An Introduction To Python - Functions, Part 2

An Introduction To Software
Development Using Python
Spring Semester, 2014
Class #18:
Functions, Part 2

Function Parameter Passing
• When a function is called, variables are created for
receiving the function’s arguments.
• These variables are called parameter variables.
• The values that are supplied to the function when it
is called are the arguments of the call.
• Each parameter variable is initialized with the
corresponding argument.
Image Credit: clipartzebra.com

Parameter Passing: An Example
1. Function Call
myFlavor = selectFlavor(yogurt)
2. Initializing functional parameter variable
3. About to return to the caller
selection = input(“Please enter flavor of yogurt you would like:”)
return selection
4. After function call
Image Credit: www.clipartpanda.com

What Happens The Next Time That
We Call This Function?
• A new parameter variable is created. (The previous parameter
variable was removed when the first call to selectFlavor
returned.)
• It is initialized with yogurt value, and the process repeats.
• After the second function call is complete, its variables are
again removed
Image Credit: www.clker.com

What If … You Try To Modify
Argument Parameters?
• When the function returns, all of its
variables are removed.
• Any values that have been assigned to
them are simply forgotten.
• In Python, a function can never change
the contents of a variable that was passed
as an argument.
• When you call a function with a variable
as argument, you don’t actually pass the
variable, just the value that it contains.
selectFlavor(yogurt):
if yogurt :
yogurt = False
else:
yogurt = True
flavor = chocolate
return (flavor)
Image Credit: www.fotosearch.com

Return Values
• You use the return statement to specify the result of a
function.
• The return statement can return the value of any expression.
• Instead of saving the return value in a variable
and returning the variable, it is often possible to eliminate the
variable and return the value of a more complex expression:
return ((numConesWanted*5)**2)
Image Credit: www.dreamstime.com

Multiple Returns
• When the return statement is processed, the
function exits immediately.
• Every branch of a function should return a
value.
if not(yogurt) :
return (noFlavor)
else:
myFlavor = input(“Enter your yogurt flavor”)
return (myFlavor)
Image Credit: www.clker.com

Multiple Returns: Mistakes
• What happens when you forget to include a
return statement on a path?
• The compiler will not report this as an error.
Instead, the special value None will be
returned from the function.
if not(yogurt) :
numNoYogurts += numNoYogurts
else:
myFlavor = input(“Enter your yogurt flavor”)
return (myFlavor)

Using Single-Line
Compound Statements
• Compounds statements in Python are generally written across
several lines.
• The header is on one line and the body on the following lines,
with each body statement indented to the same level.
• When the body contains a single statement, however,
compound statements may be written on a single line.
if digit == 1 :
return "one"
if digit == 1 : return "one"

Using Single-Line
Compound Statements
• This form can be very useful in functions that select a single
value from among a collection and return it. For example, the
single-line form used here produces condensed code that is
easy to read:
if digit == 1 : return "one"
if digit == 2 : return "two"
if digit == 3 : return "three"
if digit == 4 : return "four"
if digit == 5 : return "five"
if digit == 6 : return "six"
if digit == 7 : return "seven"
if digit == 8 : return "eight"
if digit == 9 : return "nine"
Image Credit: www.clipartbest.com

Example: Implementing A Function
• Suppose that you are helping archaeologists who
research Egyptian pyramids.
• You have taken on the task of writing a function that
determines the volume of a pyramid, given its height
and base length.
Image Credit: www.morethings.com

Inputs & Parameter Types
• What are your inputs?
– the pyramid’s height and base length
• Types of the parameter variables and the return value.
– The height and base length can both be floating-point numbers.
– The computed volume is also a floating-point number
## Computes the volume of a pyramid whose base is square.
# @param height a float indicating the height of the pyramid
# @param baseLength a float indicating the length of one
side of the pyramid’s base
# @return the volume of the pyramid as a float
def pyramidVolume(height, baseLength) :
Image Credit: egypt.mrdonn.org

Pseudocode For Calculating
The Volume
• An Internet search yields the fact that the volume of
a pyramid is computed as:
volume = 1/3 x height x base area
• Because the base is a square, we have
base area = base length x base length
• Using these two equations, we can compute the
volume from the arguments.
Image Credit: munezoxcence.blogspot.com

Implement the Function Body
• The function body is quite simple.
• Note the use of the return statement to
return the result.
def pyramidVolume(height, baseLength) :
baseArea = baseLength * baseLength
return height * baseArea / 3
Image Credit: imgkid.com

Let’s Calculate!
Question: How Tall Was the Great Pyramid?
Answer: The tallest of a cluster of three pyramids at Giza, the Great Pyramid
was originally about 146 meters tall, but it has lost about 10 meters in height
over the millennia. Erosion and burial in sand contribute to the shrinkage. The
base of the Great Pyramid is about 55,000 meters.
## Computes the volume of a pyramid whose base is a square.
13 # @param height a float indicating the height of the pyramid
14 # @param baseLength a float indicating the length of one side of the pyramid’s base
15 # @return the volume of the pyramid as a float
16 #
17 def pyramidVolume(height, baseLength)
18 baseArea = baseLength * baseLength
19 return height * baseArea / 3
20
21 # Start the program.
22 print("Volume:", pyramidVolume(146, 55000)
23 print("Volume:", pyramidVolume(136, 55000)
Image Credit: oldcatman-xxx.com

Scope Of Variables
• The scope of a variable is the part of the
program in which you can access it.
• For example, the scope of a function’s
parameter variable is the entire function.
def main() :
print(cubeVolume(10))
def cubeVolume(sideLength) :
return sideLength ** 3

Local Variables
• A variable that is defined within a function is called a
local variable.
• When a local variable is defined in a block, it
becomes available from that point until the end of
the function in which it is defined.
def main() :
sum = 0
for i in range(11) :
square = i * i
sum = sum + square
print(square, sum)
Image Credit: www.fotosearch.com

Scope Problem
• Note the scope of the variable sideLength.
• The cubeVolume function attempts to read thevariable, but it
cannot—the scope of sideLength does not extend outside the
main function.
def main() :
sideLength = 10
result = cubeVolume()
print(result)
def cubeVolume() :
return sideLength ** 3 # Error
main()
Image Credit: www.lexique.co.uk

Variable Reuse
• It is possible to use the same variable name more
than once in a program.
• Each result variable is defined in a separate function,
and their scopes do not overlap
def main() :
result = square(3) + square(4)
print(result)
def square(n) :
result = n * n
return result
main()
Image Credit: www.clipartguide.com

Global Variables
• Python also supports global variables: variables that are defined outside
functions.
• A global variable is visible to all functions that are defined after it.
• However, any function that wishes to update a global variable must
include a global declaration, like this:
• If you omit the global declaration, then the balance variable inside the
withdraw function is considered a local variable.
balance = 10000 # A global variable
def withdraw(amount) :
global balance # This function intends to update the global variable
if balance >= amount :
balance = balance - amount
Image Credit: galleryhip.com

What’s In Your Python Toolbox?
print() math strings I/O IF/Else elif While For
Lists And / Or Functions

What We Covered Today
1. Parameter passing
2. Return
3. Scope of variables
Image Credit: http://www.tswdj.com/blog/2011/05/17/the-grooms-checklist/

What We’ll Be Covering Next Time
1. Files, Part 1
Image Credit: http://merchantblog.thefind.com/2011/01/merchant-newsletter/resolve-to-take-advantage-of-these-5-e-commerce-trends/attachment/crystal-ball-fullsize/

An Introduction To Python - Functions, Part 2

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