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mailcap.getcaps() has a call to mailcap.listmailcapfiles(), which returns a list of all mailcap files found on the system.
listmailcapfiles() first looks for the MAILCAPS environment variable. If it exists, it converts the string value into a list by splitting it on ":". This will not work on platforms that use other path separators, like Windows (";").
Attached are patches that use os.pathsep instead.
For more information about the MAILCAPS variable, see Appendix A in RFC 1524 (http://tools.ietf.org/html/rfc1524.html).

Fixing typo in title

Thanks for the patches. Can you also add a test case to Lib/test/mailcap.py to cover this problem?

I'm also creating a test for mailcap (Issue 6484), which is how I found this bug. I'll have an update to that test submitted soon once I implement r.david.murray's helpful suggestions.

I just submitted new versions of test_mailcap.py in case 6484.

BTW, is there a real use case that lead you to open this report, or is it just theoretical? I’m not even sure mailcap is used on non-UNIX (and even there, I think it’s not used by recent tools).

It's just theoretical. A web search shows that a few Windows programs, like PC Pine, can use $MAILCAPS. But none of those tools look recent, like you said.

Okay. Nonetheless, splitting on os.sep doesn’t cost us anything and is IMO a good practice to follow.

New changeset 7b83d2c1aad9 by Nick Coghlan in branch 'default':
Fix #9923: mailcap now uses the OS path separator for the MAILCAP envvar. Not backported, since it could break cases where people worked around the old POSIX-specific behaviour on non-POSIX platforms.
http://hg.python.org/cpython/rev/7b83d2c1aad9 

As noted in the commit message, I didn't backport this, since it didn't seem worth risking breaking even the unlikely case that someone actually *was* using the MAILCAP environment variable on Windows.