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PYTHON PROGRAMMING
Department of computer science

• Introduction to Python and installation:
• Python is a widely used general-purpose, high level
programming language. It was initially designed by
Guido van Rossum in 1991 and developed by Python
Software Foundation. It was mainly developed for
emphasis on code readability, and its syntax allows
programmers to express concepts in fewer lines of
code. Python is a programming language that lets
you work quickly and integrate systems more
efficiently. There are two major Python versions-
Python 2 and Python 3.
• • On 16 October 2000, Python 2.0 was released with
many new features.
• • On 3rd December 2008, Python 3.0 was released
with more testing and includes new features.

• Beginning with Python programming:
• 1) Finding an Interpreter:
• Before we start Python programming, we need to
have an interpreter to interpret and run our
programs. There are certain online interpreters like
https://ide.geeksforgeeks.org/,
http://ideone.com/ or http://codepad.org/ that
can be used to start Python without installing an
interpreter.
• Windows: There are many interpreters available
freely to run Python scripts like IDLE (Integrated
Development Environment) which is installed
when you install the python software from
http://python.org/downloads/

• 2) Writing first program:
• # Script Begins
• Statement1
• Statement2
• Statement3
• # Script Ends

• Why to use Python:
• The following are the primary factors to
use python in day-to-day life:
• 1. Python is object-oriented
• Structure supports such concepts as
polymorphism, operation overloading and
multiple inheritance.
• 2. Indentation
• Indentation is one of the greatest feature
in python

• 3. It’s free (open source)
• Downloading python and installing python is free and
easy
• 4. It’s Powerful
• Dynamic typing
• Built-in types and tools
• Library utilities
• Third party utilities (e.g. Numeric, NumPy, sciPy)
• Automatic memory management
• 5. It’s Portable
• Python runs virtually every major platform used today
• As long as you have a compatable python interpreter
installed, python programs will run in exactly the same
manner, irrespective of platform.

• 6. It’s easy to use and learn
• No intermediate compile
• Python Programs are compiled automatically to an intermediate form
called byte code, which the interpreter then reads.
• This gives python the development speed of an interpreter without
the performance loss inherent in purely interpreted languages.
• Structure and syntax are pretty intuitive and easy to grasp.
• 7. Interpreted Language
• Python is processed at runtime by python Interpreter
• 8. Interactive Programming Language
• Users can interact with the python interpreter directly for writing the
programs
• 9. Straight forward syntax
• The formation of python syntax is simple and straight forward which
also makes it popular.

• Installation:
• There are many interpreters available freely to
run Python scripts like IDLE (Integrated
Development Environment) which is installed
when you install the python software from
http://python.org/downloads/
• Steps to be followed and remembered:
• Step 1: Select Version of Python to Install. Step 2:
Download Python Executable Installer. Step 3:
Run Executable Installer.
• Step 4: Verify Python Was Installed On Windows.

• Step 5: Verify Pip Was Installed. Step 6:
Add Python Path to Environment Variables
(Optional)

• Working with Python Python Code Execution:
• Python’s traditional runtime execution
model:Source code you type is translated to byte
code, which is then run by the Python Virtual
Machine (PVM). Your code is automatically
compiled, but then it is interpreted.

• There are two modes for using the Python
interpreter:
• • Interactive Mode
• • Script Mode

• Running Python in interactive mode: Without passing
python script file to the interpreter, directly execute
code to Python prompt.
• Once you‟re inside the python interpreter, then you
can start.
• >>> print("hello world")
• hello world
• #Relevant output is displayed on subsequent lines
without the >>> symbol
• >>> x=[0,1,2]
• #Quantities stored in memory are not displayed by
default.
• >>> x
• #If a quantity is stored in memory, typing its name will
display it. [0, 1, 2]
• >>> 2+3 5

The chevron at the beginning of the 1st line, i.e., the symbol >>> is a prompt the
python interpreter uses to indicate that it is ready. If the programmer types 2+6,
the interpreter replies 8.

• Running Python in script mode:
• Alternatively, programmers can store Python script
source code in a file with the .py extension, and use the
interpreter to execute the contents of the file. To
execute the script by the interpreter, you have to tell
the interpreter the name of the file. For example, if you
have a script name MyFile.py and you're working on
Unix, to run the script you have to type:
• python MyFile.py
• Working with the interactive mode is better when
Python programmers deal with small pieces of code as
you can type and execute them immediately, but when
the code is more than 2-4 lines, using the script for
coding can help to modify and use the code in future.
• Example:

