Functions. Structuring Your Python Programs

Transcription

1 Functions Structuring Your Python Programs

2 What are functions for? Breaking a program into bite-sized pieces and giving them names that make sense Putting code that would be duplicated in one place Structuring a program so smaller pieces can be used and tested independently

3 Organizing with Functions As programs (and their sections) get longer, they are harder to understand Functions give you paragraphs in your code to break out and name sections Functions should each do one clear job z 165

4 Go to school Go to UROP office Ask about Fall 08 research Attend CHEM 215 Turn in assignment Attend SI 182 Take notes Go to the bank Deposit paycheck Go to the store Get milk Get bread Check out Go home A Day in the Life......Pretty confusing, eh?

5 A Day in the Life Go to school A. Go to UROP office i. Ask about Fall 08 research B. Attend CHEM 215 i. Turn in assignment C. Attend SI 182 i. Take notes 2. Go to the bank A. Deposit paycheck 3. Go to the store A. Get milk B. Get bread C. Check out 4. Go home...a little better...

6 1. Go to school A. Go to UROP office i. Ask about Fall 08 research B. Attend CHEM 215 i. Turn in assignment C. Attend SI 182 i. Take notes 2. Go to the bank A. Deposit paycheck 3. Go to the store A. Get milk B. Get bread C. Check out 4. Go home A Day in the Life......with boxes representing functions...

7 Avoiding Code Duplication Achieves same organizational goals as simply naming a section but also: Shortens your overall program Reduces chances for errors Simplifies maintenance Enhances understandability z 168

8 Avoiding Duplication Go to refrigerator Get caffeinated beverage Go back to desk Hack until beverage gone Go to refrigerator Get caffeinated beverage Go back to desk Read python.org docs for a while Read dr-chuck.com until parched Go to refrigerator Get caffeinated beverage Go back to desk...

9 Avoiding Duplication Go to refrigerator Get caffeinated beverage Go back to desk Let s name this Quench thirst Quench thirst Hack until beverage gone Quench thirst Read python.org docs for a while Read dr-chuck.com until parched Quench thirst...

10 A Tiny Bit of Syntax def quenchthirst(): Go to refrigerator Get caffeinated beverage Go back to desk def main(): quenchthirst() Hack until beverage gone quenchthirst() Read python.org docs for a while Read dr-chuck.com until parched quenchthirst() This defines the function quenchthirst Note: Must define before use We say that these are calls to quenchthirst

11 But I already have the shakes! No more caffeine! def quenchthirst(drinktype): runtofridge() getbeverage(drinktype) runtodesk() This is called a parameter. It allows the function to operate on different values in this case, different kinds of beverages. z 174

12 But I already have the shakes! No more caffeine! def quenchthirst(drinktype): runtofridge() getbeverage(drinktype) runtodesk() def main():... quenchthirst('soda')... quenchthirst('juice') When calling functions we say we pass arguments Note: We can pass literal values or variables

13 What s with the name function, anyway? f(x) = x 2 f(x,y) = x + y These are simple mathematical functions, which is where the name comes from. In math, functions define some formula to give a result based on input values sound familiar?

14 Two Main Types Functions that do not modify their input Can be thought of as asking questions Most math and many other functions are in this category Return values based on input Functions that do modify arguments Can be thought of as giving commands These are said to have side-effects Sometimes called sub-routines

15 Functions Returning def biggestvalue(numbers): biggest = None for num in numbers: if num > biggest: biggest = num return biggest >>> lotto = [3, 42, 7, 37, 23, 13] >>> top = biggestvalue(lotto) >>> print(top) 42 >>> print(lotto) Values Returns the largest value in the list of numbers to whoever called biggestvalue. Also returns control of the program to whoever called biggestvalue [3, 42, 7, 37, 23, 13] z 177

16 Functions Modifying Arguments def bubblesort(list): for i in range(0, len(list)): for j in range(0, len(list)): if list[i] < list[j]: tmp = list[j] list[j] = list[i] list[i] = tmp >>> lotto = [3, 42, 7, 37, 23, 13] >>> bubblesort(lotto) >>> print(lotto) [3, 7, 13, 23, 37, 42] A simple sorting algorithm called bubble sort The lotto list is out of order to start After calling bubblesort, the lotto list is now sorted z 181

17 Variable Scope Variables are only accessible within their enclosing block Functions are a kind of block; if statements and for loops are others Variables with the same name in different scopes do not affect each other Global variables are the exception available from all functions z 183

18 Scope Example def one(something): print "One: ", something def two(something): print "Two: ", something print fred def test(): fred = 'Mr. Fred' answer = 42 one(answer) two(answer) >>> test() One: 42 Two: 42 Traceback (most recent call last): File "<stdin>", line 1, in? File "<stdin>", line 5, in test File "<stdin>", line 3, in two NameError: global name 'fred' is not defined

19 Cohesion Functions doing one clear job are said to be cohesive High cohesion allows you to verify independent pieces to get it right Cohesive functions don t use or depend on stuff they don t need

20 Cohesion v. Duplication Go to refrigerator Get caffeinated beverage Go back to desk Hack until beverage gone Go to refrigerator Get caffeinated beverage Go back to desk Read python.org docs for a while Read dr-chuck.com until parched Go to refrigerator Get caffeinated beverage Go back to desk...

21 Cohesion v. Duplication Go to refrigerator Get caffeinated beverage Go back to desk Let s name this Quench thirst Quench thirst Hack until beverage gone Quench thirst Read python.org docs for a while Read dr-chuck.com until parched Quench thirst...