sep 29
Homework comments : Please do test & run your code. (I'm seeing syntax errors that suggest some were typed in but not actually executed.)
Next assignment (chapter 6) is up.
functions
doctests
Discuss how you would make sure a function was
doing what you expect
Suppose you're writing a program that has something
to do with palindromes. One thing you'd need is
to have strings turned around backwards. So what
would a function look like to do that? Turns out
(google "python reverse") that you can reverse
lists easily. And we've seen before that you can
turn strings into lists and vice-versa. So you
play around at the command line and figure this out:
>>> word = "hello"
>>> chars = list(word)
>>> chars
['h', 'e', 'l', 'l', 'o']
>>> chars.reverse()
['o', 'l', 'l', 'e', 'h']
>>> import string
>>> string.join(chars, '')
'olleh'
This is a small specific piece of the larger
problem that you can tuck into a function.
Together with its docs and a test it would look like this:
def reverse_string(word):
""" Reverse a string.
input: a string, e.g. "hello"
output: the string backwards, e.g. "olleh"
>>> reverse_string("Can you hear me?")
'?em raeh uoy naC'
"""
import string
chars = list(word)
chars.reverse()
return string.join(chars, '')
if __name__ == "__main__": # This runs the doctests;
import doctest # just put these 3 lines at the
doctest.testmod() # end if you do this sort of testing.
Put this in "test.py", run it with "python test.py -v".
Also put the wrong thing in the test, and run it with "python test.py".
encode / decode example
An example : encode / decode from before, this time with functions .
- What parts should go into functions?
- What kinds of tests?
Ideas :
- top-down (look at big picture first) vs bottom-up programming (look at details first)
- test-driven programming can be very useful (write the tests first as a way to see exactly what it's supposed to do)
Class exercise :
- First remember how the program works.
- Then take 5 min to decide on some functions.
- For each function, you typically need to consider both
- the interface (how you use it, inputs & outputs), and
- implementation (how to make it work)
Here's a start of a top-down approach to the cipher problem :
"""
cipher_functions.py
"""
def get_user_input():
# fill in docs & test
return ('wwww', 'kkkk') # FIXME
def translate(word, key):
# fill in docs & test
return '????' # FIXME
def main():
print "-- cipher --\n"
(word, key) = get_user_input()
encoded = translate(word, key)
print "'%s' encrypted is '%s'" % (word, encoded)
# TODO: put stuff to run doctest here
main()
book example
- inputs?
- outputs?
- side effects?
- tests?
- docs?
default values in function arguments
def do_something(size, color="white"):
# code goes here
print "size, color = %i, %s" % (size, color)
do_something(3, "blue") # this is OK
do_something(4) # also OK; color defaults to "white"
do_something() # *not* OK; 0 args gives error
Args with defaults may only go on the *end* of the argument list ... otherwise the later ones (required) imply that the earlier ones are also required.
Note that this is a python-specific syntax.
multiple return values
def get_stuff():
return 1, "hey"
# And now use it
a, b = get_stuff()
# "Tuples" in python
other bells and whistles
Less used, and can be safely ignored on a first pass through python.
But for those who like the tricky corners...
def two_or_more_args(a, b, *c):
print "a = '" + str(a) + "'"
print "b = '" + str(b) + "'"
print "c = '" + str(c) + "'"
# and now call it.
two_or_more_args(3, "hey") # a=3, b="hey", c=[]
two_or_more_args(3, "hey", 4, "there") # a=3, b="hey", c=[4, "there"]
depending on time
... start "conditionals" (chapters 7 and 8):
value = input("What is the number? (1 to 10) ")
if value < 1 or value > 10 :
print "Oops: your number was too big or too small."
print "Your number is %i" % number
Discuss flowcharts.
Discuss comparison operators.
Coming: boolean variables.
![[paper clip]](/courses/source/wiki_images/paper_clip_tilt.png)
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