Introduction to Python

Contributors

Ratnadeep Debnath

@rtnpro

WHAT IS PYTHON?

A dynamic, interpreted, high-level language
Does not require declaring the types of variables or parameters or methods
Extremely readable
Used for solving a really wide range of problems
Object-oriented
Does not force Object-orientation
Has and requires common sense

Who all are using Python?

and the list goes on...

Why Python?

Used by many companies across the world including Google, Rackspace, Industrial Light and Magic, Flickr and many others.
A thriving community of developers and contributors and third party application developers.
PyPI (Python Package Index) lists about 33500 third-party software projects*... You name an area, there must be a Python package in it and mostly opensource and free!
Interpreted, dynamically typed, clean syntax, very easy to learn.
Learn Python in an Afternoon ;) - http://docs.python.org/2/tutorial/

Hello, world

How it's done with Java and C++

public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello, World");
    }
}

#include 
main()
{
    cout << "Hello World!";
    return 0;
}

Requires a LOT more than just common sense.

The Pythonic way

Common sense: When you want to print a string, just print it.

In Python interpreter

 >>> print "Hello, world."

As a script

Open your favorite editor and type

#! /usr/bin/env python
print "Hello, world."

and save it as hello.py. And then execute it from command line as below

$ python hello.py

Assignments

In Python, we don't declare what kind of data we want to put in a variable.

 >>> a = 12
 >>> b = 23 >>> a + b >>> 35

Examples

Multiple assignments

>>> a, b  = 10, 12>>> print a10>>> print b12

Python is fun

Let's try to swap 2 numbers

# The usual way>>> a = 10>>> b = 20>>> c = b>>> b = a>>> a = c>>> print a, b20 10

# The Pythonic way>>> a, b = 10, 20>>> a, b = b, a>>> print a, b20 10

DATATYPES

# Integer>>> a = 10# Float>>> b = 2.35# String>>> c = "Hello"# List>>> d = [1, 2, 'blah', 3.2]# Dictionary>>> f = {'a': 1, 'b': 2.3, 'c': "foo"}>>> f['b']2.3

Arithmetic operators

>>> 2 + 3
5
>>> 23 - 3
20
>>> 22.0 / 12
1.8333333333333333>>> 14 % 32>>> 4.0 // 3
1.0
>>> 4.0 / 3
1.3333333333333333

Logical operators

To do logical AND , OR we use and ,*or* keywords. x and y returns False if x is False else it returns evaluation of y. If x is True, it returns True.

>>> 1 and 4
4
>>> 1 or 4
1
>>> -1 or 4
-1
>>> 0 or 4
4

Shorthand Operators

x op = expression is the syntax for shorthand operators. It will be evaluated like x = x op expression , Few examples are

>>> a = 12
>>> a += 13
>>> a
25
>>> a /= 3
>>> a
8
>>> a += (26 * 32)
>>> a
840

Type Conversions

float(string) -> float value
int(string) -> integer value
str(integer) or str(float) -> string representation

>>> a = 8.126768
>>> str(a)
'8.126768'

If-else , the control flow

if expr1:
    do this
elif expr2:
    do that
...
...
...
else:
   do whatever

Truth value testing

if x:
    pass

DON'T

if x == True:
    pass

Try out!

1 . Write a program to find greatest of two numbers.

2. Write program which takes a number as input and if number is divisible by 3, 5 print Fizz, Buzz, FizzBuzz respectively

Functions

Defining a function

def functionname(params):
    statement1
    statement2

Sum of 2 integers

>>> def sum(a, b):

... return a + b

>>> res = sum(234234, 34453546464)

>>> res

34453780698L

Default argument value

>>> def test(a , b=-99):
...     print a
...     print b

>>> test(12, 23)
1223
>>> test(12)
12-99

Keyword arguments

>>> def func(a, b=5, c=10):
...     print 'a is', a, 'and b is', b, 'and c is', c
...
>>> func(12, 24)
a is 12 and b is 24 and c is 10
>>> func(12, c = 24)
a is 12 and b is 5 and c is 24
>>> func(b=12, c = 24, a = -1)
a is -1 and b is 12 and c is 24

Lists

>>> a = [23, 45, 1, -3434, 43624356, 234]

# Append
>>> a.append(45)

>>> a
[23, 45, 1, -3434, 43624356, 234, 45]

