def add_unique_check(self, key,
code=UNIQUE_CHECK_FAILED,
message=MESSAGES[UNIQUE_CHECK_FAILED]):
"""
Add a unique check on a single column or combination of columns.
Arguments
---------
`key` - a single field name (string) specifying a field in which all
values are expected to be unique, or a sequence of field names (tuple
or list of strings) specifying a compound key
`code` - problem code to report if a record is not valid, defaults to
`UNIQUE_CHECK_FAILED`
`message` - problem message to report if a record is not valid
"""
> if isinstance(key, basestring):
E NameError: name 'basestring' is not defined
