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Why exception from os.path.exists()?


Barry Scott <barry at barrys-emacs.org>:
> os.path.exists() is not special and I don't think should be be changed.

You are right that os.path.exists() might be logically tied to other
os.* facilities. The question is, should the application be cognizant of
the seam between the standard library and the operating system kernel?

When a Linux system call contains an illegal value, it responds with
errno=EINVAL. In Python, that's represented by the OSError exception
with e.errno=EINVAL. However, when Python encounters an illegal value
itself, it usually raises a ValueError. Is it useful for the application
to have to be prepared for OSError/EINVAL and ValueError separately? Or
should the difference be paved over by Python?

As it stands, os.path.exists() really means: the operating system
doesn't have a reason to fail os.stat() on the pathname. Python
intercedes with an exception if it can't even ask the operating system
for its opinion. That dichotomy is not suggested by the os.path.exists()
documentation. In fact, the whole point of os.path.* is to provide for
an abstraction to isolate the application from the intricacies of the
operating system specifics.

BTW, I challenge you to find a test case that tests the proper behavior
of an application if it encounters a pathname with a NUL in it. Or code
that gracefully catches a ValueError from os.path.exists().


Marko