Automate the Boring Stuff with Python
Page 5
You can see that a while statement looks similar to an if statement. The difference is in how they behave. At the end of an if clause, the program execution continues after the if statement. But at the end of a while clause, the program execution jumps back to the start of the while statement. The while clause is often called the while loop or just the loop.
Let’s look at an if statement and a while loop that use the same condition and take the same actions based on that condition. Here is the code with an if statement:
spam = 0 if spam < 5: print('Hello, world.') spam = spam + 1
Here is the code with a while statement:
spam = 0 while spam < 5: print('Hello, world.') spam = spam + 1
These statements are similar—both if and while check the value of spam, and if it’s less than five, they print a message. But when you run these two code snippets, something very different happens for each one. For the if statement, the output is simply "Hello, world.". But for the while statement, it’s "Hello, world." repeated five times! Take a look at the flowcharts for these two pieces of code, Figure 2-9 and Figure 2-10, to see why this happens.
Figure 2-9. The flowchart for the if statement code
Figure 2-10. The flowchart for the while statement code
The code with the if statement checks the condition, and it prints Hello, world. only once if that condition is true. The code with the while loop, on the other hand, will print it five times. It stops after five prints because the integer in spam is incremented by one at the end of each loop iteration, which means that the loop will execute five times before spam < 5 is False.
In the while loop, the condition is always checked at the start of each iteration (that is, each time the loop is executed). If the condition is True, then the clause is executed, and afterward, the condition is checked again. The first time the condition is found to be False, the while clause is skipped.
An Annoying while Loop
Here’s a small example program that will keep asking you to type, literally, your name. Select File▸New Window to open a new file editor window, enter the following code, and save the file as yourName.py:
➊ name = '' ➋ while name != 'your name': print('Please type your name.') ➌ name = input() ➍ print('Thank you!')
First, the program sets the name variable ➊ to an empty string. This is so that the name != 'your name' condition will evaluate to True and the program execution will enter the while loop’s clause ➋.
The code inside this clause asks the user to type their name, which is assigned to the name variable ➌. Since this is the last line of the block, the execution moves back to the start of the while loop and reevaluates the condition. If the value in name is not equal to the string 'your name', then the condition is True, and the execution enters the while clause again.
But once the user types your name, the condition of the while loop will be 'your name' != 'your name', which evaluates to False. The condition is now False, and instead of the program execution reentering the while loop’s clause, it skips past it and continues running the rest of the program ➍. Figure 2-11 shows a flowchart for the yourName.py program.
Figure 2-11. A flowchart of the yourName.py program
Now, let’s see yourName.py in action. Press F5 to run it, and enter something other than your name a few times before you give the program what it wants.
Please type your name. Al Please type your name. Albert Please type your name. %#@#%*(^&!!! Please type your name. your name Thank you!
If you never enter your name, then the while loop’s condition will never be False, and the program will just keep asking forever. Here, the input() call lets the user enter the right string to make the program move on. In other programs, the condition might never actually change, and that can be a problem. Let’s look at how you can break out of a while loop.
break Statements
There is a shortcut to getting the program execution to break out of a while loop’s clause early. If the execution reaches a break statement, it immediately exits the while loop’s clause. In code, a break statement simply contains the break keyword.
Pretty simple, right? Here’s a program that does the same thing as the previous program, but it uses a break statement to escape the loop. Enter the following code, and save the file as yourName2.py:
➊ while True: print('Please type your name.') ➋ name = input() ➌ if name == 'your name': ➍ break ➎ print('Thank you!')
The first line ➊ creates an infinite loop; it is a while loop whose condition is always True. (The expression True, after all, always evaluates down to the value True.) The program execution will always enter the loop and will exit it only when a break statement is executed. (An infinite loop that never exits is a common programming bug.)
Just like before, this program asks the user to type your name ➋. Now, however, while the execution is still inside the while loop, an if statement gets executed ➌ to check whether name is equal to your name. If this condition is True, the break statement is run ➍, and the execution moves out of the loop to print('Thank you!') ➎. Otherwise, the if statement’s clause with the break statement is skipped, which puts the execution at the end of the while loop. At this point, the program execution jumps back to the start of the while statement ➊ to recheck the condition. Since this condition is merely the True Boolean value, the execution enters the loop to ask the user to type your name again. See Figure 2-12 for the flowchart of this program.
Run yourName2.py, and enter the same text you entered for yourName.py. The rewritten program should respond in the same way as the original.
