Harsh Python

8/2/2019 Harsh Python

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What is Python?

An object- oriented, interpreted, high level

programming language

Is an object oriented scripting language

Easy to learn, read, use

Open Source

Embeddable in applications


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History

Invented in 1990 by Guido Van Rossum

The name Python stems from "Monty Python's

Flying Circus"

Python was influenced by ABC and Modula-3

First public release was in 1991


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Who is using it?

Google (various projects)

NASA (several projects)

The popular Bit Torrent peer-to-peer file sharing system is aPython program.

Yahoo! (Yahoo mail & groups)

The YouTube video sharing service is largely written inPython.

Intel, Cisco, Hewlett-Packard, Seagate, Qualcomm, and IBMuse Python for hardware testing.

Maya, a powerful integrated 3D modeling and animationsystem, provides a Python scripting API.


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Data types

Numbers flt_num= 10.0 int_num= 25

Strings my_str = Why are u using Perl?

Lists my_list = [123, 'spam', 1.23]

Tuples my_tup = (1, 4 , 32, *fo , abc+)

Dictionaries my_dict = a : 24, mo : fo-

Objects my_inst = MyClass(foo)

Modules: import myfile


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Numbers

Integers:

>>> my_int = 4

>>>my_int / 3

1

Floating Point:

>>> my_float = 5.5

>>> 20/my_float

3.6363636363636362


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Strings

Strings are immutable

There is no char type

like in C++ or Java + is overloaded to do

concatenation

>>> x = 'hello'

>>> x = x + ' there'>>> x

'hello there'


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Strings

"hello""world "helloworld" # concatenation

"hello"*3 "hellohellohello" # repetition

"hello"[0] "h" # indexing

"hello"[-1] "o" # (from end)

"hello"[1:4] ell" # slicing

len("hello") 5 # size

"e" in "hello 1 # search


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String Formatting

Similar to Cs printf

%

Can usually just use %s for everything, it will

convert the object to its String representation.

>>> "One, %d, three" % 2

'One, 2, three

>>> "%d, two, %s" % (1,3)

'1, two, 3

>>> "%s two %s" % (1, 'three')

'1 two three


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Lists

Ordered collection of

data

Data can be of different

types

Lists are mutable

Same subset operations

as Strings

>>> x = [1,'hello', (3 + 2j)]

>>> x

[1, 'hello', (3+2j)]>>> x[2]

(3+2j)

>>> x[0:2]

[1, 'hello']


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List Methods

append(x) add x to the end of the list

insert( i, x)insert x at a position i

remove(x) remove first item equal to x pop() remove item at the end of list

sort() sort the list

reverse() reverse the list


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Examples

>>> a = range(5) # [0,1,2,3,4]

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

>>> a.pop() # [0,1,2,3,4]>>> a.insert(0, 42) # [42,0,1,2,3,4]

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

>>> a.reverse() # [4,3,2,1,0]>>> a.sort() # [0,1,2,3,4]


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Tuples

Tuples are immutable

versions of lists

Coded in parentheses

>>> T = ('spam', 3.0, [11, 22, 33])

>>> T[1]

3.0

>>> T[2][1]

22

>>> x = (1,2,3)

>>> x[1:]

(2, 3)

>>> x[0]

1

>>>len(x)

3


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Dictionaries

A set of key-value pairs

Dictionaries are mutable

>>> d = {1 : 'hello', 'two' : 42, 'blah' : [1,2,3]}

>>> d

{1: 'hello', 'two': 42, 'blah': [1, 2, 3]}

>>> d['blah']

[1, 2, 3]


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Dictionaries: Add/Modify

Entries can be changed by assigning to that

entry

Assigning to a key that does not exist adds an

entry

>>> d

{1: 'hello', 'two': 42, 'blah': [1, 2, 3]}

>>> d['two'] = 99

>>> d{1: 'hello', 'two': 99, 'blah': [1, 2, 3]}

>>> d[7] = 'new entry'

>>> d

{1: 'hello', 7: 'new entry', 'two': 99, 'blah': [1, 2,

3]}


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Dictionaries: Deleting Elements

The del method deletes an element from a

dictionary

>>> d

{1: 'hello', 2: 'there', 10: 'world'}

>>> del(d[2])

>>> d{1: 'hello', 10: 'world'}


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Variables

Are not declared, just assigned

The variable is created the first time youassign it a value

Are references to objects

Everything in Python is an object

Variables must be assigned before they can be

used in expressions


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Control Flow Statements

If Statements

For Loops

While Loops


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If Statements

>>> x = 'killer rabbit'

>>> if x == 'roger':

print("how's jessica?")

elif x == 'bugs':print("what's up doc?")

else:

print('Run away! Run away!')

Run away! Run away!


