Generally for objects that Python considers false, except zero which is not considered invalid.

Examples:

>>> ne = NotEmpty(messages={'empty': 'enter something'})
>>> ne.to_python('')
Traceback (most recent call last):
  ...
Invalid: enter something
>>> ne.to_python(0)
0

Messages

badType:

The input must be a string (not a %(type)s: %(value)r)

empty:

Please enter a value

noneType:

The input must be a string (not None)