[issue18018] SystemError: Parent module '' not loaded, cannot perform relative import

[Serhiy Storchaka]

Serhiy Storchaka added the comment:


New changeset c824cc8426a16dd9f3949a3ed135523d37787bae by Serhiy Storchaka in 
branch '3.5':
[3.5] Backport bpo-30876 (GH-2639), bpo-18018 and bpo-26367. (#2677)
https://github.com/python/cpython/commit/c824cc8426a16dd9f3949a3ed135523d37787bae


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[issue18018] SystemError: Parent module '' not loaded, cannot perform relative import

[Brett Cannon]

Brett Cannon added the comment:

Thanks to everyone for providing feedback. I went with ImportError in the end 
as that's what the pure Python implementation of __import__() already raised.

--
resolution:  -> fixed
stage: test needed -> resolved
status: open -> closed

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[issue18018] SystemError: Parent module '' not loaded, cannot perform relative import

[Roundup Robot]

Roundup Robot added the comment:

New changeset c4e4886c6052 by Brett Cannon in branch 'default':
Issue #18018: Raise an ImportError if a relative import is attempted
https://hg.python.org/cpython/rev/c4e4886c6052

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[issue18018] SystemError: Parent module '' not loaded, cannot perform relative import

[Chris Angelico]

Chris Angelico added the comment:

If someone made a new way of importing and had it raise ValueError on some 
issue or other, I think there'd be complete consensus that that's the wrong 
exception. Yes, this is incompatible with Python 2 - but there are a lot of 
corner cases in the 3.3+ import code that differ from 2.7. What we have is a 
code error ("relative imports outside of packages don't make sense, dummy!") 
and the only difference is which exception is being raised (ValueError, 
SystemError, ImportError). It's not like working code has different semantics.

But even if ValueError is chosen, I would still greatly prefer that to 
SystemError. When my student told me he was getting SystemError, I started 
trying to diagnose a corrupted venv, not a buggy script.

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[issue18018] SystemError: Parent module '' not loaded, cannot perform relative import

[Brett Cannon]

Brett Cannon added the comment:

The problem is in Python/import.c: 
https://hg.python.org/cpython/file/default/Python/import.c#l1453 . While the 
Python code has a check that `package` is a truthy value, the accelerated C 
code doesn't. Adding a check that the string isn't empty or truthy should do 
the trick as there seems to already be a check for None earlier.

The other question is whether this should change back to ValueError for Python 
2 compatibility or turn into ImportError as Chris and Eric Snow have 
independently suggested?

Either way I'm only going to change this in Python 3.6 since the current 
semantics have been this way since Python 3.3, so it's beyond the realm of 
regression and into fixing a design mistake.

--
stage:  -> test needed
versions:  -Python 3.3, Python 3.4, Python 3.5

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[issue18018] SystemError: Parent module '' not loaded, cannot perform relative import

[Yongzhi Pan]

Changes by Yongzhi Pan :


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nosy: +fossilet

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[issue18018] SystemError: Parent module '' not loaded, cannot perform relative import

[Brett Cannon]

Changes by Brett Cannon :


--
assignee: flox -> brett.cannon

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[issue18018] SystemError: Parent module '' not loaded, cannot perform relative import

[Wil D'Amato]

Changes by Wil D'Amato :


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[issue18018] SystemError: Parent module '' not loaded, cannot perform relative import

[Chris Angelico]

Chris Angelico added the comment:

This still happens in current CPython, and doesn't even require a package:

rosuav@sikorsky:~$ echo 'from . import x' >syserr.py
rosuav@sikorsky:~$ python3 syserr.py
Traceback (most recent call last):
  File "syserr.py", line 1, in 
from . import x
SystemError: Parent module '' not loaded, cannot perform relative import


This just caused some confusion for a student of mine who had migrated from 
Python 2. Converting intra-package imports from "import spam" to "from . import 
spam" made them work; converting a non-package import the same way caused this 
problem. ImportError would be massively preferable.

But I'm not understanding something here. If I set __package__ inside the 
module, the behaviour changes accordingly:

rosuav@sikorsky:~$ cat syserr.py 
__package__ = 'sys'
from . import version
print(version)
rosuav@sikorsky:~$ python3 syserr.py 
3.6.0a0 (default:ac94418299bd+, Jan 15 2016, 08:44:02) 
[GCC 4.9.2]


Leaving __package__ unset has it implicitly be None. Whether it's None or '', 
the package checks should simply not be made - see three lines above the line 
Brett linked to (in today's code, that's 
https://hg.python.org/cpython/file/ac94418299b/Lib/importlib/_bootstrap.py#l925 
- hasn't changed). So I'm not sure why the checks are even happening. However, 
that probably means more about my exploration and testing than it does about 
the code; editing the text of the message doesn't result in a change, so I'm 
obviously not updating the frozen version.

--
nosy: +Rosuav
versions: +Python 3.5, Python 3.6

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[issue18018] SystemError: Parent module '' not loaded, cannot perform relative import

[Antoine Pitrou]

Antoine Pitrou added the comment:

User code should generally not see any SystemErrors (even when choosing wacky 
module names :-)), so IMHO this is a regression.

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[issue18018] SystemError: Parent module '' not loaded, cannot perform relative import

[Brett Cannon]

Brett Cannon added the comment:

The line raising the exception is 
http://hg.python.org/cpython/file/f7992397e98d/Lib/importlib/_bootstrap.py#l1518
 .

If you're sure it's a regression feel free to change what exception is raised.

--
assignee:  -> flox

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[issue18018] SystemError: Parent module '' not loaded, cannot perform relative import

[Arfrever Frehtes Taifersar Arahesis]

Changes by Arfrever Frehtes Taifersar Arahesis :


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[issue18018] SystemError: Parent module '' not loaded, cannot perform relative import

[Nick Coghlan]

Nick Coghlan added the comment:

For the specific case given, both error messages are misleading anyway - the 
problem is attempting to directly execute a module inside a package instead of 
using "-m".

However, for the case of attempting a relative import from a top level module, 
the old message is indeed preferable.

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[issue18018] SystemError: Parent module '' not loaded, cannot perform relative import

[Florent Xicluna]

New submission from Florent Xicluna:

When executing a submodule, there's a SystemError in 3.3 where we used to 
receive a ValueError.

mkdir marsu
touch marsu/__init__.py
echo "from .houba import bi" >> marsu/pilami.py

./python marsu/pilami.py

Traceback (most recent call last):
  File "marsu/pilami.py", line 2, in 
from .houba import bi
SystemError: Parent module '' not loaded, cannot perform relative import


In Python 3.2 (or Python 2.7):

./python3.2 marsu/pilami.py

Traceback (most recent call last):
  File "marsu/pilami.py", line 2, in 
from .houba import bi
ValueError: Attempted relative import in non-package

--
components: Interpreter Core
keywords: 3.3regression
messages: 189630
nosy: brett.cannon, eric.snow, flox, ncoghlan
priority: normal
severity: normal
status: open
title: SystemError: Parent module '' not loaded, cannot perform relative import
type: behavior
versions: Python 3.3, Python 3.4

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