Importing the main module twice leads to two incompatible instances

[a87580a0-fa61-4695-a31b-f532ecf7185b]

BPO	27607
Nosy	@rhettinger, @cameron-simpson, @bitdancer, @vadmium, @Vgr255, @TheLastProject, @Kwpolska
Files	issue27607.zip: Example test case

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GitHub fields: ```python assignee = None closed_at = None created_at = labels = ['interpreter-core', 'type-bug', '3.11'] title = 'Importing the main module twice leads to two incompatible instances' updated_at = user = 'https://github.com/TheLastProject' ``` bugs.python.org fields: ```python activity = actor = 'iritkatriel' assignee = 'none' closed = False closed_date = None closer = None components = ['Interpreter Core'] creation = creator = 'SylvieLorxu' dependencies = [] files = ['43877'] hgrepos = [] issue_num = 27607 keywords = [] message_count = 14.0 messages = ['271164', '271184', '271185', '271188', '271189', '271195', '271196', '271210', '271211', '271217', '271243', '271248', '271253', '271264'] nosy_count = 7.0 nosy_names = ['rhettinger', 'cameron', 'r.david.murray', 'martin.panter', 'abarry', 'SylvieLorxu', 'Kwpolska'] pr_nums = [] priority = 'normal' resolution = None stage = None status = 'open' superseder = None type = 'behavior' url = 'https://bugs.python.org/issue27607' versions = ['Python 3.11'] ```

[a87580a0-fa61-4695-a31b-f532ecf7185b]

If main imports another file, and this file imports an Enum class from main, isinstance will return false for Enum variables set in this file, causing the Enums to never match.

A proof of concept is added as a zip. I thank Kwpolska for simplifying my initial proof of concept.

To test this, unzip both files into the same directory and launch main.py with Python 3. I have succesfully been able to reproduce this issue on Python 3.4.3.

From discussion on IRC with Vgr, PEP-499 may be related to this issue: https://www.python.org/dev/peps/pep-0499/.

[bitdancer]

This is a consequence of the way __main__ works. You actually have two separate instances of the Enum class. If there is a fix for this it would be that PEP, so I'm not sure there is any point in keeping this issue open.

[a87580a0-fa61-4695-a31b-f532ecf7185b]

For what it's worth, I am not opposed to having this issue closed. As I am new to reporting anything on the Python issue tracker and was uncertain on how relevant the PEP in question was, I figured it would be better to report this anyway just in case.

[bitdancer]

Certainly reporting it is fine, if nothing else it gives a record for someone else searching for the same problem. I'm not certain why Raymond left the issue open, so I'm not closing it myself :)

[rhettinger]

I left it open so that Ethan would get a chance to see anything related to Enum.

[vadmium]

I would expect isintance() to fail like this for any class, not just Enum. I haven’t looked in your zip file, but a workaround may be to import the Enum class from a third module into both main.py and the other file. The third module should only get executed once, thus only defining one copy of the Enum class.

[a87580a0-fa61-4695-a31b-f532ecf7185b]

Importing it from a third module indeed works without issues and makes both isinstance() calls return True, yes.

[ethanfurman]

Sylvia, if you have any control over the name of your zip files, please use something shorter. ;)

The files inside are a 'main.py' and a 'module.py' -- notice there is no '__main__.py'. So it can't be executed as 'python3 -m blahblah'.

I tried renaming 'main.py' to '__main__.py' but then I get either

ImportError: cannot import name 'Module'

or

ImportError: No module named 'module'

I'm happy to check for strange Enum interactions, but I need a working test case first.

[ethanfurman]

Oh, forgot to mention I also tried testing as just 'main.py' from inside the test directory (that's what accounted for one of the two error messages).

[cd293b3e-6d38-412b-8370-a46a9aaee518]

I'm fairly sure enums aren't related to this issue, so I'm unassigning Ethan (but feel free to self-assign back if you so desire :). As I said on IRC (I'm Vgr), I think that importing a module twice is the problem here, and that PEP-499 is the way to properly solve this and related issues. Thanks for opening this issue!

I added Cameron Simpson (PEP-499's author) to the nosy list, hoping he still wants to work on this (and let us know otherwise). I would personally like to see the PEP accepted and implemented for 3.6.

[92259bf3-4b4c-47c1-a468-08a70c584615]

On 25Jul2016 02:56, Python Bug Reports \report@bugs.python.org\ wrote:

Emanuel Barry added the comment:

I'm fairly sure enums aren't related to this issue, so I'm unassigning Ethan (but feel free to self-assign back if you so desire :). As I said on IRC (I'm Vgr), I think that importing a module twice is the problem here, and that PEP-499 is the way to properly solve this and related issues. Thanks for opening this issue!

I added Cameron Simpson (PEP-499's author) to the nosy list, hoping he still wants to work on this (and let us know otherwise). I would personally like to see the PEP accepted and implemented for 3.6.

Thank you; I do want to finish off PEP-499 and get it accepted. I have been very remiss in pursuing that.

What can I do to assist here?

Cheers, Cameron Simpson \cs@zip.com.au\

[a87580a0-fa61-4695-a31b-f532ecf7185b]

My apologies, I can't get that test case to run here either. Here is a slightly older one that does reproduce the issue too.

I specifically run it with python3 main.py.

[cd293b3e-6d38-412b-8370-a46a9aaee518]

@Cameron - I don't think this issue can be solved without the PEP, so possibly that the best move would be to work on the PEP and submit it for another round on Python-Dev, to get it reviewed. Thanks!

[bitdancer]

If you have any sort of implementation of the pep, proving that it does fix this issue would be informative.