[issue46806] Overlapping PYTHONPATH may cause import already imported module

[aklajnert]

aklajnert  added the comment:

Honestly, it seems to me that the documents that you mentioned are discussing 
different problems, that may be related, but are not the same as the one I've 
described here.

I'm not arguing - you're clearly more experienced and that's not my area of 
expertise, but out of curiosity - can you mention some example use cases where 
the behavior I've described is useful?

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[issue46806] Overlapping PYTHONPATH may cause import already imported module

[Eric Snow]

Eric Snow  added the comment:

FYI, a technical solution has been discussed before: bpo-13475 and PEP 395.  
However, that does not help so much if the default behavior isn't changed.  
That would require a PEP but I expect it would be rejected because so many 
scripts already rely on the current behavior and the current behavior is useful 
in some cases.

PEP 395 also has a good discussion of the various pitfalls related to 
sys.path[0] initialization.  Furthermore, the topic is discussed in quite a few 
issues, such as bpo-44132 and bpo-29929.

Probably the best use of your time on this would be to improve the 
documentation so people will more easily avoid the problem, or at least more 
easily diagnose the situation when they stumble on it.  Again, PEP 395 is a 
good guide for this.

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[issue46806] Overlapping PYTHONPATH may cause import already imported module

[aklajnert]

aklajnert  added the comment:

I agree that adding a package directory to PYTHONPATH is not a good idea, 
however, I've stumbled on it in two completely independent companies' 
codebases. So it makes me suspect that this is may not be that rare case.

In the previous company we got bitten by this problem, and debugging took quite 
some time as this issue usually doesn't reveal immediately.

If the relative path is resolved to the same module as not relative, then the 
behavior when the same file's path but referenced in a slightly different way 
isn't seems at least inconsistent. Note that the absolute path to the module is 
exactly the same, the only thing that is different is how you reference it.

I'm happy to make an attempt to fix it if it gets acknowledged as a bug (a 
little guidance would be also helpful).

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status: pending -> open

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[issue46806] Overlapping PYTHONPATH may cause import already imported module

[Eric Snow]

Eric Snow  added the comment:

I'm leaving this "pending" in case there may be some improvement we can make to 
the documentation to address this.

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components: +Interpreter Core
nosy: +docs@python
status: open -> pending

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[issue46806] Overlapping PYTHONPATH may cause import already imported module

[Eric Snow]

Eric Snow  added the comment:

Here is more detail on what happens when "from src import user_1, user_2, 
user_3" is executed in main.py:

1. the "src" module is imported
   a. not found in sys.modules
   b. file found on a sys.path entry (the directory main.py is in)
   c. the "src" module is created with __path__ set to the src directory
   d. the module is added to sys.modules
   e. src/__init__.py is executed
2. the "src.user_1" module is imported
   a. not found in sys.modules
   b. file found relative to src.__path__
   c. module created
   d. added to sys.modules
   e. executed
3. "from .common_object import OBJECT" is resolved to "from src.common_object 
import OBJECT"
4. "src.common_object" is imported (see 2)
5. src.common_object.OBJECT is created
6. "src.user_2" is imported (see 2)
7. "src.common_object" is already found in sys.modules and used
8. "src.user_3" is imported (see 2)
9. "common_object" is imported
   a. not found in sys.modules
   b. file found on a sys.path entry (the one you added in main.py)
   c. module created
   d. added to sys.modules
   e. executed
10. common_object.OBJECT is created

So the module created at (4) is different than the one at (9), even though they 
are imported from the same file.  Consequently, the OBJECT in each is likewise 
distinct.

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[issue46806] Overlapping PYTHONPATH may cause import already imported module

[Eric Snow]

Eric Snow  added the comment:

When you run a Python script, the directory the script is in is automatically 
added to the beginning of sys.path.  This is the fundamental issue you've run 
into.

Basically, "src.common_object" is imported relative to the "src" that was 
imported relative to that automatically added sys.path entry.  However, 
"common_object" is a distinct module imported relative to the sys.path entry 
you explicitly added.

In general, adding a package's directory to sys.path is a bad idea.

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nosy: +eric.snow

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[issue46806] Overlapping PYTHONPATH may cause import already imported module

[aklajnert]

Change by aklajnert :


--
title: Overlapping PYTHONPATH may cause -> Overlapping PYTHONPATH may cause 
import already imported module

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