os.chdir() considered harmful

[Donn Cave]

Quoth Guido van Rossum <guido at beopen.com>:
| In a neat little piece of code posted by Tim, I found this gem:
|
| >         # This seems a little-known trick in Python:  changing to the
| >         # current directory allows the subsequent isdir-in-a-loop to
| >         # work on just the file name, instead of making the OS reparse
| >         # the whole darn path from the root again each time.
| >         # Depending on the OS, can save gobs of time.
|
| I have no objection to using this in a small self-contained program
| like Tim's example.  However I would like to warn that using
| os.chdir() is not always a safe practice!  The current directory is
| used for several purposes, like interpreting pathnames entered by the
| user, and importing Python modules supplied by the user.
|
| If you change the current directory, you may not find the files that
| the user asked you to open, or you may find that surprising modules
| are imported (the current directory is first in Python's module path
| when Python is invoked without a script).
|
| The current directory is also shared between threads, so in a
| multi-threaded program, it's doubly scary.
|
| My personal guideline for os.chdir() is to only use it in a main
| Python program / script, never in a module imported by others.

No argument with this, but it occurs to me that conversely, a module
imported by others should import its modules up front, and not later
during execution, because the program may indeed change its current
working directory.

	Donn Cave, donn at u.washington.edu