Add a new -X dev option: "developer mode"

[vstinner]

BPO	32043
Nosy	@warsaw, @vstinner
PRs	bpo-32043: New "developer mode": "-X dev" option #4413 bpo-32043: Rephrase -X dev documentation #4478

^{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 2017-11-21.03:03:29.377>
created_at = <Date 2017-11-16.02:43:27.272>
labels = ['interpreter-core', 'type-feature', '3.7']
title = 'Add a new -X dev option: "developer mode"'
updated_at = <Date 2017-11-21.03:03:29.376>
user = 'https://github.com/vstinner'

bugs.python.org fields:

activity = <Date 2017-11-21.03:03:29.376>
actor = 'vstinner'
assignee = 'none'
closed = True
closed_date = <Date 2017-11-21.03:03:29.377>
closer = 'vstinner'
components = ['Interpreter Core']
creation = <Date 2017-11-16.02:43:27.272>
creator = 'vstinner'
dependencies = []
files = []
hgrepos = []
issue_num = 32043
keywords = ['patch']
message_count = 7.0
messages = ['306330', '306331', '306351', '306357', '306619', '306620', '306621']
nosy_count = 2.0
nosy_names = ['barry', 'vstinner']
pr_nums = ['4413', '4478']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue32043'
versions = ['Python 3.7']

[vstinner]

I propose to add a new "developer mode": a new "-X dev" command line option.

In short:
python3 -X dev script.py
behaves as:
PYTHONMALLOC=debug python3 -W default -X faulthandler script.py

For the rationale, please see:
https://mail.python.org/pipermail/python-dev/2017-November/150514.html

[vstinner]

I rewrote Py_Main() in bpo-32030 to make it very easy to implement this new feature.

I wrote PR 4413 to implement -X dev.

[vstinner]

New changeset ccb0442 by Victor Stinner in branch 'master':
bpo-32043: New "developer mode": "-X dev" option (bpo-4413)
ccb0442

[vstinner]

Antoine Pitrou asked me to document the performance and memory overhead of -X dev:

https://mail.python.org/pipermail/python-dev/2017-November/150578.html

For example, I measured an increase of +50% on the peak memory usage (measured by tracemalloc) when running test_os.

Antoine proposed:

"""Currently, developer mode adds negligible CPU time overhead, but can
increase memory consumption significantly if many small objects are
allocated. This is subject to change in the future."""

https://mail.python.org/pipermail/python-dev/2017-November/150579.html

[vstinner]

I created bpo-32101: Add PYTHONDEVMODE=1 to enable the developer mode.

[vstinner]

New changeset bc9b6e2 by Victor Stinner in branch 'master':
bpo-32043: Rephrase -X dev documentation (bpo-4478)
bc9b6e2

[vstinner]

While the discussion is not done, I close this issue to focus on other more specific issues.

I completed the documentation and fixed warnings filters (bpo-32089). The option now also enables asyncio debug mode (bpo-32047).

It was proposed to add an environment variable to enable the developer mode: see bpo-32101 ("Add PYTHONDEVMODE=1 to enable the developer mode").

Note: I also fixed subprocess._args_from_interpreter_flags: bpo-32094.