If you pass an object of the wrong type to str.join, Python raises a
TypeError with no error message:

    Python 3.4.0b3 (default, Jan 27 2014, 02:26:41) 
    [GCC 4.2.1 Compatible Apple LLVM 5.0 (clang-500.2.79)] on darwin
    Type "help", "copyright", "credits" or "license" for more information.
    >>> ''.join(1)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError

It is unnecessarily hard to understand from this error what the
problem actually was. Which object had the wrong type? What type
should it have been? Normally a TypeError is associated with a message
explaining which type was wrong, and what it should have been. For
example:

    >>> b''.join(1)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: can only join an iterable

It would be nice if the TypeError from ''.join(1) included a message
like this.

The reason for the lack of message is that PyUnicode_Join starts out
by calling PySequence_Fast(seq, "") which suppresses the error message
from PyObject_GetIter. This commit by Tim Peters is responsible:
<http://hg.python.org/cpython/rev/8579859f198c>. The commit message
doesn't mention the suppression of the message so I can assume that it
was an oversight.

I suggest replacing the line:

    fseq = PySequence_Fast(seq, "");

in PyUnicode_Join in unicodeobject.c with:

    fseq = PySequence_Fast(seq, "can only join an iterable");

for consistency with bytes_join in stringlib/join.h. Patch attached.

The exact behavior is present in 2.7 version too. So tagging 2.7 to 3.4

Looks good to me.

New changeset b9947b68ab61 by Benjamin Peterson in branch '3.3':
give non-iterable TypeError a message (closes #20507)
http://hg.python.org/cpython/rev/b9947b68ab61

New changeset f8e1fdf79823 by Benjamin Peterson in branch '2.7':
give non-iterable TypeError a message (closes #20507)
http://hg.python.org/cpython/rev/f8e1fdf79823

New changeset b077ee45f14f by Benjamin Peterson in branch 'default':
merge 3.3 (#20507)
http://hg.python.org/cpython/rev/b077ee45f14f

There are similar issues in Objects/dictobject.c:1893, Modules/_elementtree.c:1045 and Modules/_elementtree.c:1662.