This recently caused a devastating bug in some of my code. What I have is support for the Perforce global options as a context for a perforce module. http://www.perforce.com/perforce/doc.072/manuals/cmdref/o.gopts.html#... This way one can call functions that call many perforce command and have them execute on a different client for example.
So, in module A and module B both imported the Perforce module, but they turned out not to be the same module. Module A did "with Perforce.GlobalOptions(client=client): B.function()"
B.function did not receive the new GlobalOptions because of this problem. As a result important files on the original client were overwritten (OUCH).
I would like to propose that it be made impossible in the Python source to import two instances of the same module.
-- Zachary Burns (407)590-4814 Aim - Zac256FL Production Engineer (Digital Overlord) Zindagi Games
> This recently caused a devastating bug in some of my code. What I have > is support for the Perforce global options as a context for a perforce > module. http://www.perforce.com/perforce/doc.072/manuals/cmdref/o.gopts.html#... > This way one can call functions that call many perforce command and > have them execute on a different client for example.
> So, in module A and module B both imported the Perforce module, but > they turned out not to be the same module. Module A did "with > Perforce.GlobalOptions(client=client): B.function()"
> B.function did not receive the new GlobalOptions because of this > problem. As a result important files on the original client were > overwritten (OUCH).
> I would like to propose that it be made impossible in the Python > source to import two instances of the same module.
> -- > Zachary Burns > (407)590-4814 > Aim - Zac256FL > Production Engineer (Digital Overlord) > Zindagi Games
I believe that modules are imported only once, and my toy example demonstrates that (python 2.5):
test_import.py: """testing multiple imports""" CONSTANT = 928 version = (2, 0, 9) plug_ins = [] random_text = 'some text here' def set_text(new_text): global random_text random_text = new_text plug_ins.append(new_text)
Running... In [1]: import A (2, 0, 9) ['hello!'] 928
In [2]: import B (2, 0, 9) ['hello!', 'world!'] changed!
As you can see, module A made a change to test_import.CONSTANT, and if they were different things then B would not have seen it, yet B *did* see it.
This makes me wonder if A) Perforce.GlobalOptions isn't actually setting module level variables, or B) B.function is using copies of those variables that were set when B was originally imported, so is not seeing the changes.... or C) there is yet another wrinkle here that I don't know about. ;-)
> This recently caused a devastating bug in some of my code. What I have > is support for the Perforce global options as a context for a perforce > module.http://www.perforce.com/perforce/doc.072/manuals/cmdref/o.gopts.html#... > This way one can call functions that call many perforce command and > have them execute on a different client for example.
> So, in module A and module B both imported the Perforce module, but > they turned out not to be the same module. Module A did "with > Perforce.GlobalOptions(client=client): B.function()"
> B.function did not receive the new GlobalOptions because of this > problem. As a result important files on the original client were > overwritten (OUCH).
> I would like to propose that it be made impossible in the Python > source to import two instances of the same module.
Impossible's a pretty strong word.
It's a reasonable request, but with Python's importing the way it is it'd be kind of hard to do. A Python file can be visible in multiple ways.
However, anyone who does "import __init__" (or "from . import __init__" with relative import) is asking for trouble, I can't think of any valid reason to do it, and I wouldn't mind seeing that forbidden, but it's simple to avoid. Someone probably did that because they didn't know how to import a containing package from one of its modules, failing to realize that it created a new module.
On Thu, Sep 24, 2009 at 1:38 PM, Carl Banks <pavlovevide...@gmail.com> wrote: > On Sep 24, 10:26 am, Zac Burns <zac...@gmail.com> wrote: >> Currently it is possible to import a file of one path to more than one >> 'instance' of a module. One notable example is "import __init__" from >> a package. Seehttp://stackoverflow.com/questions/436497/python-import-the-containin...
>> This recently caused a devastating bug in some of my code. What I have >> is support for the Perforce global options as a context for a perforce >> module.http://www.perforce.com/perforce/doc.072/manuals/cmdref/o.gopts.html#... >> This way one can call functions that call many perforce command and >> have them execute on a different client for example.
>> So, in module A and module B both imported the Perforce module, but >> they turned out not to be the same module. Module A did "with >> Perforce.GlobalOptions(client=client): B.function()"
>> B.function did not receive the new GlobalOptions because of this >> problem. As a result important files on the original client were >> overwritten (OUCH).
>> I would like to propose that it be made impossible in the Python >> source to import two instances of the same module.
> Impossible's a pretty strong word.
> It's a reasonable request, but with Python's importing the way it is > it'd be kind of hard to do. A Python file can be visible in multiple > ways.
