> It doesn't solve the case when an identifier or number is used as a function:

In the first case we should convert an argument to integer.

    ns = {}
    exec('''if True:
        def func(arg):
            n = int(arg)
            return {}
        '''.format(plural), ns)
    return ns['func']

Or raise an exception if argument is not integer.

In the second case I think we can just left it as is. This is not valid C expression.

Or we can add try/except in above code and convert TypeError to ValueError if this is more preferable exception.