Figure 2-12. The flowchart for the yourName2.py program with an infinite loop. Note that the X path will logically never happen because the loop condition is always True.
continue Statements
Like break statements, continue statements are used inside loops. When the program execution reaches a continue statement, the program execution immediately jumps back to the start of the loop and reevaluates the loop’s condition. (This is also what happens when the execution reaches the end of the loop.)
Trapped in an Infinite Loop?
If you ever run a program that has a bug causing it to get stuck in an infinite loop, press CTRL-C. This will send a KeyboardInterrupt error to your program and cause it to stop immediately. To try it, create a simple infinite loop in the file editor, and save it as infiniteloop.py.
while True: print('Hello world!')
When you run this program, it will print Hello world! to the screen forever, because the while statement’s condition is always True. In IDLE’s interactive shell window, there are only two ways to stop this program: press CTRL-C or select Shell ▸ restart Shell from the menu. CTRL-C is handy if you ever want to terminate your program immediately, even if it’s not stuck in an infinite loop.
Let’s use continue to write a program that asks for a name and password. Enter the following code into a new file editor window and save the program as swordfish.py.
while True: print('Who are you?') name = input() ➊ if name != 'Joe': ➋ continue print('Hello, Joe. What is the password? (It is a fish.)') ➌ password = input() if password == 'swordfish': ➍ break ➎ print('Access granted.')
If the user enters any name besides Joe ➊, the continue statement ➋ causes the program execution to jump back to the start of the loop. When it reevaluates the condition, the execution will always enter the loop, since the condition is simply the value True. Once they make it past that if statement, the user is asked for a password ➌. If the password entered is swordfish, then the break statement ➍ is run, and the execution jumps out of the while loop to print Access granted ➎. Otherwise, the execution continues to the end of the while loop, where it then jumps back to the start of the loop. See Figure 2-13 for this program’s flowchart.
Figure 2-13. A flowchart for swordfish.py. The X path will logically never happen because the loop condition is always True.
“Truthy” and “Falsey” V
alues
There are some values in other data types that conditions will consider equivalent to True and False. When used in conditions, 0, 0.0, and '' (the empty string) are considered False, while all other values are considered True. For example, look at the following program:
name = '' while not name:➊ print('Enter your name:') name = input() print('How many guests will you have?') numOfGuests = int(input()) if numOfGuests:➋ print('Be sure to have enough room for all your guests.')➌ print('Done')
If the user enters a blank string for name, then the while statement’s condition will be True ➊, and the program continues to ask for a name. If the value for numOfGuests is not 0 ➋, then the condition is considered to be True, and the program will print a reminder for the user ➌.
You could have typed not name != '' instead of not name, and numOfGuests != 0 instead of numOfGuests, but using the truthy and falsey values can make your code easier to read.
Run this program and give it some input. Until you claim to be Joe, it shouldn’t ask for a password, and once you enter the correct password, it should exit.
Who are you? I'm fine, thanks. Who are you? Who are you? Joe Hello, Joe. What is the password? (It is a fish.) Mary Who are you? Joe Hello, Joe. What is the password? (It is a fish.) swordfish Access granted.
for Loops and the range() Function
The while loop keeps looping while its condition is True (which is the reason for its name), but what if you want to execute a block of code only a certain number of times? You can do this with a for loop statement and the range() function.
In code, a for statement looks something like for i in range(5): and always includes the following:
The for keyword
A variable name
The in keyword
A call to the range() method with up to three integers passed to it
A colon
Starting on the next line, an indented block of code (called the for clause)
Let’s create a new program called fiveTimes.py to help you see a for loop in action.
print('My name is') for i in range(5): print('Jimmy Five Times (' + str(i) + ')')
The code in the for loop’s clause is run five times. The first time it is run, the variable i is set to 0. The print() call in the clause will print Jimmy Five Times (0). After Python finishes an iteration through all the code inside the for loop’s clause, the execution goes back to the top of the loop, and the for statement increments i by one. This is why range(5) results in five iterations through the clause, with i being set to 0, then 1, then 2, then 3, and then 4. The variable i will go up to, but will not include, the integer passed to range(). Figure 2-14 shows a flowchart for the fiveTimes.py program.
Figure 2-14. The flowchart for fiveTimes.py
When you run this program, it should print Jimmy Five Times followed by the value of i five times before leaving the for loop.
My name is Jimmy Five Times (0) Jimmy Five Times (1) Jimmy Five Times (2) Jimmy Five Times (3) Jimmy Five Times (4)
Note
You can use break and continue statements inside for loops as well. The continue statement will continue to the next value of the for loop’s counter, as if the program execution had reached the end of the loop and returned to the start. In fact, you can use continue and break statements only inside while and for loops. If you try to use these statements elsewhere, Python will give you an error.
