Introduction to Python Programming for Dummies

python programming language

Introduction to Python Programming for Dummies is my efforts towards breaking down for our students, computer programming using Python language. In this lecture material, you will find out the following:

  • History, audience and prerequisite for programming with the python language.
  • Features and characteristics of the python language.
  • Python environment set-up, getting python interpreter and installing python into your machines; Unix and Linux Installation, Windows installation and Macintosh installation.
  • Setting Path at Windows, Unix and Linus.
  • Running python programs.

NOTE: This Lecture notes will be regularly updated as our classes continue. So check back as often as possible to get updated lecture materials.

References:

  1. https://www.tutorialspoint.com/python/index.htm
  2. http://help.uis.cam.ac.uk/help-support/training/downloads/course-files/programming-student-files/python-courses/pythonab/pythonab-files/python3-notes.pdf
  3. http://anh.cs.luc.edu/331/notes/PythonBasics.pdf

 

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Lesson 1

History of Python

Python is said to be a general-purpose interpreted, interactive, object-oriented, and high-level programming language. It was created by Guido van Rossum during 1985- 1990 at the National Research Institute for Mathematics and Computer Science in the Netherlands. Like Perl, Python source code is also available under the GNU General Public License (GPL). This tutorial gives enough understanding on Python programming language.

Python is derived from many other languages, including ABC, Modula-3, C, C++, Algol-68, SmallTalk, and Unix shell and other scripting languages.

Python is copyrighted. Like Perl, Python source code is now available under the GNU General Public License (GPL).

Python is now maintained by a core development team at the institute, although Guido van Rossum still holds a vital role in directing its progress.

Audience

This tutorial is designed for software programmers who need to learn Python programming language from scratch.

Prerequisites
You should have a basic understanding of Computer Programming terminologies. A basic understanding of any of the programming languages is a plus.

Characteristics of Python:
Python is a high-level, interpreted, interactive and object-oriented scripting language. Python is designed to be highly readable. It uses English keywords frequently where as other languages use punctuation, and it has fewer syntactical constructions than other languages.

  • Python is Interpreted: Python is processed at runtime by the interpreter. You do not need to compile your program before executing it. This is similar to PERL and PHP.
  • Python is Interactive: You can actually sit at a Python prompt and interact with the interpreter directly to write your programs.
  • Python is Object-Oriented: Python supports Object-Oriented style or technique of programming that encapsulates code within objects.
  • Python is a Beginner’s Language: Python is a great language for the beginner-level programmers and supports the development of a wide range of applications from simple text processing to WWW browsers to games.

 

Features of Python:

Python’s features include:

  • Easy-to-learn: Python has few keywords, simple structure, and a clearly defined syntax. This allows the student to pick up the language quickly.
  • Easy-to-read: Python code is more clearly defined and visible to the eyes.
  • Easy-to-maintain: Python’s source code is fairly easy-to-maintain.
  • A broad standard library: Python’s bulk of the library is very portable and cross-platform compatible on UNIX, Windows, and Macintosh.
  • Interactive Mode: Python has support for an interactive mode which allows interactive testing and debugging of snippets of code.
  • Portable: Python can run on a wide variety of hardware platforms and has the same interface on all platforms.
  • Extendable: You can add low-level modules to the Python interpreter. These modules enable programmers to add to or customize their tools to be more efficient.
  • Databases: Python provides interfaces to all major commercial databases.
  • GUI Programming: Python supports GUI applications that can be created and ported to many system calls, libraries and windows systems, such as Windows MFC, Macintosh, and the X Window system of Unix.
  • Scalable: Python provides a better structure and support for large programs than shell scripting.

Apart from the above-mentioned features, Python has a big list of good features, few are listed below:

  • It supports functional and structured programming methods as well as OOP.
  • It can be used as a scripting language or can be compiled to byte-code for building large applications.
  • It provides very high-level dynamic data types and supports dynamic type checking.
  • IT supports automatic garbage collection.
  • It can be easily integrated with C, C++, COM, ActiveX, CORBA, and Java.

