Example is from U of T Learning to Program II Coursera course, based on their function design recipe (login req'd).
def can_vote(age):
""" (int) -> bool
Precondition: age >= 0
Return True if and only if a person aged age can vote in Canada.
The legal voting age in Canada is 18 years and older.
>>> can_vote(0)
False
>>> can_vote(18)
True
>>> can_vote(19)
True
"""
return age >= 18
# comment the following out after tests are complete
if __name__ == '__main__':
import doctest
print (doctest.testmod())