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This is a conversation with the current Python interpreter.
>>> import urlparse
>>> urlparse.urlparse(urlparse.urlunparse(urlparse.urlparse("file:////usr/bin/python")))
('file', 'usr', '/bin/python', '', '', '')
As you can see, the results are incorrect. The problem is in the urlunsplit function:
def urlunsplit((scheme, netloc, url, query, fragment)):
 if netloc or (scheme and scheme in uses_netloc and url[:2] != '//'):
 if url and url[:1] != '/': url = '/' + url
 url = '//' + (netloc or '') + url
 if scheme:
 url = scheme + ':' + url
 if query:
 url = url + '?' + query
 if fragment:
 url = url + '#' + fragment
 return url
RFC 1808 (see http://www.ietf.org/rfc/rfc1808.txt ) specifies that a URL shall have the following syntax:
<scheme>://<net_loc>/<path>;<params>?<query>#<fragment>
The problem with the current version of urlunsplit is that it tests if there are already two slashes before the 'url' section before outputting a URL. This is incorrect because (1) RFC 1808 clearly specifies at least three slashes between the end of the scheme portion and the beginning of the path portion and (2) this method will strip the first few slashes from an arbitrary path portion, which may require those slashes. Removing that url[:2] != '//' causes urlunsplit to behave correctly when dealing with urls like file:////usr/bin/python .

Some other issues with the urlparse module. Several constant lists defined at the beginning of the module should be sets because they are only used for testing if certain strings are in them. Also, urlunsplit() uses the + operator way too much, creating strings that are immediately thrown away. IMO, the alternative is actually more readable. Attaching a patch (diff -u urlparse.py urlparse.py.new > urlparse.diff).
File Added: urlparse.diff

Hi Thomas,
Verified the Bug with Python 2.5 and verified the fix as well. Works fine.
>>> urlparse(urlunparse(urlparse('file:////home/ors')))
('file', '', '//home/ors', '', '', '')
>>> urlparse(urlunparse(urlparse('file://///home/ors')))
('file', '', '///home/ors', '', '', '')
>>> urlparse(urlunparse(urlparse('file://////home/ors')))
('file', '', '////home/ors', '', '', '')
>>> urlparse(urlunparse(urlparse(urlunparse(urlparse('file://////home/ors')))))
('file', '', '////home/ors', '', '', '')
>>>

This issue no longer exists. I verified the bug report on the trunk and urlparse() and urlunparse methods behave properly on subsquent usages on the same url.
>>> urlparse.urlparse(urlparse.urlunparse(urlparse.urlparse('file:///home/ors/Letter.txt')))
ParseResult(scheme='file', netloc='', path='/home/ors/Letter.txt', params='', query='', fragment='')
Can be closed as Invalid.

I believe this was closed incorrectly. The original test case includes four slashes, not three, and still fails in 3.5. However Issue 23505 has been opened in the mean time which should cover this.