Importing Modules

What are 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.

Importing Modules

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.

import random
for i in range(5):
    print(random.randint(1, 10))

When you save your Python scripts, take care not to give them a name that is used by one of Python’s modules, such as random.py, sys.py, os.py, or math.py.

When you run this program, the output will look something like this:

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.

`from import` Statements

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 import random form of the statement.

Ending a Program Early with the `sys.exit()` function

The last flow control concept to cover is how to terminate the program. Programs always terminate if the program execution reaches the bottom of the instructions. However, you can cause the program to terminate, or exit, before the last instruction 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.

import sys

while True:
    print('Type exit to exit.')
    response = input()
    if response == 'exit':
        sys.exit()
    print('You typed ' + response + '.')

print('This line will never be executed.')

Run this program and see how it works.

This program has an infinite loop with no break statement inside. The only way this program will end is if the execution reaches the sys.exit() call. When response is equal to exit, the line containing the sys.exit() call is executed. Since the response variable is set by the input() function, the user must enter exit in order to stop the program.

Importing Modules

What are Modules?​