>>> 'def'.isidentifier()
True

According to the documentation, the reserved words are classified as identifiers:
http://docs.python.org/3/reference/lexical_analysis.html#keywords

There's an easy workaround:

>>> from keyword import iskeyword
>>> def is_valid_identifier(s):
...     return s.isidentifier() and not iskeyword(s)
... 
>>> is_valid_identifier('def')
False

Or maybe 'is_usable_identifier' :)

Hmm. I were going to use this method for re's named group (see issue14462). There is a possibility that some third-party code uses it for checking on general Unicode-aware identifiers. The language specifification says that keywords is a subset of identifiers. However in most places in stdlib (collections.namedtuple, unittest.mock, inspect.Parameter) is_usable_identifier() should be used instead of isidentifier().

I already use it in the regex module for named groups. I don't think it would ever be a problem in practice because the names are invariably handled as strings.

IMHO str.isidentifier() should not return False for Python keywords, since it's often used in contexts where Python keywords would otherwise be valid.  A keyword arg to exclude keywords could be added though, assuming there are enough use cases.

New changeset a7fe48dfbfe9 by Raymond Hettinger in branch '3.3':
Issue 17447:  Clarify that str.isidentifier doesn't check for reserved keywords.
http://hg.python.org/cpython/rev/a7fe48dfbfe9

I've added a clarification to the docs.  Closing this report as invalid.