Last modified on 30 September 2014, at 21:35

Non-Programmer's Tutorial for Python 3/Defining Functions

Creating FunctionsEdit

To start off this chapter I am going to give you an example of what you could do but shouldn't (so don't type it in):

a = 23
b = -23
if a < 0:
    a = -a
if b < 0:
    b = -b
if a == b:
    print("The absolute values of", a, "and", b, "are equal.")
    print("The absolute values of", a, "and", b, "are different.")

with the output being:

The absolute values of 23 and 23 are equal.

The program seems a little repetitive. Programmers hate to repeat things -- that's what computers are for, after all! (Note also that finding the absolute value changed the value of the variable, which is why it is printing out 23, and not -23 in the output.) Fortunately Python allows you to create functions to remove duplication. Here is the rewritten example:

a = 23
b = -23
def absolute_value(n):
    if n < 0:
        n = -n
    return n
if absolute_value(a) == absolute_value(b):
    print("The absolute values of", a, "and", b, "are equal.")
    print("The absolute values of", a, "and", b, "are different.")

with the output being:

The absolute values of 23 and -23 are equal.

The key feature of this program is the def statement. def (short for define) starts a function definition. def is followed by the name of the function absolute_value. Next comes a '(' followed by the parameter n (n is passed from the program into the function when the function is called). The statements after the ':' are executed when the function is used. The statements continue until either the indented statements end or a return is encountered. The return statement returns a value back to the place where the function was called. We already have encountered a function in our very first program, the print function. Now we can make new functions.

Notice how the values of a and b are not changed. Functions can be used to repeat tasks that don't return values. Here are some examples:

def hello():
def area(width, height):
    return width * height
def print_welcome(name):
    print("Welcome", name)
w = 4
h = 5
print("width =", w, " height =", h, " area =", area(w, h))

with output being:

Welcome Fred
width = 4  height = 5  area = 20

That example shows some more stuff that you can do with functions. Notice that you can use no arguments or two or more. Notice also when a function doesn't need to send back a value, a return is optional.

Variables in functionsEdit

When eliminating repeated code, you often have variables in the repeated code. In Python, these are dealt with in a special way. So far all variables we have seen are global variables. Functions have a special type of variable called local variables. These variables only exist while the function is running. When a local variable has the same name as another variable (such as a global variable), the local variable hides the other. Sound confusing? Well, these next examples (which are a bit contrived) should help clear things up.

a = 4
def print_func():
    a = 17
    print("in print_func a = ", a)
print("a = ", a)

When run, we will receive an output of:

in print_func a = 17
a = 4

Variable assignments inside a function do not override global variables, they exist only inside the function. Even though a was assigned a new value inside the function, this newly assigned value was only relevant to print_func, when the function finishes running, and the a's values is printed again, we see the originally assigned values.

Here is another more complex example.

