Add PYTHONDEVMODE=1 to enable the developer mode

[vstinner]

BPO	32101
Nosy	@ncoghlan, @pitrou, @vstinner
PRs	bpo-32043: Rephrase -X dev documentation #4478 bpo-32101: Add PYTHONDEVMODE environment variable #4624 bpo-32101: Fix tests for PYTHONDEVMODE=1 #4821

^{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 2017-11-30.10:49:45.114>
created_at = <Date 2017-11-21.02:58:57.806>
labels = ['type-feature', '3.7']
title = 'Add PYTHONDEVMODE=1 to enable the developer mode'
updated_at = <Date 2017-12-12.22:15:02.111>
user = 'https://github.com/vstinner'

bugs.python.org fields:

activity = <Date 2017-12-12.22:15:02.111>
actor = 'vstinner'
assignee = 'none'
closed = True
closed_date = <Date 2017-11-30.10:49:45.114>
closer = 'vstinner'
components = []
creation = <Date 2017-11-21.02:58:57.806>
creator = 'vstinner'
dependencies = []
files = []
hgrepos = []
issue_num = 32101
keywords = ['patch']
message_count = 6.0
messages = ['306618', '306625', '307195', '307284', '307286', '308162']
nosy_count = 3.0
nosy_names = ['ncoghlan', 'pitrou', 'vstinner']
pr_nums = ['4478', '4624', '4821']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue32101'
versions = ['Python 3.7']

[vstinner]

In the review of the PR 4478, Antoine Pitrou proposed to add an environment variable to enable the new Python "developer mode" to inherit the developer mode in child Python processes.

I proposed to add PYTHONDEVMODE=1. Nick Coghlan proposed PYTHONXOPTIONS=dev.

While -X is a new "catch all" for new CPython options, Python got new PYTHONxxx environment variables to enable new options:

• -X faulthandler: PYTHONFAULTHANDLER=1
• -X tracemalloc=N: PYTHONTRACEMALLOC=n
• -X importtime: PYTHONPROFILEIMPORTTIME=1 -- note that the names are different in this case, I don't know why

I have a preference for PYTHONDEVMODE=1.

If we start to use PYTHONXOPTIONS, users may want to use PYTHONXOPTIONS=tracemalloc, and then we have to decide which environment variable has the preference :-(

Moreover, tracemalloc takes an argument, whereas environment variables with a value containing '=' are rare and so likely to cause issues in programs not carefully written to handle such variables. (Environment variables are inherited by all processes, not only Python processes.)

[ncoghlan]

Another argument against PYTHONXOPTIONS is that each implementation is free to decide which -X options it wants to support, and we don't really define what they're supposed to do with options they don't recognise.

By contrast, there's an established behaviour for unknown environment variables: implementations that don't support them just ignore them.

So +1 for PYTHONDEVMODE=1 from me - that also gives us more flexibility if we decide to add arguments to the option later.

[vstinner]

While implementing a new PYTHONDEVMODE environment variable, I found that it would help to also add a new sys.flags.dev_mode flag. So I added it.

So for example, asyncio uses sys.flags.dev_mode rather than checking for 'dev' in sys._xoptions *and* check if the environment variable is set or not.

[vstinner]

New changeset 5e3806f by Victor Stinner in branch 'master':
bpo-32101: Add PYTHONDEVMODE environment variable (bpo-4624)
5e3806f

[vstinner]

Thanks Antoine and Nick for reviews and the feedback!

[vstinner]

New changeset 721e25c by Victor Stinner in branch 'master':
bpo-32101: Fix tests for PYTHONDEVMODE=1 (bpo-4821)
721e25c