Python Quick Guide

Python Overview:

Python is a high-level, interpreted, interactive and object oriented-scripting language.

• Python is Interpreted
• Python is Interactive
• Python is Object-Oriented
• Python is Beginner's Language
  

Python was developed by Guido van Rossum in the late eighties and early nineties at the National Research Institute for Mathematics and Computer Science in the Netherlands.
Python's feature highlights include:

• Easy-to-learn
• Easy-to-read
• Easy-to-maintain
• A broad standard library
• Interactive Mode
• Portable
• Extendable
• Databases
• GUI Programming
• Scalable

Getting Python:

The most up-to-date and current source code, binaries, documentation, news, etc. is available at the official website of Python:
Python Official Website : https://www.python.org/
You can download the Python documentation from the following site. The documentation is available in HTML, PDF, and PostScript formats.
Python Documentation Website : www.python.org/doc/

First Python Program:

Interactive Mode Programming:

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

root# python
Python 2.5 (r25:51908, Nov  6 2007, 16:54:01)
[GCC 4.1.2 20070925 (Red Hat 4.1.2-27)] on linux2
Type "help", "copyright", "credits" or "license" for more info.
>>>

Type the following text to the right of the Python prompt and press the Enter key:

>>> print "Hello, Python!";

This will produce 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 following identifier naming convention for Python:

• Class names start with an uppercase letter and all other identifiers with a lowercase letter.
• Starting an identifier with a single leading underscore indicates by convention that the identifier is meant to be 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 reserved words in Python. These reserved words may not be used as constant or variable or any other identifier names.

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:

One of the first caveats programmers encounter when learning Python is the fact that there are 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. Both blocks in this example are fine:

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

However, the second block in this example will generate an error:

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

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 can be 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 physical line end are part of the comment, and the Python interpreter ignores them.

#!/usr/bin/python

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

This will produce following result:

Hello, Python!

A comment may be 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.

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:

Groups of individual statements making up a single code block are called suites in Python.
Compound or complex statements, such as if, while, def, and class, are those which 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.

Example:

if expression : 
   suite
elif expression : 
   suite 
else : 
   suite

Python - Variable Types:

Variables are nothing but reserved memory locations to store values. This means that when you create a variable you reserve some space in memory.
Based on the data type of a variable, the interpreter allocates memory and decides what can be stored in the reserved memory. Therefore, by assigning different data types to variables, you can store integers, decimals, or characters in these variables.

Assigning Values to Variables:

The operand to the left of the = operator is the name of the variable, and the operand to the right of the = operator is the value stored in the variable. For example:

counter = 100          # An integer assignment
miles   = 1000.0       # A floating point
name    = "John"       # A string

print counter
print miles
print name

Standard Data Types:

Python has five standard data types:

• Numbers
• String
• List
• Tuple
• Dictionary

Python Numbers:

Number objects are created when you assign a value to them. For example:

var1 = 1
var2 = 10

Python supports four different numerical types:

• int (signed integers)
• long (long integers [can also be represented in octal and hexadecimal])
• float (floating point real values)
• complex (complex numbers)

Here are some examples of numbers:

int	long	float	complex
10	51924361L	0.0	3.14j
100	-0x19323L	15.20	45.j
-786	0122L	-21.9	9.322e-36j
080	0xDEFABCECBDAECBFBAEL	32.3+e18	.876j
-0490	535633629843L	-90.	-.6545+0J
-0x260	-052318172735L	-32.54e100	3e+26J
0x69	-4721885298529L	70.2-E12	4.53e-7j

Python Strings:

Strings in Python are identified as a contiguous set of characters in between quotation marks.

Example:

str = 'Hello World!'

print str          # Prints complete string
print str[0]       # Prints first character of the string
print str[2:5]     # Prints characters starting from 3rd to 6th
print str[2:]      # Prints string starting from 3rd character
print str * 2      # Prints string two times
print str + "TEST" # Prints concatenated string

Python Lists:

Lists are the most versatile of Python's compound data types. A list contains items separated by commas and enclosed within square brackets ([]).

#!/usr/bin/python

list = [ 'abcd', 786 , 2.23, 'john', 70.2 ]
tinylist = [123, 'john']

print list          # Prints complete list
print list[0]       # Prints first element of the list
print list[1:3]     # Prints elements starting from 2nd to 4th
print list[2:]      # Prints elements starting from 3rd element
print tinylist * 2  # Prints list two times
print list + tinylist # Prints concatenated lists

Python Tuples:

A tuple is another sequence data type that is similar to the list. A tuple consists of a number of values separated by commas. Unlike lists, however, tuples are enclosed within parentheses.
Tuples can be thought of as read-only lists.

tuple = ( 'abcd', 786 , 2.23, 'john', 70.2  )
tinytuple = (123, 'john')

print tuple           # Prints complete list
print tuple[0]        # Prints first element of the list
print tuple[1:3]      # Prints elements starting from 2nd to 4th
print tuple[2:]       # Prints elements starting from 3rd element
print tinytuple * 2   # Prints list two times
print tuple + tinytuple # Prints concatenated lists