One way to "reuse" code is by creating a for loop or a while loop. However, Python has two more ways: functions and classes. You'll learn about functions here. Guess what? You've already learnt the necessary precursors, so we can jump right in!
Let's say that we need to have a program that asks, say, 5 people for their names, displays different messages for each, and adds them to a list. We could do this with copy/paste:
firstNames = []
lastNames = []
first = input("Person 1's First Name:")
last = input("Person 1's Last Name:")
print("So great to see you, %s %s!" %(first,last))
firstNames.append(first)
lastNames.append(last)
first = input("Person 2's First Name:")
last = input("Person 2's Last Name:")
print("Go %s %s!" %(first,last))
firstNames.append(first)
lastNames.append(last)
first = input("Person 3's First Name:")
last = input("Person 3's Last Name:")
print("%s %s, you're awesome!" %(first,last))
firstNames.append(first)
lastNames.append(last)
first = input("Person 4's First Name:")
last = input("Person 4's Last Name:")
print("Woohoo for %s %s!" %(first,last))
firstNames.append(first)
lastNames.append(last)
first = input("Person 5's First Name:")
last = input("Person 5's Last Name:")
print("You're awesome, %s %s!" %(first,last))
firstNames.append(first)
lastNames.append(last)
That's reeeeeeeeeeeeeeeeeeeeeeeally long, 32 lines. Here's a shorter method:
num = 0
def hi(message):
num = num + 1
first = input("Person %s's First Name:" %(num))
last = input("Person %s's Last Name:" %(num))
print(message %(first,last))
firstNames.append(first)
lastNames.append(last)
hi("So great to see you, %s %s!")
hi("Go %s %s!")
hi("%s %s, you're so awesome!")
hi("Woohoo for %s %s!")
hi("You're awesome, %s %s!")
This is only 13 lines, a huge improvement. You'll learn how to do this right below.
The format of a function is simple:
def name(params):
code...
code...
For example, let's make a simple function that prints your name. We'll call it hello. It should be able to take your first and last name. Here it is:
def hello(firstname,lastname):
print("Hello, %s %s!" %(firstname,lastname))
Let's test our new function.
def hello(firstname,lastname):
print("Hello, %s %s!" %(firstname,lastname))
hello("Bob","Junior") # This should print "Hello, Bob Junior!".
Wow! But what if we want to just provide one name, or even none? What should we do? If we run our current function with zero or one parameter(s), Python just gives us an error!
Fortunately, Python has a solution called optional arguments. We can provide default values that the user can override by typing them when the user calls the function.
Let's look at our hello function's first line, the line with def. It looks like this:
def hello(firstname,lastname):
firstname and lastname are both parameters of the function. We can make them optional parameters by providing a default value:
def hello(firstname,lastname = None):
Did you notice that the lastname parameter now has an = None after it? This tells Python to make it an optional parameter. Of course, we'll need to alter our function a bit, unless we want to display Hello, Bob None! Here's our new function:
def hello(firstname,lastname = None):
if lastname == None:
print("Hello, %s!" %(firstname))
else:
print("Hello, %s %s! %(firstname,lastname))
This function checks whether we provided a lastname. If we didn't, it just shows the first name.
What if we want our hello function to give us the string that we need to print? Python lets us do that with the return keyword. Take a look at the code below:
def hello(firstname,lastname = None):
if lastname == None:
return "Hello, " + str(firstname) + "!"
else:
return "Hello, " + str(firstname) + " " + str(lastname) + "!"
The return command tell Python to exit the function. If we provide the return command with something, Python has the function return the value, meaning that the function is "replaced" with the value.
Running the code below will get the string from hello and print it:
print(hello("Bob","Junior"))
You can pass functions as arguments in Python by writing them as a parameter without parentheses. For example, look at this code:
def a():
pass
def b(f):
f()
b(a)
We define a function a, which takes no parameters, and a function b, which takes one function as a parameter. We then run b(a), calling b on our function a.
You might want to make a function that randomly runs one of two functions. For example, either:
print("Adding 2 to a")
a = a + 2
or:
a = a - 5
print("Subtracting 5 from a")
We could do this with an if block, but we want to try making a function that does this for us. Here's how:
01 from random import randint
02 a = 0
03
04 def add():
05 print("Adding 2 to a")
06 a = a + 2
07
08 def subtract():
09 a = a - 5
10 print("Subtracting 5 from a")
11
12 def random(f1,f2):
13 if randint(0,1) == 0:
14 f1()
15 else:
16 f2()
17
18 random(add,subtract)
Let's explain what this code means:
Line 1 imports randint from the random module.
Line 2 creates the variable a.
Lines 4 - 6 define the function add(), which:
Line 5 prints "Adding 2 to a"
Line 6 adds 2 to a.
Lines 8 - 10 define the function subtract(), which:
Line 9 subtracts 5 from a.
Line 10 prints "Subtracting 5 from a".
Lines 12 - 16 define the function random(f1,f2), which:
Line 13 comes up with a random number between 0 and 1. If the number is equal to 0:
Line 14 runs the first function given.
Otherwise:
Line 16 runs the second function given.
Line 18 then calls random on our functions add and subtract.
Create a function that asks the user for their age, then prints "You are (age) years old."
Adjust the function you made in challenge 1 to return the string instead of printing the string.
Adjust the function you made in challenge 2 to accept age as a parameter instead of using input().
Adjust the function you made in challenge 3 to use input() if no age is given.
Params: Short for parameters.
Args (*)
Kwargs (**)
Create a function that takes unlimited arguments, then prints each argument given.
Create a function that takes unlimited KEYWORDED arguments, then prints each key=value pair given.
Read the section on using events with turtle.