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Lesson 2

Installation and documentation

If you use Mac OS X or Linux, then Python should already be installed on your computer by
default. Windows users can download the latest version by visiting the Python home page, at
http://www.python.org.
Here, you will also find loads of documentation and other useful information.

Invoking your Python program:

After installation, you need to go to the Disk Operating System DOS Command prompt. This is where you’ll execute your Python programs. Do the following:

Click Start button >> Select “All Programs” >> Click “Accessories” >> Select “Command Prompt” and a black screen of DOS is opened for you. python programAt this point, enter this command: CD\ and press the enter key.
This takes you to the root directory of your system; where the Python interpreter is installed. This is important so that you will be able to correctly call up the interpreter to execute any of your programs.

Python Environment Setup : Setting up PATH

Programs and other executable files can be in many directories, so operating systems provide a search path that lists the directories that the OS searches for executables.

The path is stored in an environment variable, which is a named string maintained by the operating system. This variable contains information available to the command shell and other programs. The path variable is named as PATH in Unix or Path in Windows (Unix is casesensitive; Windows is not).

Setting path at Windows

To add the Python directory to the path for a particular session in Windows −

At the command prompt − type path %path%;C:\Python and press Enter.

Note − C:\Python is the path of the Python directory. The Path inside “” refers to the address of the folder where your python and other files are stored.

Python Basic Syntax

First Python Program

Let us execute programs in different modes of programming.

Interactive Mode Programming

Invoking the interpreter without passing a script file as a parameter brings up the following prompt −

python
Python 2.4.3 (#1, Nov 11 2010, 13:34:43)
[GCC 4.1.2 20080704 (Red Hat 4.1.2-48)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>>

Type the following text at the Python prompt and press the Enter:

>>> print "Hello, Python!"

Python Basic Syntax 

If you are running new version of Python, then you would need to use print statement with parenthesis as in print (“Hello, Python!”);. However in Python version 2.4.3, this produces the following result:

Hello, Python!

Script Mode Programming

Invoking the interpreter with a script parameter begins execution of the script and continues until the script is finished. When the script is finished, the interpreter is no longer active.

Let us write a simple Python program in a script. Python files have extension .py. Type the following source code in a test.py file:

print "Hello, Python!"

We assume that you have Python interpreter set in PATH variable. Now, try to run this program as follows −

$ python test.py

This produces the following result:

Hello, Python!

Let us try another way to execute a Python script. Here is the modified test.py file −

#!/usr/bin/python

print "Hello, Python!"

We assume that you have Python interpreter available in /usr/bin directory. Now, try to run this program as follows −

$ chmod +x test.py     # This is to make file executable
$./test.py

This produces the following result −

Hello, Python!

Python Identifiers

A Python identifier is a name used to identify a variable, function, class, module or other object. An identifier starts with a letter A to Z or a to z or an underscore (_) followed by zero or more letters, underscores and digits (0 to 9).

Python does not allow punctuation characters such as @, $, and % within identifiers. Python is a case sensitive programming language. Thus, Manpower and manpower are two different identifiers in Python.

Here are naming conventions for Python identifiers −

  • Class names start with an uppercase letter. All other identifiers start with a lowercase letter.
  • Starting an identifier with a single leading underscore indicates that the identifier is private.
  • Starting an identifier with two leading underscores indicates a strongly private identifier.
  • If the identifier also ends with two trailing underscores, the identifier is a language-defined special name.

Reserved Words

The following list shows the Python keywords. These are reserved words and you cannot use them as constant or variable or any other identifier names. All the Python keywords contain lowercase letters only.

and exec not
assert finally or
break for pass
class from print
continue global raise
def if return
del import try
elif in while
else is with
except lambda yield

Lines and Indentation

Python provides no braces to indicate blocks of code for class and function definitions or flow control. Blocks of code are denoted by line indentation, which is rigidly enforced.

