== A little gotcha with _os.path.join_

For my sins, I am one of those people who doesn't always read Python's
fine documentation carefully enough. For example, I wrote a large chunk
of code using the helpful _os.path.join()_ before I noticed a little
gotcha: it's *very* helpful. In particular:

> Joins one or more path components intelligently. ~~If any component is
> an absolute path, all previous components are thrown away, and
> joining continues~~.

(Right there in black and white in the [[fine documentation
http://www.python.org/doc/current/lib/module-os.path.html]].)

This means that you cannot do:

>  fpath = os.path.join(root, untrusted)

At least, you can't do this *safely* and have it do what you probably
think it does.

The other gotcha to remember for _os.path.join_ is that no matter
how convenient it looks on a Unix machine, *urls are not paths*. If
you use _os.path.join()_ on URLs and ever run on a Windows machine, the
results are not likely to be pleasant. You probably want the [[urlparse
module http://www.python.org/doc/current/lib/module-urlparse.html]]'s
_urljoin()_ function, but note that it behaves much like _os.path.join_
when handled absolute second parts, so:

> _>>> urljoin("!http://host/a/b", "/d/e/f") \\
> '!http://host/d/e/f'_

This is convenient if you are expecting it.