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Introduction

From wikipedia

Python is a widely used high-level, general-purpose, interpreted, dynamic programming language. Its design philosophy emphasizes code readability, and its syntax allows programmers to express concepts in fewer lines of code than possible in languages such as C++ or Java. The language provides constructs intended to enable clear programs on both a small and large scale

Guido van Rossum is the author of Python programming language, and continues to oversee the Python development process


Installation

  • Get Python for your OS from official website - https://www.python.org/
    • Most Linux distributions come with Python by default
  • See also this guide for more detail as well as how to set up virtual environment, how to use pip (NEVER use sudo pip unless you know what you are doing)

  • Examples presented here is for Unix-like systems, Python version 3 and uses bash shell
  • You can also run Python code online
    • pythontutor - code execution in Python 2 and 3 versions, visualizing code flow and sample programs are among its features
    • jupyter - web application that allows you to create and share documents that contain live code, equations, visualizations and explanatory text
    • ideone - online compiler and debugging tool which allows you to compile source code and execute it online in more than 60 programming languages
    • Python Interpreter shell
  • It is assumed that you are familiar with command line. If not, check out this basic tutorial on ryanstutorials and this list of curated resources for Linux

Hello World example

Let's start with simple a Python program and how to run it

#!/usr/bin/python3

print("Hello World")

The first line has two parts

  • /usr/bin/python3 is the path of Python interpreter
  • #! called as shebang, directs the program loader to use the interpreter path provided

The third line prints the message Hello World with a newline character added by default by the print function

Running Python program

You can write the program using text editor like gedit, vim or other editors
After saving the file, give execute permission and run the program from a terminal

$ chmod +x hello_world.py

$ ./hello_world.py
Hello World

To find out path and version of Python in your system

$ type python3
python3 is /usr/bin/python3

$ python3 --version
Python 3.4.3

If you happen to follow a book/tutorial on Python version 2 or coming with Perl experience, it is a common mistake to forget () with print function

#!/usr/bin/python3

print "Have a nice day"
  • Depending on type of error, it may be easy to spot the mistake based on error messages printed on executing the program
  • In this example, we get the appropriate Missing parentheses message
$ ./syntax_error.py 
  File "./syntax_error.py", line 3
    print "Have a nice day"
                          ^
SyntaxError: Missing parentheses in call to 'print'
  • single line comments start with #
    • #! has special meaning only on first line of program
  • we will see multiline comments in later chapters
#!/usr/bin/python3

# Greeting message
print("Hello World")

Further Reading


Python Interpreter

  • It is generally used to execute snippets of Python language as a means to learning Python or for debugging purposes
  • The prompt is usually >>>
  • Some of the topics in coming chapters will be complemented with examples using the Python Interpreter
  • A special variable _ holds the result of last printed expression
  • One can type part of command and repeatedly press Up arrow key to match commands from history
  • Press Ctrl+l to clear the screen, keeping any typed command intact
  • exit() to exit
$ python3
Python 3.4.3 (default, Oct 14 2015, 20:28:29) 
[GCC 4.8.4] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> print("hi")
hi
>>> abc
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'abc' is not defined
>>> num = 5
>>> num
5
>>> 3 + 4
7
>>> 12 + _
19
>>> exit()

Further Reading


Python Standard Library

The library contains built-in modules (written in C) that provide access to system functionality such as file I/O that would otherwise be inaccessible to Python programmers, as well as modules written in Python that provide standardized solutions for many problems that occur in everyday programming.

Some of these modules are explicitly designed to encourage and enhance the portability of Python programs by abstracting away platform-specifics into platform-neutral APIs