Nauč se Python > Kurzy > Beginners course PyLadies > Functions > Custom functions


In the previous lesson we were working with functions that were written by someone else - they are already built-in in Python.

from math import pi


Today we will learn how to code our own functions which will be helpful when we need to run tasks repeatedly.

It's not hard:

def  find_perimeter( width ,  height ): 
    "Returns the rectangle's perimeter of the given sides" 
    return  2 * (width  +  height)

print ( find_perimeter(4 ,  2))

How does it work?

You define a function with the command def. Right after that you have to write name of the function, parentheses (which may contain arguments) and then, of course, a colon.

We have already said that after a colon, everything that belongs to (in our case) the function must be indented. The indented code is called function body and it contains the commands that the function performs. In the body you can use various commands including if, loop, etc.

The body can start with a documentation comment which describes what the function is doing.

A function can return a value with the return command.

def  print_score(name, score): 
    print (name, 'score is', score) 
    if score > 1000:
        print('World record!') 
    elif score > 100: 
    elif score > 10: 
    elif score > 1: 
        print('At least something. ') 
        print('Maybe next time. ')

print_score('Your', 256) 
print_score('Denis', 5)

When you call a function, the arguments you write in parentheses are assigned to the corresponding variables in the function definition's parentheses. So when you call our new function with print_score('Your', 256), imagine that, internally, it assigns the values like this:

name = 'Your'
score = 256

print (name, 'score is', score) 
if score > 1000:
    ... #etc.


The return command terminates the function and returns the calculated value out of the function. You can use this command only in functions.

It behaves similar to the break command that terminates loops.

def yes_or_no(question):
    "Returns True or False, depending on the user's answers."
    while True:
        answer = input(question)
        if answer == 'yes':
            return True
        elif answer == 'no':
            return False
            print('What do you want!! Just  type "Yes" or "No".')

if yes_or_no('Do you want to play a game?'):
    print('OK, but you have to program it first.')
    print('That is sad.' )

Same as if and break, return is a command, not a function. That's why return has no parentheses after it.

Try to write a function that returns the area of an ellipse with given dimensions. The formula is A = πab, where a and b are the lengths of the axes. Then call the function and print the result.


Return or print?

The last program could be also written like that:

from math import pi

def ellipse(a, b): 
    print('The area is', pi * a * b) # Caution, 'print' instead of 'return'!

ellipse(3, 5)

The program works this way, too. But it loses one of the main advantages that functions have - when you want to use the value differently than to print it.

A function that returns its result can be used as part of other calculations:

def elliptical_cylinder(a, b, hight):
    return ellipse(a, b) * hight

print(elliptical_cylinder(3, 5, 3))

But if our ellipse function just printed the result, we wouldn't be able to calculate the area of elliptical cylinder this way.

The reason why return is better than print is that a function can be re-used in many different situations. When we don't actually want to know the intermediate results, we can't use functions with print.

Using return, we can re-use the same function, for example, in graphic games, web applications, or even to control a robot.

It is similar with input: If I hardcoded input into a function, I could use it only in situations where there's a user with keyboard present. That's why it's always better to pass arguments to a function, and call input outside of the function:

from  math import pi

def ellipse(a, b): 
    """This reusable function returns only the result - the ellipse's area with a and b axes"""
    #This is only the calculation
    return pi * a * b

#print and input are "outside" the reusable function!
x = input('Enter length of 1st axis: ')
y = input('Enter length of 2nd axis: ')
print('The ellipsis area is', ellipse(x, y),'cm2.')

There are of course exceptions: A function that directly generates a text can be written with print, or a function that processes text information. But when the function calculates something it's better to not have print and input inside it.


When the function does not end with an explicit return, the value that it returns is automatically None.

None is a value that is already "inside" Python (same as True and False). It's literally "none, nothing".

def nothing():
    "This function isn't doing anything."


Local variables

Congratulations! You can now define your own functions! Now we have to explain what local and global variables are.

A function can use variables from "outside":

pi = 3.1415926  # a variable defined outside the function

def circle_area(radius):
    return pi * radius ** 2


But every variable and argument that is defined within the function body are brand new and they share nothing with "outside" code.

Variables that are defined inside a function body are local variables, because they work only locally inside the function. For example, the following won't work how you would expect:

x = 0  # Assign value to global variable x

def set_x(value):
    x = value  # Assign value to local variable x


Variables that are not local are global variables - they exist throughout the whole program. But if a function defines a local variable with the same name, this local variable will only
have the value that was assigned within the function.

Let's look at an example. Before you run the next program, try to guess how it will behave. Then run it, and if it did something different than you expected, try to explain why. There is a catch! :)

from math import pi
area = 0
a = 30

def ellipse_area(a, b):
    area = pi * a * b  # Assign value to 'area`
    a = a + 3  # Assign value to 'a`
    return area

print(ellipse_area(a, 20))

Now try to answer the following questions:

  • Is the variable pi local or global?
  • Is the variable area local or global?
  • Is the variable a local or global?
  • Is the variable b local or global?


If it seems confusing and complicated just avoid naming variables (and function's arguments) within a function the same as those outside.

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