I tried to use Zero-width joiner (U+200D) as part of an identifier.
It produce an exception like this:

SyntaxError: invalid character in identifier

I have attached the Python file which produce this error.

Zero-width joiner (U+200D) is a Unicode control character:
http://en.wikipedia.org/wiki/Unicode_control_characters

Why do you think this is a bug?

On a further look at this issue, I understood Python cannot use all
Unicode control characters as identifiers.  But for many international
languages, without some control characters like ZWJ & ZWNJ [1], it won't
be possible to construct all characters with proper visual
representation.  So, if Python really want to support international
characters as identifiers (for some reason), ZWJ & ZWNJ are unavoidable,
may be some other characters also.

 [1] http://en.wikipedia.org/wiki/Zero-width_joiner
     http://en.wikipedia.org/wiki/Zero-width_non-joiner

I think RFC-3454 [1] can be used as a base for selecting the control
characters which can be used as a valid identifier character.

 [1] http://www.rfc-editor.org/rfc/rfc3454.txt

Valid identifiers should begin with a letter or '_' and contain only
letters, numbers and '_'. This probably means that only the Unicode
characters that belong to the categories Ll, Lu (Letter Lower/Upper
case), Nd (Number, Decimal Digit) and Pc (Punctuation, Connector) - and
possibly other categories like Lm, Lt, No and Nl - are valid.

Some examples:
>>> ａ－ｂ = 5 # U+FF0D, Cat: Pd, FULLWIDTH HYPHEN-MINUS
SyntaxError: invalid character in identifier
>>> a＃ = 5 # U+FF03, Cat: Po, FULLWIDTH NUMBER SIGN
SyntaxError: invalid character in identifier
>>> a）b = 5 # U+FF09, Cat: Pe, FULLWIDTH RIGHT PARENTHESIS
SyntaxError: invalid character in identifier
>>> ａ＿ｂ = 5 # U+FF3F, Cat: Pc, FULLWIDTH LOW LINE
>>> ａ＿ｂ
5
>>> a﹍b﹎c﹏d = 5 # U+FE4D, U+FE4E, U+FE4F, Cat: Pc
>>> a﹍b﹎c﹏d
5

The definition of a word in the new re module (actually targetted at
Python 2.7) is currently a sequence of L&, N&, M& and Pc.

I suppose ideally we want the definitions of a word and an identifier to
be basically the same, except that an identifier can't start with N&.

See PEP 3131 for a specification what is an identifier in Python.

Closing this as "won't fix".