# Insert
>>> a.insert(0, 1) # 1 added at the 0th position of the list

>>> a
[1, 23, 45, 1, -3434, 43624356, 234, 45]

>>> a.insert(0, 111)

>>> a
[111, 1, 23, 45, 1, -3434, 43624356, 234, 45]

# Count the length of the list
>>> a.count(45)
2

# Remove a element from the list
>>> a.remove(234)

>>> a
[111, 1, 23, 45, 1, -3434, 43624356, 45]

# Reverse a list
>>> a.reverse()

>>> a
[45, 43624356, -3434, 1, 45, 23, 1, 111]

# Append a list into another list
>>> b = [45, 56, 90]
>>> a.append(b)

>>> a
[45, 43624356, -3434, 1, 45, 23, 1, 111, [45, 56, 90]]
>>> a[-1]
[45, 56, 90]

# Extend a into b
>>> a.extend(b) #To add the values of b not the b itself
>>> a
[45, 43624356, -3434, 1, 45, 23, 1, 111, [45, 56, 90], 45, 56, 90]
>>> a[-1]
90

# Sort a list
>>> a.sort()
>>> a
[-3434, 1, 1, 23, 45, 45, 45, 56, 90, 111, 43624356, [45, 56, 90]]

# Delete a element in list
>>> del a[-1]
>>> a
[-3434, 1, 1, 23, 45, 45, 45, 56, 90, 111, 43624356]

# Basic Data Structure
>>> a = [1, 2, 3, 4, 5]
>>> a.append(1)
>>> a
[1, 2, 3, 4, 5, 1]

>>> a.pop(0)
1
>>> a.pop(0)
2
>>> a
[3, 4, 5, 1]

>>> l = [1, 2, 3, 4, 5, 6, 7] [1, 2, 3, 4, 5, 6, 7]

>>> l[:2] #first two elements
[1, 2]

>>> l[2:] #exclude first two elements
[3, 4, 5, 6, 7]

>>> l[::2] #every second element[1, 3, 5, 7]

>>> l[::1] #every element[1, 2, 3, 4, 5, 6, 7]

>>> l[::3] #every third element
[1, 4, 7]

>>> l[::10] #every tenth element
[1]

>>> l[::-1]
[7, 6, 5, 4, 3, 2, 1]

Try Out!

>>> l[1:7:2]

>>> [][:2]

>>> [1][:2]

List Comprehensions

# List Comprehensions 
>>> a = [1, 2, 3]
>>> [x ** 2 for x in a]
[1, 4, 9]
>>> z = [x + 1 for x in [x ** 2 for x in a]]
>>> z
[2, 5, 10]

Looping

While Loop

while condition:
    statement1
    statement2

>>> n = 0>>> while n < 11:
...     print n
...     n += 1
...
0
1
2
3
4
5
6
7
8
9
10

For Loop

>>> a = ['Python', 'is', 'powerful']

>>> for x in a:
...     print x,
...
Python is powerful

>>> range(1, 5)
[1, 2, 3, 4]

>>> range(1, 15, 3)
[1, 4, 7, 10, 13]

>>> range(10)
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

>>> range(10, 1, -2)
[10, 8, 6, 4, 2]

For...else...

>>> for i in range(0, 5):
...     print i
... else:
...     print "Bye bye"
...
0
1
2
3
4
Bye bye

Try Out!

1. A program to print the even numbers between 1-100

Sets

# Set: Another data-structure with no duplicate items
>>> a = set('abcthabcjwethddda')
>>> a
set(['a', 'c', 'b', 'e', 'd', 'h', 'j', 't', 'w'])

# Set Operations
>>> a = set('abracadabra')
>>> b = set('alacazam')

>>> a                                  # unique letters in a
set(['a', 'r', 'b', 'c', 'd'])

>>> a - b                              # letters in a but not in b
set(['r', 'd', 'b'])

>>> a | b                              # letters in either a or b
set(['a', 'c', 'r', 'd', 'b', 'm', 'z', 'l'])

>>> a & b                              # letters in both a and b
set(['a', 'c'])

>>> a ^ b                              # letters in a or b but not both
set(['r', 'd', 'b', 'm', 'z', 'l'])

