"""
 doctest_example.py

 This shows 
   (1) python documentation strings, and
   (2) a test of a function within such a string.

 Here the function is trivial : it simply doubles its argument.

 The whole program is also trivial: it asks for a number,
 and then prints what double() does to it.

 The doc strings are the text within the triple quotes.
 They're stored within <thing>.__doc__ and will be printed
 if you ask for help(<thing>) in an interactive python session.

 The tests are exactly what you would see by running an
 interactive python session and evaluating the function.

 Jim Mahoney | Sep 2018
"""

def double(x):
    """ Return 2*x
    >>> double(3)
    6.1
    >>> double('a')
    'aa'
    """
    return 2*x

def main():
    number = input("What is the number? ")
    print("Twice " + str(number) + " is " + str(double(number)))

def _test():
    """ Run the tests in this file,
        i.e. the doc string parts with with ">>> ... " as if at a python prompt.
        Google 'python doctest' for the details.
        Usage:
         $ python <filename>       # 1. Invoke python on a file as usual.
         $                         #    No warnings means success.
         $ python <filename> -v    # 2. Invoke with -v (verbose) flag
         Trying:                   #    to see details.
           double(3)
         Expecting:
           6
         ok
         ...
    """
    import doctest
    doctest.testmod()   # test a module
                        # (By default, this module, e.g. this file.)

_test()   # run tests
main()    # run the main program