[issue17953] sys.modules cannot be reassigned

[Eric Snow]

Eric Snow added the comment:

For the record, issue12633 has some more discussion on this.

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nosy: +eric.snow

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[issue17953] sys.modules cannot be reassigned

[Yogesh Chaudhari]

Yogesh Chaudhari added the comment:

That sounds good. I will open up an issue (if not for anything else other than 
to get more eyes on this issue)

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[issue17953] sys.modules cannot be reassigned

[R. David Murray]

R. David Murray added the comment:

I think that is a reasonable suggestion for an enhancement to patchcheck 
(though I think Brett had a good reason to add a news entry in this particular 
case).  You could open a new issue with that suggestion.  On the other hand, 
patchcheck always warns about other stuff that it can't so easily realize 
aren't really needed, so it may not be considered a worthwhile enhancement.

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[issue17953] sys.modules cannot be reassigned

[Yogesh Chaudhari]

Yogesh Chaudhari added the comment:

You are most welcome Brett. I am just starting to contribute with this doc fix. 
Didn't think a small detail in it will generate such a long debate in the 
python-committers mailing list. 

Is there a plan in action for 'fixing' Misc/NEWS. I now realize that this 
change is too small to warrant an entry there, however, as someone new to the 
list, I simply followed http://docs.python.org/devguide/patch.html. Doing a 
make patchcheck warns about no changes to Misc/NEWS. 

IMHO it would be great if this tool can look at the files changed (or maybe 
at-least just the directory, or even, simply number of lines modified) and 
decide if an entry is required in Misc/News. Of-course this is from my 
perspective. I would like to get your view on this and help about the solution 
if possible.

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[issue17953] sys.modules cannot be reassigned

[Brett Cannon]

Brett Cannon added the comment:

Thanks for the patch, Yogesh, but I went with different wording that I came up 
with on the train the other day while offline. But you did remind me to update 
Misc/NEWS (which led to a discussion on python-committers), so thanks for that.

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resolution:  -> fixed
status: open -> closed

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[issue17953] sys.modules cannot be reassigned

[Roundup Robot]

Roundup Robot added the comment:

New changeset 4f8160e45cb7 by Brett Cannon in branch '3.3':
Issue #17953: document that sys.modules shouldn't be replaced (thanks
http://hg.python.org/cpython/rev/4f8160e45cb7

New changeset b60ae4ebf981 by Brett Cannon in branch 'default':
merge fix for issue #17953
http://hg.python.org/cpython/rev/b60ae4ebf981

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[issue17953] sys.modules cannot be reassigned

[Yogesh Chaudhari]

Yogesh Chaudhari added the comment:

Documentation added for sys.modules

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keywords: +patch
nosy: +Yogesh.Chaudhari
Added file: http://bugs.python.org/file30305/issue17953.patch

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[issue17953] sys.modules cannot be reassigned

[Brett Cannon]

Brett Cannon added the comment:

Sounds good to me.

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[issue17953] sys.modules cannot be reassigned

[Terry J. Reedy]

Terry J. Reedy added the comment:

"sys.modules
This is a dictionary that maps module names to modules which have already 
been loaded. This can be manipulated to force reloading of modules and other 
tricks."

How about adding at the end
"However, replacing or clearing it may cause Python to fail."

or to be slightly more explicit "However, replacing it or deleting critical 
entries may cause Python to fail."

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nosy: +terry.reedy
stage: test needed -> needs patch

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[issue17953] sys.modules cannot be reassigned

[Brett Cannon]

Brett Cannon added the comment:

There is no good way to solve this. At the C level there interpreter struct has 
two key fields, sysdict and modules. The former is sys.__dict__ and the latter 
is sys.modules. But when you re-assign sys.modules you then break the 
assumption that sys.modules is the same dict as that contained in 
interp->modules. And this all goes out the window as the C code is expected to 
use interp->modules while the Python code in importlib only has access to 
sys.modules. The reason this used to "work" is your new dictionary was 
completely ignored and so we basically a no-op from the perspective of import 
(done in Python 2.7 but same result in any version up to Python 3.3)::

  >>> import sys
  >>> original_modules = sys.modules
  >>> new_modules = sys.modules.copy()
  >>> sys.modules = new_modules
  >>> import pkg
  >>> 'pkg' in original_modules
  True
  >>> 'pkg' in new_modules
  False

What really needs to happen is that sys.modules needs to be documented as 
something that cannot be replaced. If you really want to update it cleanly then 
do ``sys.modules.clear(); sys.modules.update(new_modules)``, but even that is 
tricky because removing certain modules will flat-out break Python.

I have updated the issue to be a documentation one and added Python 3.4 to the 
affected versions.

--
components: +Documentation -Interpreter Core
keywords: +easy
stage:  -> test needed
type: behavior -> 
versions: +Python 3.4

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[issue17953] sys.modules cannot be reassigned

[Antoine Pitrou]

Antoine Pitrou added the comment:

I wouldn't call it a bug personally. The modules dictionary is used in all 
kinds of places in the interpreter; you can change the dictionary's contents, 
but not swap it with another one.

It's just a pity that we can't forbid reassignment altogether.

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nosy: +pitrou

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[issue17953] sys.modules cannot be reassigned

[Christian Heimes]

Changes by Christian Heimes :


--
assignee:  -> brett.cannon
keywords: +3.3regression
nosy: +brett.cannon

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[issue17953] sys.modules cannot be reassigned

[ProgVal]

New submission from ProgVal:

In Python 3.3 (I did not test with 3.4), sys.modules cannot be reassigned 
without breaking the import mechanism; while it works with Python <= 3.2.

Here is how to reproduce the bug:

progval@Andromede:/tmp$ mkdir test_imports
progval@Andromede:/tmp$ echo "from . import module" > test_imports/__init__.py
progval@Andromede:/tmp$ echo "print('foo')" > test_imports/module.py
progval@Andromede:/tmp$ python3.3
Python 3.3.1 (default, Apr  6 2013, 13:58:40) 
[GCC 4.7.2] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import sys
>>> sys.modules = dict(sys.modules)
>>> import test_imports
Traceback (most recent call last):
  File "", line 1, in 
  File "./test_imports/__init__.py", line 1, in 
from . import module
SystemError: Parent module 'test_imports' not loaded, cannot perform relative 
import
>>> 
progval@Andromede:/tmp$ python3.2
Python 3.2.3 (default, May  6 2013, 01:46:35) 
[GCC 4.7.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import sys
>>> sys.modules = dict(sys.modules)
>>> import test_imports
foo
>>>

--
components: Interpreter Core
messages: 188901
nosy: Valentin.Lorentz
priority: normal
severity: normal
status: open
title: sys.modules cannot be reassigned
type: behavior
versions: Python 3.3

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