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CGIHTTPServer._url_collapse_path_split should probably live in a more 
general library and become a documented API. Perhaps in urlparse?

I'd Wrote a patch for this.
Perhaps some unittests are needed.

Yes, unit tests are always needed for any change.
Gregory, why is a function that does two things (collapse and split) a
good candidate for a public API? It results in a pretty awkward name :)
I'm not saying it's necessarily a bad idea, just looking for motivation.

Actually urlparse.urljoin implements RFC 2396
Is it true that 'CGIHTTPServer._url_collapse_path_split' is just a
inverted 'urlparse.urljoin' ?
"""
>>> urlparse.urljoin('http://a/b/c/','g')
'http://a/b/c/g'
>>> urlparse.url_collapse_path_split('http://a/b/c/g')
('/http:/a/b/c', 'g')
>>> urlparse.urlsplit('http://a/b/c/g')
SplitResult(scheme='http', netloc='a', path='/b/c/g', query='', fragment='')
"""
And there is existing function 'urlparse.urlsplit'.
I think "CGIHTTPServer._url_collapse_path_split" is just a customized
version of latter, and should be rewritten using 'urlparse.urlsplit'.

urlparse.urljoin and urlparse.urlsplit do not do what is required for 
this function. urljoin does not collapse paths. urlsplit has nothing 
to do with paths.
I agree r.david.murray that it is odd that it does two functions at once 
(the collapse and the split). I wrote it specifically for its current 
use case when checking paths to cgi scripts.
The unittests for it describe the exact behavior it needs to implement. 
Trying to implement a separate collapse function is approximately the 
same amount of code because the edge cases such as
'/a/b/' and '/a/b/c/..' which both need to result in it returning 
('/a/b', '') instead of ('/a', 'b') are why it made sense to keep as a 
single function for its current use.
Unittests for the function already exist in Lib/test/test_httpservers.py 
to describe its full behavior.

Yes, the parsing best be done in urlparse.
As this function claims to do step 6 of RFC2396, I am surprised why it
cannot be done by using existing urlparse functions itself.(Because, the
tests for RFC2396 compliance covers those cases too).

I've tried investigating this but I've got lost so I'll have to pass the buck. I've got confused because of the code moving around in Python 2 and the library changes going to Python 3 :-(

Some ideas for this sort of function (or perhaps a few related functions):
* Allow OS-independent handling of the path. E.g. the CGI server still has to check if each segment is a CGI script, the rest becomes PATH_INFO. Perhaps return a sequence of path segments rather than a string.
* Allow easy conversion to a valid OS path (or perhaps a pathlib object)
* Ensure that invalid and special OS filenames trigger an error (e.g. "NUL" or unsupported characters, including backslashes ,円 on Windows). Like a stricter version of pathlib’s is_reserved().
* Handle percent decoding
Also, see the SimpleHTTPRequestHandler.translate_path() method, and Issue 14567. There seems to be a lot of redundancy.