Python intro

Introduction to Python

Abhinav Upadhyay
<abhinav@socialtwist.com>

What is Python
● A dynamically typed, object oriented
programmnig language
● Created by Guido van Rossum in 1989

Features
● High-level
● Object Oriented
● Scalable
● Extensible
● Portable
● Readable
● Managed runtime
● Interpreted and Byte-compiled.

Difference from C Style Languages
● Forget ; and {}
● Embrace indentation

Getting Started
● “Hello, World” in Python:

print 'Hello, world!'

Getting Started
● Redirecting your output

file = open('output', 'w')
print >> file, 'Hello, world!'

Making your program interactive
● Getting user input:

name = raw_input('Enter your name: ')
print name

Comments in Python
● Single line comments --> #
● Multiline comments --> ''' your comment '''
● Docstrings

Operators
● Numerical operators: +, -, /, //, *, **, %

● Comparision operators: <, >, <=, >=, ==, !=, <>

● Logical operators: and, or, not

Numerical types in Python
● int
– long
– float

● bool

● complex

String handling in Python
● Creating a string:

my_string = “this is a string”

my_2nd_string = 'string using single
quotes'

my_3rd_string = ''' triple quotes also
work'''

● String operators:
+ --> Performs concatenation
str1 = 'a'
str2 = 'b'
print a + b
Output: 'ab'
* --> Performs repitition
str1 = 'a'
print a * 5
Output: 'aaaaa'

● String operators:

% --> Format operator
age = 20
str1 = 'Your age is %dn' % age
Output: 'Your age is 20'

● String Manipulation:
– Indices starting from 0
– Indices from last element starting with -1

str1 = 'hello, world'
print str1[0]
Output: 'h'
print str[-1]
Output: 'd'
Print str[-2]
Output: 'l'

Slicing:
● Use the slicing operator [] to slice the string into smaller
parts
● Syntax: str[m:n]
– This will print the characters of the string starting from the mth position till
the nth position (but excluding the nth character)

str1 = '0123456789'
print str1[1:4]
Output: 123

Built-in functions of Python
● Functions which are built-in to the Python
interpreter and are always available
● Examples: pow(), print(), open(), int(), str(),
etc.

● Some useful BIFs:
– type(), help(), dir()

Common Python Data-Structures
● Lists:
– Array like data-structure
– Objects stored in sequential order
– Indices starting from 0
– Can store arbitrary number of objects (unlike fixed
length arrays in other languages)
– Can also store objects of different types in the
same list.

● Tuples:
– Similar to lists, with two visible differences:
● Lists use [], while tuples use () in their syntax
● Tuples are immutable data-structures

● Dictionaries:
– Hash tables
– Can store any number of objects
– {} are used for creating dictionaries

Loops in Python
● For loop:
Used for iterating over a sequence of items
for item in list:
print item

Loops in Python
● While loop:
Used for executing a suite of code a number
of times.
Count = 0
while count < 10:
print count
count += 1

Conditionals
● If-elif-else
if <condition>:
#statement1
#statement2
elif <condition 2>:
#statement3
#statement4
else:
pass

File handling

F = open('file1.txt', 'r')
for eachLine in F:
print F
F.close()

f = open('file2.txt', 'w')
f.write('hello, worldn')
f.close()

Functions in Python
● The 'def' keyword
● The return value
● Returning multiple values

Python intro

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