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The error message for os.path.join() is misleading when the first argument is None. The message should probably say something about mixing "None" and strings.
Python 3.3.0b1 (default:f954ee489896, Jul 16 2012, 22:42:29) 
[GCC 4.2.1 Compatible Apple Clang 3.1 (tags/Apple/clang-318.0.58)] on darwin
>>> import os
>>> os.path.join(None, 'a')
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File ".../cpython/Lib/posixpath.py", line 89, in join
 "components.") from None
TypeError: Can't mix strings and bytes in path components.

I propose the following patch (against 3.2) that does the right thing.

I'd be more inclined to tighten up the check for the "can't mix" message to something like:
 valid_types = all(isinstance(s, (str, bytes, bytearray)) for s in (a, ) + p)
 if valid_types:
 # Must have a mixture of text and binary data
 raise TypeError("Can't mix strings and bytes in path components.")
 raise
If people pass in something that isn't a valid argument *at all*, then I'm fine with just letting the underlying exception pass through.
The str/bytes case is just worth special-casing because either on their own *are* valid arguments.

Yeah, we talked about that exact think with Antoine on IRC. New proposal.

Looks good to me

New changeset 5553a53a230a by Hynek Schlawack in branch '3.2':
#15377: Make posixpath.join() more strict when checking for str/bytes mix
http://hg.python.org/cpython/rev/5553a53a230a
New changeset d087ef80372d by Hynek Schlawack in branch 'default':
#15377: Make posixpath.join() more strict when checking for str/bytes mix
http://hg.python.org/cpython/rev/d087ef80372d 

Fine! :)