a_var = 10
b_var = 15
e_var = 25
def a_func(a_var):
    print("in a_func a_var = ", a_var)
    b_var = 100 + a_var
    d_var = 2 * a_var
    print("in a_func b_var = ", b_var)
    print("in a_func d_var = ", d_var)
    print("in a_func e_var = ", e_var)
    return b_var + 10
 c_var = a_func(b_var)
 print("a_var = ", a_var)
 print("b_var = ", b_var)
 print("c_var = ", c_var)
 print("d_var = ", d_var)
 in a_func a_var =  15
 in a_func b_var =  115
 in a_func d_var =  30
 in a_func e_var =  25
 a_var =  10
 b_var =  15
 c_var =  125
 d_var = 
 Traceback (most recent call last):
  File "C:\", line 19, in <module>
    print("d_var = ", d_var)
 NameError: name 'd_var' is not defined
In this example the variables <code>a_var</code>, <code>b_var</code>, and <code>d_var</code> are all local variables when they are inside the function <code>a_func</code>.  After the statement <code>return b_var + 10</code> is run, they all cease to exist. The variable <code>a_var</code> is automatically a local variable since it is a parameter name.  The variables <code>b_var</code> and <code>d_var</code> are local variables since they appear on the left of an equals sign in the function in the statements <code>b_var = 100 + a_var</code> and <code>d_var = 2 * a_var</code> .
Inside of the function <code>a_var</code> has no value assigned to it.  When the function is called with <code>c_var = a_func(b_var)</code>, 15 is assigned to <code>a_var</code> since at that point in time <code>b_var</code> is 15, making the call to the function <code>a_func(15)</code>.  This ends up setting <code>a_var</code> to 15 when it is inside of <code>a_func</code>.
As you can see, once the function finishes running, the local variables
<code>a_var</code> and <code>b_var</code> that had hidden the global variables of the same name are gone.  Then the statement <code>print("a_var = ", a_var)</code> prints the value <code>10</code> rather than the value <code>15</code> since the local variable 
that hid the global variable is gone.  
Another thing to notice is the <code>NameError</code> that happens at the end.  This appears since the variable <code>d_var</code> no longer exists since <code>a_func</code> finished.  All the local variables are deleted when the function exits.  If you want to get something from a function, then you will have to use <code>return something</code>.
One last thing to notice is that the value of <code>e_var</code> remains unchanged inside <code>a_func</code> since it is not a parameter and it never appears on the left of an equals sign inside of the function <code>a_func</code>.  When a global variable is accessed inside a function it is the global variable from the outside.
Functions allow local variables that exist only inside the function and 
can hide other variables that are outside the function.
=== Examples ===
<source lang="python">
#! /usr/bin/python
#-*-coding: utf-8 -*-
# converts temperature to Fahrenheit or Celsius
def print_options():
    print(" 'p' print options")
    print(" 'c' convert from Celsius")
    print(" 'f' convert from Fahrenheit")
    print(" 'q' quit the program")
def celsius_to_fahrenheit(c_temp):
    return 9.0 / 5.0 * c_temp + 32
def fahrenheit_to_celsius(f_temp):
    return (f_temp - 32.0) * 5.0 / 9.0
choice = "p"
while choice != "q":
    if choice == "c":
        c_temp = float(input("Celsius temperature: "))
        print("Fahrenheit:", celsius_to_fahrenheit(c_temp))
        choice = input("option: ")
    elif choice == "f":
        f_temp = float(input("Fahrenheit temperature: "))
        print("Celsius:", fahrenheit_to_celsius(f_temp))
        choice = input("option: ")
    elif choice == "p": #Alternatively choice != "q": so that print when anything unexpected inputed
        choice = input("option: ")

Sample Run:

 'p' print options
 'c' convert from celsius
 'f' convert from fahrenheit
 'q' quit the program
option: c
Celsius temperature: 30 
Fahrenheit: 86.0
option: f
Fahrenheit temperature: 60
Celsius: 15.5555555556
option: q

#! /usr/bin/python
#-*-coding: utf-8 -*-
# calculates a given rectangle area
def hello():
def area(width, height):
    return width * height
def print_welcome(name):
    print('Welcome,', name)
def positive_input(prompt):
    number = float(input(prompt))
    while number <= 0:
        print('Must be a positive number')
        number = float(input(prompt))
    return number
name = input('Your Name: ')
print('To find the area of a rectangle,')
print('enter the width and height below.')
w = positive_input('Width: ')
h = positive_input('Height: ')
print('Width =', w, ' Height =', h, ' so Area =', area(w, h))

Sample Run:

Your Name: Josh
Welcome, Josh

To find the area of a rectangle,
enter the width and height below.

Width: -4
Must be a positive number
Width: 4
Height: 3
Width = 4  Height = 3  so Area = 12


Rewrite the program from the Examples above to have a separate function for the area of a square, the area of a rectangle, and the area of a circle (3.14 * radius**2). This program should include a menu interface.

def square(L):
    return L * L
def rectangle(width , height):
    return width * height
def circle(radius):
    return 3.14159 * radius ** 2
def options():
    print("s = calculate the area of a square.")
    print("c = calculate the area of a circle.")
    print("r = calculate the area of a rectangle.")
    print("q = quit")
print("This program will calculate the area of a square, circle or rectangle.")
choice = "x"
while choice != "q":
    choice = input("Please enter your choice: ")
    if choice == "s":
        L = float(input("Length of square side: "))
        print("The area of this square is", square(L))
    elif choice == "c":
        radius = float(input("Radius of the circle: "))
        print("The area of the circle is", circle(radius))
    elif choice == "r":
        width = float(input("Width of the rectangle: "))
        height = float(input("Height of the rectangle: "))
        print("The area of the rectangle is", rectangle(width, height))
    elif choice == "q":
        print(" ",end="")
        print("Unrecognized option.")

Non-Programmer's Tutorial for Python 3
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