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No Braces???

Python uses indentation instead of braces todetermine the scope of expressions

All lines must be indented the same amount

to be part of the scope (or indented more ifpart of an inner scope)

This forces the programmer to use proper

indentation since the indenting is part of theprogram!


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While Loops

While Loop:

x = 1

while x < 10 :

print x

x = x + 1

>>> import whileloop

1

23

4

5

6

7

89

>>>In whileloop.py


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For Loops

>>>for varin sequence:

statements

range() function generateslists of numbers :

range (5) -> [0,1,2,3,4]

Example:

mylist=*hello,hi,hey,!+;

for i in mylist:

print i


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Grouping Indentation

In Python In C

for i in range(20):

if i%3 == 0:

print i

if i%5 == 0:

print "Bingo!"

print "---"

for (i = 0; i < 20; i++)

{if (i%3 == 0) {

printf("%d\n", i);if (i%5 == 0) {

printf("Bingo!\n"); }}

printf("---\n");}

0

Bingo!

------

---

3

---

---

---

6

---

------

9

---

---

---

12

---

---

---

15

Bingo!

---

---

---

18

---

---


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Loop Control Statements

break Jumps out of the closest enclosing loop

continue Jumps to the top of the closest enclosing loop


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Functions

Use the 'def' statement

Function body follows; indented!

Basic Def:

def name ([arg 1 , arg 2, +):

statement 1

.

statement n

[return [expression]]


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Example Function

def greet(name):

print 'hello', name

greet('Jack')

greet('Jill')

greet('Bob')


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The return Statement:

The statement return [expression] exits a function, optionally

passing back an expression to the caller.

# Function definition is here

def sum( arg1, arg2 ):

# Add both the parameters and return them.total = arg1 + arg2

print "Inside the function : ", total

return total;

# Now you can call sum function

total = sum( 10, 20 );

print "Outside the function : ", total


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Defining a Class

A class is a special data type which defines how to

build a certain kind of object

Classes are designed to create and manage newobjects

Instances are objects that are created which follow

the definition given inside of the class

You just define the class and then use it


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Methods in Classes

Define a methodin a class by including

function definitions within the scope of the

class block

There must be a special first argument self

in allof method definitions which gets bound

to the calling instance

There is usually a special method called

__init__in most classes


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Example

>>> class ExampleClass:def __init__(self, some_message):

self.message = some_message

print "New Class instance created, with message:"

print self.message

>>> first_instance = ExampleClass("message1")

New Class instance created, with message:

message1

>>> second_instance = ExampleClass("message2")

New Class instance created, with message:

message2


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Instantiating Objects

There is no new keyword as in Java.

Just use the class name with ( ) notation and

assign the result to a variable

__init__ serves as a constructor for the

class. Usually does some initialization work

The arguments passed to the class name are

given to its __init__() method


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Constructor: __init__

An __init__ method can take any number

of arguments.

__init__ is a kind of constructor, when an

instance of a class is created.

However, the first argument self in the

definition of __init__ is special


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Self

The first argument of every method is a referenceto the current instance of the class

By convention, we name this argument self

In __init__, selfrefers to the object currentlybeing created; so, in other class methods, it refers

to the instance whose method was called

Similar to the keyword this in Java or C++ But Python uses selfmore often than Java uses

this


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Deleting instances: No Need to free

When you are done with an object, you donthave to delete or free it explicitly.

Python has automatic garbage collection.

Python will automatically detect when all ofthe references to a piece of memory havegone out of scope. Automatically frees thatmemory.

Theres also no destructor method forclasses


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Importing and Modules

Use classes & functions defined in another file

A Python module is a file with the same name (plus the .py

extension)

Like Java import, C++ include Three formats of the command:

import somefile

from somefile import *

from somefile import className

The difference? What gets imported from the file and what

name refers to it after importing


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import

import somefile

Everything in somefile.py gets imported.

To refer to something in the file, append the textsomefile. to the front of its name:

somefile.className.method(abc)

somefile.myFunction(34)


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from import *

from somefile import *

Everything in somefile.py gets imported

Take care! Using this import command can easily

overwrite the definition of an existing function or

variable!

className.method(abc)

myFunction(34)


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from import

from somefile import className

Only the item className in somefile.py gets imported.

Take care! Overwrites the definition of this name if

already defined in the current namespace!

className.method(abc) imported

myFunction(34) Not imported


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Compared to Java

Code up to 5 times shorter and more readable

Multiple inheritance

Quicker development

no compilation phase

less typing

Yes, it may run a bit slower

but development is much faster

and Python uses less memory (studies show)


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Compared to C/C++

Python code is often 5-10 times shorter than

equivalent C++ code!

No compilation phase

No need to declare the variables.

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Harsh Python