[issue42564] "from .__init__ import ..." syntax imports a duplicate module

[Gregory Szorc]

Gregory Szorc  added the comment:

Who uses this syntax? 
https://github.com/search?l=Python&q=%22from+.__init__+import%22&type=Code says 
a lot of random code, surprisingly/sadly.

As for python-ideas, thanks for the suggestion: I may very well craft an email!

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[issue42564] "from .__init__ import ..." syntax imports a duplicate module

[Brett Cannon]

Brett Cannon  added the comment:

You could propose your backwards-incompatible proposals on python-ideas, Greg, 
and see if you get any uptake.

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[issue42564] "from .__init__ import ..." syntax imports a duplicate module

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

What is the problem? What real code imports __init__?

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[issue42564] "from .__init__ import ..." syntax imports a duplicate module

[Gregory Szorc]

Gregory Szorc  added the comment:

I worked around this in PyOxidizer by stripping a trailing `.__init__` from 
module names when resolving the indexed resource data. This allows the import 
to work since it can find the data now, but it also preserves the double module 
object, which isn't ideal IMO.

My preferred solution would be to either ban `__init__` in module name 
components or strip trailing `.__init__` from the name in `find_spec()`, 
effectively normalizing it away. Either of these would be backwards 
incompatible. Could either of these be considered for 3.10?

It's worth noting that `__init__` could potentially occur in the interior of 
the module name. e.g. `foo.__init__.bar`. This would require filenames like 
`foo/__init__/bar.py`. I wouldn't be surprised if this exists somewhere in the 
wild.

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[issue42564] "from .__init__ import ..." syntax imports a duplicate module

[Brett Cannon]

Brett Cannon  added the comment:

I agree with Serhiy; don't do this. The only way we could fix this would be to 
always set a `__init__` module for every package implicitly, but then that 
would break anyone who wanted to clear out a package in sys.modules as the 
`__init__` reference in sys.modules becomes a dangling reference.

--
resolution:  -> wont fix
stage:  -> resolved
status: open -> closed

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[issue42564] "from .__init__ import ..." syntax imports a duplicate module

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

Other example is:

>>> import sys
>>> import xml
>>> import xml.__init__
>>> sys.modules['xml']

>>> sys.modules['xml.__init__']

>>> sys.modules['xml'] is sys.modules['xml.__init__']
False

I'm not sure we should do anything about it other than say "Don't do this."

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nosy: +brett.cannon, eric.snow, ncoghlan, serhiy.storchaka

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[issue42564] "from .__init__ import ..." syntax imports a duplicate module

[Gregory Szorc]

New submission from Gregory Szorc :

(Rereporting from https://github.com/indygreg/PyOxidizer/issues/317.)

$ mkdir foo
$ cat > foo/__init__.py < test = True
> EOF
$ cat > foo/bar.py < from .__init__ import test
> EOF
$ python3.9
Python 3.9.0 (default, Nov  1 2020, 22:40:00)
[GCC 10.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import foo.bar
>>> import sys
>>> sys.modules['foo']

>>> sys.modules['foo.__init__']


I am surprised that `from .__init__` even works, as `__init__` isn't a valid 
module name.

What appears to be happening is the path based importer doesn't recognize the 
`__init__` as special and it falls back to its regular file probing technique 
to locate a module derive from the path. It finds the `__init__.py[c]` file and 
imports it.

A consequence of this is that the explicit `__init__` import/module exists as a 
separate module object under `sys.modules`. So you can effectively have the 
same file imported as 2 module objects living under 2 names. This could of 
course result in subtle software bugs, like module-level variables not updating 
when you expect them to. (This could also be a feature for code relying on this 
behavior, of course.)

I only attempted to reproduce with 3.9. But this behavior has likely existed 
for years.

--
components: Interpreter Core
messages: 382464
nosy: indygreg
priority: normal
severity: normal
status: open
title: "from .__init__ import ..." syntax imports a duplicate module
type: behavior
versions: Python 3.9

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