> However, anyone who does "import __init__" (or "from . import > __init__" with relative import) is asking for trouble, I can't think > of any valid reason to do it, and I wouldn't mind seeing that > forbidden, but it's simple to avoid. Someone probably did that > because they didn't know how to import a containing package from one > of its modules, failing to realize that it created a new module.
There are corner cases. The corner case that I ran into was that there were two ways to find the module on PATH because one value of PATH was over another. Since then this problem has been removed and it wasn't too much trouble to work around - but finding the problem was a real pain.
I am not intimately familiar with the import code and trust your judgment that it is difficult. If people are in agreement that this should be changed though it could be put in a list somewhere waiting for some ambitious person to figure out the implementation, no?
Personally I think it would be worthwhile.
-- Zachary Burns (407)590-4814 Aim - Zac256FL Production Engineer (Digital Overlord) Zindagi Games
Zac Burns wrote: > On Thu, Sep 24, 2009 at 1:38 PM, Carl Banks <pavlovevide...@gmail.com> wrote: >> On Sep 24, 10:26 am, Zac Burns <zac...@gmail.com> wrote: >>> Currently it is possible to import a file of one path to more than one >>> 'instance' of a module. One notable example is "import __init__" from >>> a package. Seehttp://stackoverflow.com/questions/436497/python-import-the-containin...
>>> This recently caused a devastating bug in some of my code. What I have >>> is support for the Perforce global options as a context for a perforce >>> module.http://www.perforce.com/perforce/doc.072/manuals/cmdref/o.gopts.html#... >>> This way one can call functions that call many perforce command and >>> have them execute on a different client for example.
>>> So, in module A and module B both imported the Perforce module, but >>> they turned out not to be the same module. Module A did "with >>> Perforce.GlobalOptions(client=client): B.function()"
>>> B.function did not receive the new GlobalOptions because of this >>> problem. As a result important files on the original client were >>> overwritten (OUCH).
>>> I would like to propose that it be made impossible in the Python >>> source to import two instances of the same module. >> Impossible's a pretty strong word.
>> It's a reasonable request, but with Python's importing the way it is >> it'd be kind of hard to do. A Python file can be visible in multiple >> ways.
>> However, anyone who does "import __init__" (or "from . import >> __init__" with relative import) is asking for trouble, I can't think >> of any valid reason to do it, and I wouldn't mind seeing that >> forbidden, but it's simple to avoid.
/__init__.py is basically an implementation hack to make a directory also 'be' a file. Use at one own risk, I say.
> There are corner cases. The corner case that I ran into was that there > were two ways to find the module on PATH because one value of PATH was > over another. Since then this problem has been removed and it wasn't > too much trouble to work around - but finding the problem was a real > pain.
> I am not intimately familiar with the import code and trust your > judgment that it is difficult. If people are in agreement that this > should be changed though it could be put in a list somewhere waiting > for some ambitious person to figure out the implementation, no?
1. It would slow down all imports, at least a bit.
2. It would kill code that intentionally makes use of duplicate modules (but this could be considered exploitation of a bug, perhaps). It would also make forced module reloads harder, it not impossible. Currently, just delete the entry in sys.modules.
3. The language itself does not specify how and where from an implementation 'initializes' a module on first import. Indeed, CPython has at least three options (.py, .zip, and .dll or .pyd (Windows)), with hooks for more. Lets a take the request as specifically preventing the creation of duplicate module objects from a particular .py file.
One implementatin *might* be add a set to sys, say sys.mod_files for x.py or x.pyc files already used to initialize a module. The .py or .pyc or .pyo would be stripped but the name otherwise should be the absolute path. (Including drive letter, on Windows).
This would not cover the case when files are symlinked (or copied). For *nix, a set of inode numbers could be used, but not for Windows. I suspect there might be other system-specific problems I have not thought of.
> I would like to propose that it be made impossible in the Python > source to import two instances of the same module.
A fully-automatic solution is more difficult than it might seem at first: http://www.python.org/dev/peps/pep-0328/ But there is a simple code-discipline solution: never ever use relative imports, even between code in the same package.
We got "bit" by double-imports (a mix of relative, absolute, and even cross-imports A "import B" and B "import A") early on in one of our projects. The symptom was that the imported module would be initialized *twice*, once for a relative import and once for an absolute. This is not a happy situation for pseudo-singletons like the "logging" module --- esp. if one is hacking the internals! :-) We no longer use relative imports *EVER*, even within the same package.
Perhaps Perforce is doing something tricky with scoping or importing, and you've just managed to stumble across this trickiness because of a double-import. Good luck, these things are a bugger to debug.