As another for loop example, consider this story about the mathematician Karl Friedrich Gauss. When Gauss was a boy, a teacher wanted to give the class some busywork. The teacher told them to add up all the numbers from 0 to 100. Young Gauss came up with a clever trick to figure out the answer in a few seconds, but you can write a Python program with a for loop to do this calculation for you.
➊ total = 0 ➋ for num in range(101): ➌ total = total + num ➍ print(total)
The result should be 5,050. When the program first starts, the total variable is set to 0 ➊. The for loop ➋ then executes total = total + num ➌ 100 times. By the time the loop has finished all of its 100 iterations, every integer from 0 to 100 will have been added to total. At this point, total is printed to the screen ➍. Even on the slowest computers, this program takes less than a second to complete.
(Young Gauss figured out that there were 50 pairs of numbers that added up to 100: 1 + 99, 2 + 98, 3 + 97, and so on, until 49 + 51. Since 50 × 100 is 5,000, when you add that middle 50, the sum of all the numbers from 0 to 100 is 5,050. Clever kid!)
An Equivalent while Loop
You can actually use a while loop to do the same thing as a for loop; for loops are just more concise. Let’s rewrite fiveTimes.py to use a while loop equivalent of a for loop.
print('My name is') i = 0 while i < 5: print('Jimmy Five Times (' + str(i) + ')') i = i + 1
If you run this program, the output should look the same as the fiveTimes.py program, which uses a for loop.
The Starting, Stopping, and Stepping Arguments to range()
Some functions can be called with multiple arguments separated by a comma, and range() is one of them. This lets you change the integer passed to range() to follow any sequence of integers, including starting at a number other than zero.
for i in range(12, 16): print(i)
The first argument will be where the for loop’s variable starts, and the second argument will be up to, but not including, the number to stop at.
12 13 14 15
The range() function can also be called with three arguments. The first two arguments will be the start and stop values, and the third will be the step argument. The step is the amount that the variable is increased by after each iteration.
for i in range(0, 10, 2): print(i)
So calling range(0, 10, 2) will count from zero to eight by intervals of two.
0 2 4 6 8
The range() function is flexible in the sequence of numbers it produces for for loops. For example (I never apologize for my puns), you can even use a negative number for the step argument to make the for loop count down instead of up.
for i in range(5, -1, -1): print(i)
Running a for loop to print i with range(5, -1, -1) should print from five down to zero.
5 4 3 2 1 0
Importing Modules
All Python programs can call a basic set of functions called built-in functions, including the print(), input(), and len() functions you’ve seen before. Python also comes with a set of modules called the standard library. Each module is a Python program that contains a related group of functions that can be embedded in your programs. For example, the math module has mathematics-related functions, the random module has random number–related functions, and so on.
Before you can use the functions in a module, you must import the module with an import statement. In code, an import statement consists of the following:
The import keyword
The name of the module
Optionally, more module names, as long as they are separated by commas
Once you import a module, you can use all the cool functions of that module. Let’s give it a try with the random module, which will give us access to the random.randint() function.
Enter this code into the file editor, and save it as printRandom.py:
import random for i in range(5): print(random.randint(1, 10))
When you run this program, the output will look something like this:
4 1 8 4 1
The random.randint() function call evaluates to a random integer value between the two integers that you pass it. Since randint() is in the random module, you must first type random. in front of the function name to tell Python to look for this function inside the random module.
Here’s an example of an import statement that imports four different modules:
import random, sys, os, math
Now we can use any of the functions in these four modules. We’ll learn more about them later in the book.
from import Statements
&nb
sp; An alternative form of the import statement is composed of the from keyword, followed by the module name, the import keyword, and a star; for example, from random import *.
With this form of import statement, calls to functions in random will not need the random. prefix. However, using the full name makes for more readable code, so it is better to use the normal form of the import statement.
Ending a Program Early with sys.exit()
The last flow control concept to cover is how to terminate the program. This always happens if the program execution reaches the bottom of the instructions. However, you can cause the program to terminate, or exit, by calling the sys.exit() function. Since this function is in the sys module, you have to import sys before your program can use it.
Open a new file editor window and enter the following code, saving it as exitExample.py:
import sys while True: print('Type exit to exit.') response = input() if response == 'exit': sys.exit() print('You typed ' + response + '.')
Run this program in IDLE. This program has an infinite loop with no break statement inside. The only way this program will end is if the user enters exit, causing sys.exit() to be called. When response is equal to exit, the program ends. Since the response variable is set by the input() function, the user must enter exit in order to stop the program.