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Announcements
References for today: 3.1-3.6 (functions); 4.9
(docstring for a function); 14.9 (writing a module)
No reading for next time: Our treatment of objects
differs significantly from the book's. Slides by D. Gries, L. Lee, S. Marschner, W. White
last time
From command sequences to functions
Given: variable data contains a string with at least two ','s.
Example: data="LL, '14, 1-800-OPYTHON, 1-555-TYPHOON"
Goal: print the part of the string after the 2nd ',', stripping whitespace from ends. Put commands in file string_puzzle_soln.py
At command line, type "python string_puzzle_soln.py" But users (including other programmers) want to write things like:
gvr_phone = phone("GvR, '14, 1-800-OPYTHON")
Can store output in a variable
User not forced to have variable named data
Can use the same function twice in one line
phone(some_data_string) == phone(other_data_string)
Calling a Function Already in a Module
Example: fix the capitalization of words with two caps,
like "THanks".
Lucky us: someone has written a file string.py which
defines a module string holding various functions.
on some computers: /Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/string.py
import string # Tell Python to we want access to this module.
# DON'T say "string.py" or you'll get an error.
# Built-in modules don't have to be in current dir. print string.capwords("THanks") specify that function capwords is from module string
Anatomy of a Function Definition
header
body
(indented)
specification,
as a docstring
declaration of parameter (input variable) named s
def
capwords(s):
"""Returns: Title-cased copy of s.
Example: "aBc dEf " -> "Abc Def"
Precondition: s is a string"""
temp = 5
return (sep or '
Return statement (optional). Contains expression whose value
is to be the result from the
').join(x.capitalize()
function call.
for x in s.spli...)
Writing a Function, Step One:
Header and Specification
Example input strings: 'RAISMAN, 6.7, 9.1,0'
"Ponor, 7 , 9, 0"
Single summary line, followed by blank line.
(More detail can be added in separate paragraphs)
def
defdscore(info):
dscore(info):
"""Returns: the difficulty score, as float, represented in info.
Precondition: info is a string with commas separating its
component values: last name, difficulty score, execution score, penalty."""
Precondition: assumptions about the argument values
Parameters: Variables Holding Input Values
def dscore(info):
"""Returns: difficultyWhen
score,you
as acall
float,a function,
represented
insupply
info.
you
arguments: input values.
Precondition: info is aex:
string
with commas separating 6.7,
its 9, 0')
gym.dscore('Raisman,
component values: last
name,
difficulty
score,inexecution
These
values
are stored
the function's
score, penalty.""" corresponding parameters: input variables
used within the function. Ex: info
startcomma = info.index(',')
[...]
A Specification is a Contract
Preconditions are a promise that:
§  if the arguments satisfy the preconditions, the function
works as described in the specification;
§  but, if the user's arguments violate the precondition, all
bets are off.
So write these contracts carefully! Common sources of software errors:
§  Preconditions not documented properly
§  Functions used in ways that violate preconditions
Testing Program "Correctness"
• 
• 
• 
• 
Bug: Error in a program. (Always expect them!)
Debugging: Process of finding bugs and removing them. Testing: Process of analyzing, running program, looking for bugs.
Test case: A set of input values, together with the expected output.
Organizing Test Cases: Unit Tests
A unit test is a module that tests another module.
1.  It imports the other module (so it can access it)
2.  It imports the cornelltest module (provided by us)
3.  It defines one or more test procedures
• 
• 
Evaluate the function(s) on the test cases
Compare the result to the expected value
4.  It has special code that calls the test procedures
Example unit test excerpt: testgym.py
Expected answer
# test procedure
The value produced
goes first.
def test_dscore ():
by the function being
"""Test procedure for gym.dscore(n)"""
tested goes second.
cornelltest.assert_floats_equal(6.7,
gym.dscore('Raisman, 6.7, 9.0, 0'))
Quits Python
Code below this line is executed
if not equal
we do "python testgym.py" on
# Application code
command line
if __name__ == '__main__':
test_dscore()
Message printed out only
print 'Module gym is working correctly'
if all test cases pass (i.e.,
no test function causes an
error)
An Important, If Counterintuitive, Principle
Write test cases for a function from its specification —
even before writing the function’s body. def number_vowels(w):
"""Returns: number of vowels in word w.
Precondition: w a string with at least one letter and only letters"""
pass # nothing here yet!
You need to be clear about what you're trying to solve
before you try to solve it! (Unless you're doing research.) Debugging with Print Statements
Print statements expose the values of variables, so
you can check if they have the value you expect.
print 'in this solution, df is "' + df + '"' Don't leave these in your finished code! They
reduce readability.