>>> a
set(['a', 'c', 'b', 'e', 'd', 'h', 'j', 'q', 't', 'w'])
>>> a.add('p')
>>> a
set(['a', 'c', 'b', 'e', 'd', 'h', 'j', 'q', 'p', 't', 'w'])

Strings

>>> s = "I am Indian"
>>> s
'I am Indian'
>>> s = 'I am Indian'
>>> s = "Here is a line \
... splitted in two lines"
>>> s
'Here is a line split in two lines'
>>> s = "Here is a line \n split in two lines"
>>> s
'Here is a line \n split in two lines'
>>> print s
Here is a line
split in two lines

>>> s = """ This is a
... multiline string, so you can
... write many lines"""
>>> print s
This is a
multiline string, so you can
write many lines

# String Methods
>>> s = "We all love Python"

>>> s.split(" ")
['We', 'all', 'love', 'Python']

>>> x = "Nishant:is:waiting"
>>> x.split(':')
['Nishant', 'is', 'waiting']

>>> "-".join("GNU/Linux is great".split(" "))
'GNU/Linux-is-great'

>>> s = "  abc\n "        #Strips whitespaces and newlines
>>> s.strip()
'abc'

>>> s = "faulty for a reason"

>>> s.find("for")
7

>>> s.find("fora")
-1

>>> s.startswith("fa") #To check if the string startswith fa or not True>>> s.endswith("reason") # True

>>> s = "sayan chowdhury"
>>> s.title()
'Sayan Chowdhury'

>>> z = s.upper()
>>> z
'SAYAN CHOWDHURY'
>>> z.lower()
'sayan chowdhury'

>>> s = "I am A pRoGraMMer"
>> s.swapcase()
'i AM a PrOgRAmmER'

Dictionaries

>>> data = {'kushal':'Fedora', 'kart_':'Debian', 'Jace':'Mac'}
>>> data
{'kushal': 'Fedora', 'Jace': 'Mac', 'kart_': 'Debian'}
>>> data['kart_']
'Debian'

To add more data to it by simply

>>> data['parthan'] = 'Ubuntu'
>>> data
{'kushal': 'Fedora', 'Jace': 'Mac', 'kart_': 'Debian', 'parthan': 'Ubuntu'}

To delete any particular key:value pair

>>> del data['kushal']
>>> data
{'Jace': 'Mac', 'kart_': 'Debian', 'parthan': 'Ubuntu'

To check if any key is there in the dictionary or not you can use 'in' keyword.

>>> 'Soumya' in data
False

If you want to loop through a dict use iteritems() method.


    >>> data
{'Kushal': 'Fedora', 'Jace': 'Mac', 'kart_': 'Debian', 'parthan': 'Ubuntu'}
>>> for x, y in data.iteritems():
...     print "%s uses %s" % (x, y)
...
Kushal uses Fedora
Jace uses Mac
kart_ uses Debian
parthan uses Ubuntu

>>> data['foo']
Traceback (most recent call last):
File "", line 1, in 
KeyError: 'foo'
>>> data.get('foo', None)
None

Fun exercise

Let's find all Pycon US 2013 videos' download links :)

Are you ready for the challenge?

http://pyvideo.org/category/33/pycon-us-2013/files

Solution

from BeautifulSoup import BeautifulSoup 
import requests 
 
url = 'http://pyvideo.org/category/33/pycon-us-2013/files' 
req = requests.get(url) 
data = req.text 
soup = BeautifulSoup(data) 
 
for link in soup.findAll('a', href=True): 
    # if link['href'].startswith(
	#'http://s3.us.archive.org/nextdayvideo/psf/pycon2013/'):
    if link.get('href', '').startswith(
            'http://s3.us.archive.org/nextdayvideo/psf/pycon2013/'): 
        #print link.get('href') 
        print link.get('href').split('?Signature')[0]

Links you can use

http://pypi.python.org - Python Package Index
http://docs.python.org/2/tutorial/ - The “official” Python tutorial
http://wiki.python.org/moin/MostPopularPythonProjects -
http://www.python.org/community/workshops/ - Python
http://wiki.python.org/moin/ - The Python “MoinMoin” Wiki
http://docs.python.org/devguide/ - All about developing Python
http://mail.python.org/mailman/listinfo - A list of all Python
http://pymbook.readthedocs.org/en/latest/ - Python for you & me
http://pythonbooks.revolunet.com/
http://git.io/7W2LkQ - Kracekumar