Introduction to Python

Introduction to Python

Python was developed by Guido Van Rossum at the National Research Institute for Mathematics and Computer Science in Netherlands during 1985-1990. Python is derived from many other languages, including ABC, Modula-3,C,C++,Algol-68,SmallTalk,Unix shell and other scripting languages. Rossum was inspired by Monty Python's Flying Circus, a BBC comedy series and he wanted the name of his new language to be short, unique and mysterious. Hence he named it python. It is general-purpose interpreted, interactive, object-oriented, and high-level programming language. Python source code is available under the GNU General Public License(GPL) and is now maintained by a core development team at the National Research Institute.

Features of Python

5.      Scalable

6.      Extendable

7.      Dynamic

8.      GUI Programming and Database

9.      Broad Standard Library

1.      Simple and easy-to-learn

2.      Interpreted and interactive

3.      Object-oriented

4.      Portable

How to Run Python

There are three different ways to start python

a) Using Interactive Interpreter

            You can start python from Unix, DOS, or any other system that provides you a command-line interpreter or shell window. Get into command line of python. For Unix/Linux, you can get into interactive mode by typing $python or python%. For windows/DOS it is c:>python.

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

            $ python

     python 2.7.10 (default, sep 27 2015, 18:11:38)

     [GCI 5.1.1 20150422 (Red Hat 5.1.1-1)] on linux2

     Type "help", "copyright", "credits" or "license" for more

     information

     >>>

Type the following text at python prompt and press enter:

            >>>print("programming in python")

The result will be as given below

            programming in python

     >>>

b) Scripts From Command Line

This method invokes the interpreter with a script parameter which begins the execution of the script and continues until the script is finished. When the script is finished, the interpreter is no longer active. a python script can be executed at command line by invoking the interpreter on your application, as follows.

            for Unix/Linux $python script.py or python% script.py

     for windows/DOS c:>python script.py

c) Integrated Development Environment         

You can run python from Graphical User Interface(GUI) environment as well, if you have a GUI application on your system that supports python. IDLE is the integrated Development Environment(IDE) for UNIX and pythonWin is the first window interface for python.

Identifiers

A python identifier is a name used to identify a variable, function, class, module or any other object. Python is case sensitive and hence uppercase and lowercase letters are considered distinct. The following are rules for naming an identifier in python.

1.      Identifier can be combination of letters in lowercase(a to z) or uppercase(A to Z) or digit(0-9) or an underscore(_). For example total and Total is different.

2.      Reserved keywords cannot be used as an identifier.

3.      Identifiers cannot begin with a digit.

4.      Special symbols like !,@,#, etc cannot be used in an identifier. For example sum@ is an invalid identifier.

5.      Identifier can be of any length.

Reserved Keywords

These are keywords reserved by the programming language and prevent the user or the programmer from using it as it as an identifier in a program. There are 33 keywords in python 3.3. This number may vary with different versions. To retrieve the keywords in python the following code can be given at the prompt.

              >>> import keyword    

     >>> print(keyword.kwlist)          

 The following list in table shows the python keywords.

False	class	finally	is	return
None	continue	for	lambda	try
True	def	from	nonlocal	while
and	del	global	not	with
as	elif	if	or	yield
assert	else	import	pass	break
except	in	raise

Variables

Variables are reserved memory locations to store values. 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.

           Python variables do not need explicit declaration to reserve memory space. The declaration happens automatically when you assign a value to a variable. The equal sign(=) is used to assign 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 that variable.

Example program

a = 100

b = 1000

print(a)

print(b)

output

            100

     1000

Comments in Python

Comments are very important while writing a program. It describes what the source code has done. Comments are for programmers for better understanding of a program. In python , we use the hash(#) symbol to start writing a comment. All the character after the #and up to the end of the physical line are part of the comment.

Example program

     >>>#this is demo of comment

     >>>print("hello")

For multiline comments use triple quotes, either ''' or """. the statements within the start and end of triple quotes are considered as comment.

Indentation in Python

Most of the programming languages like c, c++ use braces{} to define a block of code. Python uses indentation. A code block starts with indentation and ends with the first unindented line. The amount of indentation can be decided by the programmer, but it ,must be consistent throughout the block. Generally four whitespaces are used for indentation.

Example program

            if True:

          print("correct")

     else:

          print("Wrong")

Multi-line statement

Instructions that a python interpreter can execute are called statements. In python, end of a statement is marked by a newline character, but we can make statement extend over multiple lines with the line continuation character(\).

           

grand_total = first_item +\

              second_item+\           

              third_item

Input, Output and Import Functions

Displaying the Output

The function used to print output on a screen is print statement where you can pass zero or more expression separated by commas. The print function converts the expression you pass into a string and writes the result to standard output.

Example program

     >>>a=2

     >>>print("the value of a is:",a)

     the value of a is 2

     >>>

By default a space is added after the text and before the value of variable a. The syntax of print function is given below

print *objects, sep=' ', end='\n', file=sys.stdout,flush=False

            Here objects are the value to be printed . sep is the separator used between the values. spaces character is the default separator. end is printed after  printing all the values. The default value of end is a newline. file is the object where the values are printed and its default value is sys.stdout(screen). Flush determines whether the output stream needs to be flushed for any waiting output.