The number of spaces in the indentation is variable, but all statements within the block must be indented the same amount. For example −

if True:
    print "True"
else:
  print "False"

However, the following block generates an error −

if True:
    print "Answer"
    print "True"
else:
    print "Answer"
  print "False"

Thus, in Python all the continuous lines indented with same number of spaces would form a block. The following example has various statement blocks −

Note: Do not try to understand the logic at this point of time. Just make sure you understood various blocks even if they are without braces.

#!/usr/bin/python

import sys

try:
  # open file stream
  file = open(file_name, "w")
except IOError:
  print "There was an error writing to", file_name
  sys.exit()
print "Enter '", file_finish,
print "' When finished"
while file_text != file_finish:
  file_text = raw_input("Enter text: ")
  if file_text == file_finish:
    # close the file
    file.close
    break
  file.write(file_text)
  file.write("\n")
file.close()
file_name = raw_input("Enter filename: ")
if len(file_name) == 0:
  print "Next time please enter something"
  sys.exit()
try:
  file = open(file_name, "r")
except IOError:
  print "There was an error reading file"
  sys.exit()
file_text = file.read()
file.close()
print file_text

Multi-Line Statements

Statements in Python typically end with a new line. Python does, however, allow the use of the line continuation character (\) to denote that the line should continue. For example −

total = item_one + \
        item_two + \
        item_three

Statements contained within the [], {}, or () brackets do not need to use the line continuation character. For example −

days = ['Monday', 'Tuesday', 'Wednesday',
        'Thursday', 'Friday']

Quotation in Python

Python accepts single (‘), double (“) and triple (”’ or “””) quotes to denote string literals, as long as the same type of quote starts and ends the string.

The triple quotes are used to span the string across multiple lines. For example, all the following are legal −

word = 'word'
sentence = "This is a sentence."
paragraph = """This is a paragraph. It is
made up of multiple lines and sentences."""

Comments in Python

A hash sign (#) that is not inside a string literal begins a comment. All characters after the # and up to the end of the physical line are part of the comment and the Python interpreter ignores them.

#!/usr/bin/python

# First comment
print "Hello, Python!" # second comment

This produces the following result −

Hello, Python!

You can type a comment on the same line after a statement or expression −

name = "Madisetti" # This is again comment

You can comment multiple lines as follows −

# This is a comment.
# This is a comment, too.
# This is a comment, too.
# I said that already.

Using Blank Lines

A line containing only whitespace, possibly with a comment, is known as a blank line and Python totally ignores it.

In an interactive interpreter session, you must enter an empty physical line to terminate a multiline statement.

Waiting for the User

The following line of the program displays the prompt, the statement saying “Press the enter key to exit”, and waits for the user to take action −

#!/usr/bin/python

raw_input("\n\nPress the enter key to exit.")

Here, “\n\n” is used to create two new lines before displaying the actual line. Once the user presses the key, the program ends. This is a nice trick to keep a console window open until the user is done with an application.

Multiple Statements on a Single Line

The semicolon ( ; ) allows multiple statements on the single line given that neither statement starts a new code block. Here is a sample snip using the semicolon −

import sys; x = 'foo'; sys.stdout.write(x + '\n')

Multiple Statement Groups as Suites

A group of individual statements, which make a single code block are called suites in Python. Compound or complex statements, such as if, while, def, and class require a header line and a suite.

Header lines begin the statement (with the keyword) and terminate with a colon ( : ) and are followed by one or more lines which make up the suite. For example −

if expression : 
   suite
elif expression : 
   suite 
else : 
   suite

Command Line Arguments

Many programs can be run to provide you with some basic information about how they should be run. Python enables you to do this with -h −

$ python -h
usage: python [option] ... [-c cmd | -m mod | file | -] [arg] ...
Options and arguments (and corresponding environment variables):
-c cmd : program passed in as string (terminates option list)
-d     : debug output from parser (also PYTHONDEBUG=x)
-E     : ignore environment variables (such as PYTHONPATH)
-h     : print this help message and exit

[ etc. ]

You can also program your script in such a way that it should accept various options. Command Line Arguments is an advanced topic and should be studied a bit later once you have gone through rest of the Python concepts.

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Lesson 3