[issue36375] PEP 499 implementation: "python -m foo" binds the main module as both __main__ and foo in sys.modules

[Nick Coghlan]

Nick Coghlan  added the comment:

Leaving the relationship between pickle and __name__ alone wasn't an oversight, 
as folks already rely on that to gracefully transition from single-file modules 
to multi-file packages without breaking pickle compatibility in either 
direction. The trick is to do a "from ._submodule import *" in the package's 
__init__.py (exposing all the names), and then a "__name__ = __package__" in 
the submodule itself.

Thus the second snippet above, as a way to port code that was specifically 
relying on the double import to provide pickle compatibility without risking 
introducing other unintended incompatibility problems.

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[issue36375] PEP 499 implementation: "python -m foo" binds the main module as both __main__ and foo in sys.modules

[Eric Snow]

Eric Snow  added the comment:

Exactly. :)

I'd expect PEP 499 to specify changing __module__ of classes and functions from 
__main__ to the module name (__spec__.name).  This aligns closely with the 
whole point of the PEP. :)  As a bonus, it will simplify things for pickling 
(which doesn't inherently deal well with __main__).

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[issue36375] PEP 499 implementation: "python -m foo" binds the main module as both __main__ and foo in sys.modules

[Cameron Simpson]

Cameron Simpson  added the comment:

On 25Nov2019 17:38, Python Bug Reports  wrote:
>Eric Snow  added the comment:
>
>FWIW, I have some feedback on the PEP.  (See msg357448.)  Can we discuss here 
>or should I open a mailing list thread?

Let's discuss it here unless it looks like we need wider input. This is 
related to issue37292 (_xxsubinterpreters: Can't unpickle objects 
defined in __main__), yes?

With PEP499, "python -m foo" should have __name__=='__main__' and 
__spec__.name=='foo'. Were you thinking the __module__ should come from 
__spec__.name (and that the PEP should make that clear)?

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[issue36375] PEP 499 implementation: "python -m foo" binds the main module as both __main__ and foo in sys.modules

[Eric Snow]

Eric Snow  added the comment:

FWIW, I have some feedback on the PEP.  (See msg357448.)  Can we discuss here 
or should I open a mailing list thread?

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[issue36375] PEP 499 implementation: "python -m foo" binds the main module as both __main__ and foo in sys.modules

[Cameron Simpson]

Cameron Simpson  added the comment:

Just a remark about the preamble comment: it reads to me as though PEP499 is a 
misfeature. Possibly change:

  We actually want the double import, so remove the alias if it exists.

to:

  This module is unusual, and actually wants the double import.
  Therefore we remove the __spec__.name alias if it exists
  so that a subsequent import by name will make a distinct instance.

Verbose, but at least I know the intent.

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[issue36375] PEP 499 implementation: "python -m foo" binds the main module as both __main__ and foo in sys.modules

[Cameron Simpson]

Cameron Simpson  added the comment:

I want to start with an apology. I have become a little swamped by work and 
didn't let anyone know I've made little time for this since March.

To my naive eye Nick's snippet looks like it would work for pdb; I became a 
little stalled there trying to understand how pdb managed the main module.

I totally lack the expertise to address pickle; I've never used it and the long 
history of feature updates sugests to me that I cannot get a deep enough 
understanding in a meaningful time.

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[issue36375] PEP 499 implementation: "python -m foo" binds the main module as both __main__ and foo in sys.modules

[Nick Coghlan]

Nick Coghlan  added the comment:

[Belatedly updating this issue with the current status as of March]

Cameron's implementation generally looks good, but there are couple of 
compatibility/migration questions that we need to consider, as spelled out in 
the PEP update that added me as BDFL-Delegate: 
https://github.com/python/peps/pull/946/files

* We need a generic porting guide entry to handle projects that turn out to 
have been relying on their name *not* being bound in sys.modules. For example, 
adding this preamble:

if __name__ == "__main__":
# To prevent inadvertent double imports, the -m
# switch in Python 3.9+ defaults to aliasing __main__
# under the executed module's import name. We actually
# want the double import, so remove the alias if it exists
import sys
_main_module = sys.modules.get(__name__)
_spec_module = sys.modules.get(__spec__.name)
if _main_module is _spec_module:
sys.modules.pop(__spec__.name)

We'd test the above snippet by adding it to the `pdb` module (and reverting the 
other compatibility changes to that module)

* We need to understand the implications for pickle compatibility, and provide 
a porting guide snippet, similar to the one above for explicitly requesting the 
double-import behaviour. For example:

if __name__ == "__main__":
# To prevent inadvertent double imports, the -m
# switch in Python 3.9+ defaults to aliasing __main__
# under the executed module's import name. We need
# pickle to use the real module name for objects from
# __main__ though, so we set the import name here
_running_as_main = True
__name__ = __spec__.name

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[issue36375] PEP 499 implementation: "python -m foo" binds the main module as both __main__ and foo in sys.modules

[Greg Price]

Change by Greg Price :


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nosy: +Greg Price

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[issue36375] PEP 499 implementation: "python -m foo" binds the main module as both __main__ and foo in sys.modules

[Cameron Simpson]

Cameron Simpson  added the comment:

New PR 12490 attached with a fix for test_pdb. More extensive comments are in 
the leading comment on the PR itself. - Cameron

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[issue36375] PEP 499 implementation: "python -m foo" binds the main module as both __main__ and foo in sys.modules

[Cameron Simpson]

Change by Cameron Simpson :


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pull_requests: +12442

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[issue36375] PEP 499 implementation: "python -m foo" binds the main module as both __main__ and foo in sys.modules

[Cameron Simpson]

Cameron Simpson  added the comment:

I've withdrawn the PR; I hadn't run the full test suite and there are things to 
fix. - Cameron

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[issue36375] PEP 499 implementation: "python -m foo" binds the main module as both __main__ and foo in sys.modules

[Cameron Simpson]

Change by Cameron Simpson :


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keywords: +patch
pull_requests: +12408
stage:  -> patch review

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[issue36375] PEP 499 implementation: "python -m foo" binds the main module as both __main__ and foo in sys.modules

[Cameron Simpson]

New submission from Cameron Simpson :

This issue is to track the implementation of PEP 499 which we hope to get into 
the upcoming 3.8 release. I've made it because cpython PRs want a bpo number in 
the subject line.

I'll link in the proposed PR once I've filed off a few grammar issues I've just 
noticed (documentation English grammar).

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components: Interpreter Core
messages: 338425
nosy: cameron, ncoghlan
priority: normal
severity: normal
status: open
title: PEP 499 implementation: "python -m foo" binds the main module as both 
__main__ and foo in sys.modules
type: enhancement
versions: Python 3.8

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