[Python-Dev] how important is setting co_filename for a module being imported to what __file__ is set to?

Brett Cannon brett at python.org
Mon Aug 31 02:34:06 CEST 2009

On Sun, Aug 30, 2009 at 17:24, Guido van Rossum<guido at python.org> wrote:
> On Sun, Aug 30, 2009 at 4:28 PM, Brett Cannon<brett at python.org> wrote:
>> I am going through and running the entire test suite using importlib
>> to ferret out incompatibilities. I have found a bunch, although all
>> rather minor (raising a different exception typically; not even sure
>> they are worth backporting as anyone reliant on the old exceptions
>> might get a nasty surprise in the next micro release), and now I am
>> down to my last failing test suite: test_import.
>> Ignoring the execution bit problem (http://bugs.python.org/issue6526
>> but I have no clue why this is happening), I am bumping up against
>> TestPycRewriting.test_incorrect_code_name. Turns out that import
>> resets co_filename on a code object to __file__ before exec'ing it to
>> create a module's namespace in order to ignore the file name passed
>> into compile() for the filename argument. Now I can't change
>> co_filename from Python as it's a read-only attribute and thus can't
>> match this functionality in importlib w/o creating some custom code to
>> allow me to specify the co_filename somewhere (marshal.loads() or some
>> new function).
>> My question is how important is this functionality? Do I really need
>> to go through and add an argument to marshal.loads or some new
>> function just to set co_filename to something that someone explicitly
>> set in a .pyc file? Or I can let this go and have this be the one
>> place where builtins.__import__ and importlib.__import__ differ and
>> just not worry about it?
> ISTR that Bill Janssen once mentioned a file replication mechanism
> whereby there were two names for each file: the "canonical" name on a
> replicated read-only filesystem, and the longer "writable" name on a
> unique master copy. He ended up with the filenames in the .pyc files
> being pretty bogus (since not everyone had access to the writable
> filesystem). So setting co_filename to match __file__ (i.e. the name
> under which the module is being imported) would be a nice service in
> this case.
> In general this would happen whenever you pre-compile a bunch of .py
> files to .pyc/.pyo and then copy the lot to a different location. Not
> a completely unlikely scenario.

Well, to get this level of compatibility I am going to need to add
some magical API somewhere then to overwrite a code object's "file"
location. Blah.

I will either add an argument to marshal.loads to specify an
overriding file path or add an imp.exec that takes a file path
argument to override the code object with.

> (I was going to comment on the execution bit issue but I realized I'm
> not even sure if you're talking about import.c or not. :-)

So it turns out a bunch of execution/write bit stuff has come up in
Python 2.7 and importlib has been ignoring it. =) Importlib has simply
been opening up the bytecode files with 'wb' and writing out the file.
But test_import tests that no execution bit get set or that a write
bit gets added if the source file lacks it. I guess I can use
posix.chmod and posix.stat to copy the source file's read and write
bits and always mask out the execution bits. I hate this low-level
file permission stuff.


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