• Numeric Data types:
• The data stored in memory can be of many types.
For example, a student roll number is stored as a
numeric value and his or her address is stored as
alphanumeric characters. Python has various
standard data types that are used to define the
operations possible on them and the storage
method for each of them.
• Int:
• Int, or integer, is a whole number, positive or
negative, without decimals, of unlimited length.
• >>> print(24656354687654+2)
• 24656354687656
• >>> print(20)
• 20

• # To verify the type of any object in
Python, use the type() function:
• >>> type(10)
• <class 'int'>
• >>> a=11
• >>> print(type(a))
• <class 'int'>

• Float:
• Float, or "floating point number" is a number,
positive or negative, containing one or more
decimals.
• Float can also be scientific numbers with an "e" to
indicate the power of 10.
• >>> y=2.8
• >>> y
• 2.8
• >>> y=2.8
• >>> print(type(y))
• <class 'float'>
• >>> type(.4)
• <class 'float'>

• >>> 2.
• 2.0
• Example:
• x = 35e3
• y = 12E4
• z = -87.7e100
• print(type(x))
• print(type(y))
• print(type(z))
• Output:
• <class 'float'>
• <class 'float‘>
• <class 'float'>

• Boolean:
• Objects of Boolean type may have one of
two values, True or False:
• >>> type(True)
• <class 'bool'>
• >>> type(False)
• <class 'bool'>

• String:
• 1. Strings in Python are identified as a contiguous set
of characters represented in the quotation marks.
Python allows for either pairs of single or double
quotes.
• • 'hello' is the same as "hello".
• • Strings can be output to screen using the print
function.
• For example: print("hello").
• >>> print(“VSR college") VSR college
• >>> type(“VSR college")
• <class 'str'>
• >>> print(‘VSR college')
• VSR college
• >>> " "
• ' '

• A string is a group/a sequence of characters. Since
Python has no provision for arrays, we simply use
strings. This is how we declare a string. We can use
a pair of single or double quotes. Every string
object is of the type „str‟.
• >>> type("name")
• <class 'str'>
• fruit = 'banana'
• >>> letter = fruit[1]
• The second statement selects character number 1
from fruit and assigns it to letter. The expression in
brackets is called an index. The index indicates
which character in the sequence we want

• String slices:
• A segment of a string is called a slice.
Selecting a slice is similar to selecting a
character: Subsets of strings can be taken
using the slice operator ([ ] and [:]) with
indexes starting at 0 in the beginning of the
string and working their way from -1 at the
end. Slice out substrings, sub lists, sub Tuples
using index. Syntax:[Start: stop: steps]
• Slicing will start from index and will go up to
stop in step of steps.
• Default value of start is 0,

• Stop is last index of list
• And for step default is 1
• For example 1−
• 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 5th
• print str[2:]
• # Prints string starting from 3rd character
• print str * 2
• # Prints string two times
• print str + "TEST"
• # Prints concatenated string

• Output:
• Hello World!
• H
• llo
• llo World!
• Hello World!Hello World!
• Hello World!TEST

• Example 2:
• >>> x='computer'
• >>> x[1:4]
• 'omp'
• >>> x[3:]
• 'puter'
• >>> x[:5]
• 'compu'
• >>> x[-1]
• 'r'
• >>> x[-3:]
• 'ter'
• >>> x[:-2]
• 'comput'
• >>> x[::-2]
• 'rtpo'
• >>> x[::-1]
• 'retupmoc'

• Immutability:
• It is tempting to use the [] operator on the left side of an
assignment, with the intention of changing a character in
a string.
• For example:
• >>> greeting=‘VSR college!'
• >>> greeting[0]='n'
• TypeError: 'str' object does not support item assignment
The reason for the error is that strings are immutable,
which means we can‟t change an existing string.
• The best we can do is creating a new string that is a
variation on the original:
• >>> greeting = 'Hello, world!'
• >>>new_greeting = 'J' + greeting[1:]
• >>>new_greeting
• 'Jello, world!'
• Note: The plus (+) sign is the string concatenation
operator and the asterisk (*) is the repetition operator

• Note: All the string methods will be returning
either true or false as the result
• 1. isalnum():
• Isalnum() method returns true if string has at
least 1 character and all characters are
alphanumeric and false otherwise.
• Syntax:
• String.isalnum()
• Example:
• >>> string="123alpha"
• >>>string.isalnum()
• True

• 2. isalpha():
• isalpha() method returns true if string has
at least 1 character and all characters are
alphabetic and false otherwise.
• Syntax:
• String.isalpha()
• Example:
• >>> string="nikhil"
• >>>string.isalpha()
• True

• 3. isdigit():
• isdigit() returns true if string contains only
digits and false otherwise.
• Syntax:
• String.isdigit()
• Example:
• >>> string="123456789"
• >>>string.isdigit()
• True

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