[Python-Dev] Change to sys.path[0] for Python 2.3

[James C. Ahlstrom]

Gordon McMillan wrote:
> 
> Jim "Fortran" Ahlstrom wrote:

> > This insertion occurs after a number of imports have already
> > occurred.  Specifically, it occurs after the import of site, os,
> > and sitecustomize.  This is confusing.
> 
> Why? site happens before Python even thinks about
> sys.argv[0]. By it's very name it's about "how this installation
> should behave", not "how a script in this directory behaves".

Adding the directory of the Python script occurs after a number of
imports have already happened.  This is not necessary.  The directory
of the script is known.  It is confusing because the programmer sees
that the script directory is the first item of sys.path, and so
concludes that [s]he can put scripts there and have them imported.
This sometimes works, but fails for os, site, sitecustomize and a
few others.  There is no reason for this other that accidental
details of the implementation.

Think of documenting imports.  We would need to explain that
sys.argv[0] is Special, and different from other items.  Yuk.
The programmer may indeed manipluate sys.path, but at least
Python's default path should be simple.

BTW, is there any documentation on the details of imports, even
a description of sys.path?  We need an "invocation and imports"
manual (which I guess I just volunteered to write).

> > If no path component is given, the zero length string is
> > inserted.  But if the current working directory later
> > changes, this is no longer valid.  If we want the directory
> > of the script to be sys.path[0], then an absolute path should be
> > inserted.
> 
> Some people os.chdir() just for this effect. I don't think I mind if
> they experience some pain <wink>.

This is a problem for zip imports and directory caching.  If an item
of sys.path is a relative path, and getcwd() changes, then it is
difficult (as in slow, not as in impossible) to get caching to
work.  Think of sys.path item "./archive.zip".  There is a dictionary
full of items starting with "./", and then CWD changes.  I would have
to recognize a relative path for any item of sys.path, and call
getcwd() for each one, all for each supported OS.  Not fun.

Using my current code, zip imports fail if a relative path is given
and getcwd() changes.  This is a problem especially on Windows, as
normally the CWD changes to the directory of an opened file.

I think most of the practical problems go away if sys.path[0] is
an absolute path.  Then I can either make relative paths work
with the penalty that imports will be slower, or write documentation
that sys.path[] must only contain absolute paths.

In either case, I think sys.path[0] should be an absolute path.  Again,
think about documenting imports.  What is sys.path[0]?  It is the
directory of the script.  This is a fixed directory that doesn't change.

JimA