PYTHON

A brief Introduction for beginners


@whoami ?




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 59056 third-party software projects*... You name an area, there must be a Python package in it and mostly open source 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 <iostream>
using namespace std;
int 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

 Get your feet wet




VARIABLES & DATA TYPES

  • Identifiers
  • Keywords
  • Assignments
  • Operators  & Expressions

IDENTIFIERS


identifier ::= (letter|"_") (letter | digit | "_")*letter ::= lowercase | uppercaselowercase ::= "a"..."z"uppercase ::= "A"..."Z"digit ::= "0"..."9"


So, while abc is a valid identifier, 1abc is not :)

KEYWORDS


and        del       from      not     whileas         elif      global    or      withassert     else      if        pass    yieldbreak      except    import    printclass      exec      in        raisecontinue   finally   is        returndef        for       lambda    try

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 !

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





Operators


  • Arithmetic: +, -, /, *, %, //
  • Relational: >, <, >=, <=, ==, !=
  • Logical: and, or
  • Shorthand: +=, /=, -=, *=



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


Expressions

Expressions are made of operators and operands. An expression is like 2 + 3 .
Generally while writing expressions we put spaces before and after every operator so that the code becomes clearer to read, like
a = 234 * (45 - 56.0 / 34)
How expressions are evaluated
(6 + 3) - 12 / 3 + 3 * 2 -1
9 - 12 / 3 + 3 * 2 -1
9 - 4 + 3 * 2 - 1
9 - 4 + 6 - 1
5 + 6 - 1
11 - 1
10



Type Conversions


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

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




control flow


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

 

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



Local & global variables

#!/usr/bin/env python
def change(b):
    a = 90
    print a
a = 9
print "Before the function call ", a
print "inside change function",
change(a)
print "After the function call ", a

           The output 
$ ./local.py
Before the function call  9
inside change function 90
After the function call  9


An example for global variable usage 
#!/usr/bin/env python
def change(b):
    global a
    a = 90
    print a
a = 9
print "Before the function call ", a
print "inside change function",
change(a)
print "After the function call ", a



Default argument value


>>> def test(a , b=-99):
...     if a > b:
...         return True
...     else:
...         return False 

>>> test(12, 23)
False
>>> test(12)
True



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

Guess the output of this ;) 
>>> def func(a, b=13, v):
...     print a, b, v
...






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]



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







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 "
>>> s.strip()
'abc' 
>>> s = "faulty for a reason"
>>> s.find("for")
7
>>> s.find("fora")
-1 

>>> s.startswith("fa") #To check if the string starts with "fa" or not>>> s.endswith("reason") # True 
>>> s = "manoj pandey"
>>> s.title()
'Sayan Chowdhury' 
>>> z = s.upper()
>>> z
'MANOJ PANDEY'
>>> z.lower()
'manoj pandey'
>>> s = "I am A pRoGraMMer"
>> s.swapcase()
'i AM a PrOgRAmmER'




File-Handling

>>> f = open('foo.txt', 'w') 
>>> f.write("First line") 
>>> f.close() 
>>> f = open('foo.txt', 'r') 
>>> f.readline()
'First line' 
>>> f.readline()
'' 
>>> f = open('foo.txt', 'a') 
>>> f.write('Second line') 
>>> f.close() 
>>> f = open('foo.txt', 'r') 
>>> f.readline()
'First lineSecond line'

>>> f = open('foo.txt', 'a') 
>>> f.write("New line\n") 
>>> f.write("One more new line")
>>> f.close()        
>>> f = open('foo.txt', 'r') 
>>> f.readline()
'First lineSecond lineNew line\n' 
>>> f.readline()
'One more new line'
>>> f.readline()
''
>>> f.close() 
>>> f = open('foo.txt') 
>>> f.readlines()
['First lineSecond lineNew line\n', 'One more new line'] 

>>> f = open('foo.txt', 'w') 
>>> f.writelines(["1\n", "2\n"])
>>> f.close() 
>>> f = open('foo.txt') 
>>> f.readlines()
['1\n', '2\n'] 
>>> f.close()




MODULES

An example

"""
Bars Module
============
This is an example module with provide different ways to print bars.
"""
def starbar(num):
    """
    Prints a bar with *
    :arg num: Length of the bar
    """
    print '*' * num

def hashbar(num):
    """
    Prints a bar with #
    :arg num: Length of the bar
    """
    print '#' * num

def simplebar(num):
    """
    Prints a bar with -
    :arg num: Length of the bar
    """
    print '-' * num 

>>> import bars>>> bars.hashbar(10)##########

Importing modules


>>> from bars import simplebar, starbar
>>> simplebar(10)
>>> ----------

# NEVER DO THIS>>> from bars import *



Submodules

We can have many submodules inside a module.

mymodule
|-- bars.py
|-- __init__.py
`-- utils.py


Default modules

help:
>>> help(str)
os:
>>> import os
>>> os.getpid()
14875
>>> os.getcwd()
'/Users/rtnpro'
>>> os.chdir('/tmp')
>>> os.getcwd()
'/tmp'



Example

def view_dir(path='.'):
    """
    This function prints all files and directories in the given directory.
    :args path: Path to the directory, default is current directory
    """
    names = os.listdir(path)
    names.sort()
    for name in names:
        print name,
Using the view_dir() example:
>>> view_dir('/')
.readahead bin boot dev etc home junk lib lib64 lost+found media mnt opt
proc root run sbin srv sys tmp usr var




Fun exercise

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

Are you ready for the challenge?

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 - 
  • MostPopularPythonProjects
  • http://www.python.org/community/workshops/ - Python 
  • Conferences and Workshops
  • 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





try:
    run_a_workshop()
    print("Good job guys")
except PeopleDidNotLearn:
    print("We have to try harder")
finally:
    print("Heeeell yeah, It's finished")

A brief introduction to Python

By Manoj Pandey

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A brief introduction to Python

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