[issue23203] Aliasing import of sub-{module, package} from the package raises AttributeError on import.

[Nick Coghlan]

Nick Coghlan  added the comment:

Heh, apparently I forgot how IMPORT_FROM currently works some time between 2015 
and 2017 :)

I agree this is out of date now, as the requested behaviour was already 
implemented for 3.7

--
resolution:  -> out of date
stage:  -> resolved
status: open -> closed
superseder:  -> Treat `import a.b.c as m` as `m = sys.modules['a.b.c']`

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[issue23203] Aliasing import of sub-{module, package} from the package raises AttributeError on import.

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

Is anything left to do with this issue after issue30024?

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[issue23203] Aliasing import of sub-{module, package} from the package raises AttributeError on import.

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

Nick's suggestion was implemented in issue30024.

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nosy: +serhiy.storchaka

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[issue23203] Aliasing import of sub-{module, package} from the package raises AttributeError on import.

[Nick Coghlan]

Nick Coghlan added the comment:

I'm suggesting we change this part of the bytecode emitted for import x.y.z as 
bar:

  6 IMPORT_NAME  0 (x.y.z)
  9 LOAD_ATTR1 (y)
 12 LOAD_ATTR2 (z)
 15 STORE_NAME   3 (bar)

to instead emit:

  6 IMPORT_NAME  0 (x.y)
  9 IMPORT_FROM  1 (z)
 12 STORE_NAME   2 (bar)
 15 POP_TOP

The degenerate case of import x as y would be unchanged, only cases which 
currently emit LOAD_ATTR instructions would be modified.

I haven't looked at the practical details yet, but the key would be to use 
IMPORT_FROM to do the name resolution, rather than LOAD_ATTR, thus enabling the 
fallback to sys.modules for the eager lookup.

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[issue23203] Aliasing import of sub-{module, package} from the package raises AttributeError on import.

[Brett Cannon]

Brett Cannon added the comment:

That seems reasonable. I guess first step is a patch and then seeing if it 
passes the test suite.

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[issue23203] Aliasing import of sub-{module, package} from the package raises AttributeError on import.

[Arfrever Frehtes Taifersar Arahesis]

Changes by Arfrever Frehtes Taifersar Arahesis arfrever@gmail.com:


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[issue23203] Aliasing import of sub-{module, package} from the package raises AttributeError on import.

[Brett Cannon]

Brett Cannon added the comment:

I think I would need to see exactly how you want the bytecode to change and 
then think over any backwards-compatibility issues since this is doesn't come 
up very often.

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[issue23203] Aliasing import of sub-{module, package} from the package raises AttributeError on import.

[Nick Coghlan]

Nick Coghlan added the comment:

You can see the difference between the two cases in the bytecode:

 dis.dis(import x.y.z)
  1   0 LOAD_CONST   0 (0)
  3 LOAD_CONST   1 (None)
  6 IMPORT_NAME  0 (x.y.z)
  9 STORE_NAME   1 (x)
 12 LOAD_CONST   1 (None)
 15 RETURN_VALUE 
 dis.dis(import x.y.z as bar)
  1   0 LOAD_CONST   0 (0)
  3 LOAD_CONST   1 (None)
  6 IMPORT_NAME  0 (x.y.z)
  9 LOAD_ATTR1 (y)
 12 LOAD_ATTR2 (z)
 15 STORE_NAME   3 (bar)
 18 LOAD_CONST   1 (None)
 21 RETURN_VALUE

The aliased version needs to bind the innermost object immediately, so it 
fails, since foo.bar doesn't get set until *after* the import is finished.

The version without the alias succeeeds, as it doesn't attempt to eagerly 
access the attribute before it gets set by the interpreter.

To better handle a similar situation with eager attribute lookups during 
import, issue 17636 changed IMPORT_FROM to fall back to looking at sys.modules 
when a module attribute it is looking for is missing.

Brett, Eric, perhaps it would be worth changing the bytecode emitted in the 
import x.y.z as name case to match that for from x.y import x as name?

 dis.dis(from x.y import z as bar)   
  1   0 LOAD_CONST   0 (0)
  3 LOAD_CONST   1 (('z',))
  6 IMPORT_NAME  0 (x.y)
  9 IMPORT_FROM  1 (z)
 12 STORE_NAME   2 (bar)
 15 POP_TOP
 16 LOAD_CONST   2 (None)
 19 RETURN_VALUE

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[issue23203] Aliasing import of sub-{module, package} from the package raises AttributeError on import.

[Ezio Melotti]

Changes by Ezio Melotti ezio.melo...@gmail.com:


--
nosy: +brett.cannon, eric.snow, ezio.melotti, ncoghlan
type:  - behavior

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[issue23203] Aliasing import of sub-{module, package} from the package raises AttributeError on import.

[Franck Michea]

New submission from Franck Michea:

Hi, for those of you that prefer to read an example, you can read that 
commented demonstration of the bug[1].

Today I discovered what I think is a bug in the import system. Here is the 
basic setup:

We have three nested packages: foo - bar - baz. The bar package imports 
foo.bar.baz. We try to import foo.bar. This works well unless we try to alias 
the foo.bar.baz import in foo.bar with the import ... as ... syntax. In that 
case the file will be read and executed but when we get out of the import 
statement, then python throws:

Traceback (most recent call last):
  File string, line 1, in module
  File foo_alias_mod/bar/__init__.py, line 1, in module
import foo_alias_mod.bar.baz as name_does_not_matter
AttributeError: 'module' object has no attribute 'bar'

This works whether baz is a package or a module actually. It does not matter if 
it's from the interpreter, or in a file, ... I've seen it trigger with 2.7.5, 
2.7.9, 3.4.5, tip, so I guess this has been here for some time.

Please read the link below for a complete demo, and you can always download the 
tarball[2] to test yourself.

[1]: Commented demonstration: http://98810f8c06.net/wtf_python.html
[2]: Tarball for test: http://98810f8c06.net/wtf_python-demo.tar.bz2

--
components: Interpreter Core
messages: 233719
nosy: franck
priority: normal
severity: normal
status: open
title: Aliasing import of sub-{module,package} from the package raises 
AttributeError on import.

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