Reading the Input

Python provides two built-in functions to read from standard input(keyboard). These functions are raw_input and input.

Example program

            str = input("enter your name:")

     print("your name is: ",str)

output

     enter your name: abc

     your name is: abc

Import function

When the program grows bigger or when there are segments of code that is frequently used, it can be stored in different modules. A module is a file containing python definition and statements. Python modules have a file name and end with expression .py. Definition inside a module can be imported to another module or the interactive interpreter in python. We use the import keyword to do this .

Example program

            >>>import math

     >>>math.pi

     3.141592653589793

Operators

Operators are the constructs which can manipulate the value of operands. Consider the expression a = b+c, here a, b and c are operands and +, = are operators. There are different types of operators.

1.      Arithmetic operators

2.      Comparison operators

3.      Assignment operators

4.      Logical operators

5.      Bitwise operators

6.      Membership operators

7.      Identity operators

Arithmetic Operators

Arithmetic operators are used for performing basic arithmetic operations. The following table shows the arithmetic operators supported by python

Operator	Operation	Description
+	Addition	Adds values on either side of operator
-	Subtraction	Subtracts right hand operand from left hand operand
*	Multiplication	Multiplies values on either side of the operator
/	Division	Divides left hand operand by the right hand operand
%	Modulus	Divides left hand operand by right hand operand and returns remainder
**	Exponent	Performs exponential calculation on operands
//	Floor division	The division of Operands where the result is the quotient in which the digits after the decimal point are removed

Comparison Operator

These operators compare the values on either sides of them and decide the relation among them. They are also called as relational operators. The following table shows the comparison operators supported by python

Operator	Description
==	If the values of two operands are equal, then the condition becomes true.
!=	If the values of two operands are not equal, then the condition becomes true.
>	If the values of left operands is greater than the value of right operand, then the condition becomes true.
<	If the values of left operands is lesser than the value of right operand, then the condition becomes true.
>=	If the values of left operands is greater than or equal to the value of right operand, then the condition becomes true.
<=	If the values of left operands is lesser than or equal to the value of right operand, then the condition becomes true.

Assignment Operators

Python provides various assignment operators. Various shorthand operators for addition, subtraction, multiplication, division, modulus, exponent and floor division are also supported by python. The following table shows the assignment operators supported by python

Operator	Description
=	Assigns values from right side operand to left side operand
+=	It adds right operand to the left operand and assign the result to left operand
-+	It subtracts right operand from the left operand and assign the result to left operand
*=	It multiplies right operand with the left operand and assign the result to left operand
/=	It divides right operand by the left operand and assign the result to left operand
%=	It takes modulus using two operands and assign the result to left operand
**=	Performs exponential calculation on operands and assign values to the left
//=	It performs floor division on operators and assign value to the left operand

Bitwise Operators

Bitwise operator works on bits and perform bit by bit operation. The following table shows the bitwise operators supported by python.

Operator	Operation	Description
&	Binary AND	Operator copies a bit to the result if it exists in both operands
|	Binary OR	It copies a bit if it exists in either operand.
^	Binary XOR	It copies the bit if it is set in one operand but not both
~	Binary ones complement	It is unary and ahs the effect of 'flipping' bits.
<<	Binary Left Shift	The left operands value is moved left by the number of bits specified by the right operand.
>>	Binary Right Shift	The left operands value is moved right by the number of bits specified by the right operand.

Logical Operators

Table shows the various logical operators supported by python language.

Operator	Operation	Description
and	Logical And	If both the operands are true then condition becomes true
or	Logical OR	If any of the operand is true then condition becomes true
not	Logical NOT	Used to reverse the logical state of its operand

Membership Operators

Python's membership operators test for membership in a sequence, such as strings, lists, and tuples. There are two membership operators supported by python which are described in the following table.

Operator	Description
in	Evaluates to true if the variable on either side of the operator point to the same object and false otherwise
not in	Evaluates to true if it does not finds a variable in the specifies sequence and false otherwise.

Identity Operator

Identity operators compare the memory locations of two objects. There are two identity operators as shown in the following table

Operator	Description
is	Evaluates to true if the variables on either side of the operator point to the same objects and false otherwise
is not	Evaluates to false if the variables on either side of the operator point to the same objects and true otherwise

Operator Precedence

The following table lists all operators from highest precedence to lowest.

Operator	Description
**	Exponentiation
~,+,-	Complement, unary plus and minus
*,/,%,//	Multiply, divide, modulo and floor division
+,-	Addition and subtraction
>>,<<	Right and left bitwise shift
&	Bitwise AND
^,|	Bitwise exclusive 'OR' and regular 'OR'
<=,< >,>=	Comparison operators
< >, ==,!=	Equality operators
=,%=,/=,//=,-=,+=,*=,**=	Assignment operators
is, is not	Identity operators
in, not in	Membership operators
not, or, and	Logical operators