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 2017-10-17.10:03:43.361>
created_at = <Date 2017-10-09.13:44:19.479>
labels = []
title = '[2.7] Add PYTHONSHOWREFCOUNT environment variable to Python 2.7'
updated_at = <Date 2017-10-17.20:13:15.247>
user = 'https://github.com/vstinner'

bugs.python.org fields:

activity = <Date 2017-10-17.20:13:15.247>
actor = 'vstinner'
assignee = 'none'
closed = True
closed_date = <Date 2017-10-17.10:03:43.361>
closer = 'vstinner'
components = []
creation = <Date 2017-10-09.13:44:19.479>
creator = 'vstinner'
dependencies = []
files = []
hgrepos = []
issue_num = 31733
keywords = ['patch']
message_count = 5.0
messages = ['303952', '303962', '304491', '304496', '304526']
nosy_count = 5.0
nosy_names = ['vstinner', 'benjamin.peterson', 'ezio.melotti', 'serhiy.storchaka', 'cstratak']
pr_nums = ['3932', '4019']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue31733'
versions = ['Python 2.7']

[vstinner]

When Python 2.7 is built in debug mode, it displays the total number of references when the program finishes or after each statement in the interactive interpreter.

Example:

haypo@selma$ ./python
Python 2.7.14+ (heads/2.7:cc4b6f1c62, Oct  9 2017, 15:30:11) 
[GCC 7.2.1 20170915 (Red Hat 7.2.1-2)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> 1+1
2
[45025 refs]
>>> 2+2
4
[45025 refs]
>>> 
[45025 refs]
[21655 refs]

While I'm Python core developer working on reference leaks for years, I *never* used this value. I only use "python -m test -R 3:3 test_xxx" to track reference leaks.

I really liked Python 3.4 which made this option an opt-in, python3 -X showrefcount: bpo-17323 (commit 1f8898a).

I propose to change Python 2.7 behaviour to not print this value by default, but add a new PYTHONSHOWREFCOUNT environment variable to display it. The value is displayed if PYTHONSHOWREFCOUNT is set. Other similar existing variables:

• PYTHONTHREADDEBUG
• PYTHONDUMPREFS
• PYTHONMALLOCSTATS

https://docs.python.org/2.7/using/cmdline.html#debug-mode-variables

I prefer to add a new environment style rather than adding a new -X command line option. IMHO an environment variable is less intrusive. For example, older Python 2.7 versions completely ignore unknown environment variables, whereas "python2.7 -X showrefcount ..." currently logs the "-X is reserved for implementation-specific arguments" message into stderr.

Attached PR adds PYTHONSHOWREFCOUNT.

[vstinner]

Since this issue is on the thin border between "feature" and "bug fix", I would like to get the feedback of Benjamin on this change.

@benjamin: Are you ok to make such change late in the Python 2.7 cycle, in Python 2.7.15?

[vstinner]

New changeset 3c082a7 by Victor Stinner in branch '2.7':
bpo-31733: Add PYTHONSHOWREFCOUNT env var (GH-3932)
3c082a7

[vstinner]

Python 2.7.15 now requires to set PYTHONSHOWREFCOUNT env var to dump "[xxx refs]".

[vstinner]

New changeset 355393e by Victor Stinner in branch '2.7':
[2.7] bpo-31733, bpo-31692: Document 2 new env vars in What's New in Python 2.7 (GH-4019)
355393e