Modules

Python comes with many built-in modules.

A module is imported using the import statement.

The following example imports the time module and prints the current time using the asctime function defined in that module.

import time
print(time.asctime())

The os module provides functions related to files and processes.

The following example prints the current working directory.

import os
print(os.getcwd())

The listdir funtion lists all the files in a given directory.

import os

print(os.listdir("."))

Python comes with many built-in modules. We'll explore lot more modules in the upcoming lessons.

Reading Command-line arguments

The sys module keeps trak of the command-line arguments passed to a program.

import sys
print(sys.argv)

The above example prints all the command-line arguments passed to the program. As you can see the sys.argv will always be a list with the program name as first argument, followed by all the arguments passed to it.

Please note that the elements of sys.argv will always be strings.

Try changing the arguments to something else.

Example: echo

Let's try to implement a simple version of the echo command of unix.

The echo command prints the command-line arguments passed to it.

$ echo hello
hello

$ echo hello world
hello world

We'll implement a simple version that just prints the first argument.

=== echo.py
import sys
print(sys.argv[1])

Command-line arguments are most common way to take user inputs. We are going to use this a lot more in the examples of upcoming lessons.

Problem: Write a program square.py that takes a number as command-line argument and prints its square.

$ python square.py 2
4
$ python square.py 5
25