Python PPT by Sushil Sir.pptx

What Is Python?
 Created in 1990 by Guido van Rossum


About the origin of Python, Van Rossum wrote in 1996:
Over six years ago, in December 1989, I was looking for a
"hobby" programming project that would keep me occupied
during the week around Christmas. My office … would be
closed, but I had a home computer, and not much else on my
hands. I decided to write an interpreter for the new scripting
language I had been thinking about lately: a descendant of
ABC that would appeal to Unix/C hackers. I chose Python as a
working title for the project, being in a slightly irreverent
mood (and a big fan of Monty Python's Flying Circus).


Is Python A Scripting Language?
 Usually thought of as one
 But this is mainly a marketing issue
 People think of scripting languages as being
easy to learn, and useful.
 But Python is a well worked out coherent
dynamic programming language
 And there is no reason not to use it for a wide
range of applications.

DesignPhilosophy
>>> import this
The Zen of Python, by Tim Peters
Beautiful is better than ugly.
Explicit is better than implicit.
Simple is better than complex.
Complex is better than complicated.
Flat is better than nested.
Sparse is better than dense.
Readability counts.
Special cases aren't special enough to break the rules.
Although practicality beats purity.
Errors should never pass silently.
Unless explicitly silenced.
In the face of ambiguity, refuse the temptation to guess.
There should be one-- and preferably only one --obvious way to do it.
Although that way may not be obvious at first unless you're Dutch.
Now is better than never.
Although never is often better than *right* now.
If the implementation is hard to explain, it's a bad idea.
If the implementation is easy to explain, it may be a good idea.
Namespaces are one honking great idea -- let's do more of those!

Why Python?
 Designed to be easy to learn and master
 Clean, clear syntax
 Very few keywords
 Highly portable
 Runs almost anywhere - high end servers and
workstations, down to windows CE
 Uses machine independent byte-codes
 Extensible
 Designed to be extensible using C/C++, allowing
access to many external libraries

Most obvious and notorious
features
 Clean syntax plus high-level data types
 Leads to fast coding (First language in many
universities abroad!)
 Uses white-space to delimit blocks
 Humans generally do, so why not the
language?
 Try it, you will end up liking it
 Variables do not need declaration
 Although not a type-less language

Productivity!
 Reduced development time
 code is 2-10x shorter than C, C++, Java
 Improved program maintenance
 code is extremely readable
 Less training
 language is very easy to learn

What is it used for?
 rapid prototyping
 web scripting
 throw-away, ad hoc programming
 steering scientific applications
 extension language
 XML processing
 database applications
 GUI applications
 A Glue Language

PYTHON COMPARED TO
OTHER LANGUAGES

Python vs. Perl
 Easier to learn
 important for occasional users
 More readable code
 improved code maintenance
 Fewer “magical” side effects
 More “safety” guarantees
 Better Java integration
 Less Unix bias

Python vs. Tcl
 Real datatypes, object-orientation
 More differentiated syntax
 Much faster (even than Tcl 8.x)
 Less need for C extensions
 C extensions don’t redefine syntax
 hence fewer extension conflicts
 Better Java integration
 Python uses Tk as de-facto GUI std

Python vs. Java
 Code 5-10 times more concise
 Dynamic typing
 Much quicker development
 no compilation phase
 less typing
 Yes, it runs slower
 but development is so much faster!
 Use Python with Java: Jython!

Python Programs
 Python programs and modules are written
as text files with traditionally a .py
extension
 Each Python module has its own discrete
namespace
 Name space within a Python module is a
global one.

Python Programs
 Python modules and programs are
differentiated only by the way they are
called
 .py files executed directly are programs (often
referred to as scripts)
 .py files referenced via the import statement
are modules

Python Programs
 Thus, the same .py file can be a
program/script, or a module
 This feature is often used to provide
regression tests for modules
 When module is executed as a program, the
regression test is executed
 When module is imported, test functionality is
not executed

Variables and Types(1 of 3)
 Variables need no declaration
 >>> a=1
>>>
 As a variable assignment is a statement,
there is no printed result
 >>> a
1
 Variable name alone is an expression, so
the result is printed

 Variables must be created before they can
be used
 >>> b
Traceback (innermost last):
File "<interactive input>", line
1, in ?
NameError: b
>>>
 Python uses exceptions - more detail later

 Objects always have a type
 >>> a = 1
>>> type(a)
<type 'int'>
>>> a = "Hello"
>>> type(a)
<type 'string'>
>>> type(1.0)
<type 'float'>

Assignment versus Equality
Testing
 Assignment performed with single =
 Equality testing done with double = (==)
 Sensible type promotions are defined
 Identity tested with is operator.
 >>> 1==1
1
>>> 1.0==1
1
>>> "1"==1
0

Simple Data Types
 Strings
 May hold any data, including embedded
NULLs
 Declared using either single, double, or triple
quotes
 >>>
>>>
s
s
= "Hi there"
'Hi there'
>>> s = "Embedded
>>> s
"Embedded 'quote'"
'quote'"

Simple Data Types
 Triple quotes useful for multi-line strings
 >>> s = """ a long
... string with "quotes" or
anything else"""
>>> s
' a long012string with "quotes" or
anything else'
>>> len(s)
45

Simple Data Types
 Integer objects implemented using C
longs
 Like C, integer division returns the floor
 >>> 5/2
2
 Float types implemented using C doubles
 No point in having single precision since
execution overhead is large anyway

Simple Data Types
 Long Integers have unlimited size
 Limited only by available memory
 >>> long = 1L << 64
>>> long ** 5
21359870359209100823950217061695521146027045223
56652769947041607822219725780640550022962086936
576L

HighLevel Data Types
 Lists hold a sequence of items
 May hold any object
 Declared using square brackets
 >>> l = []# An empty list
>>> l.append(1)
>>> l.append("Hi there")
>>> len(l)
2

High Level Data Types
 >>> l
[1, 'Hi there']
>>>
>>> l = ["Hi there", 1, 2]
>>> l
['Hi there', 1, 2]
>>> l.sort()
>>> l
[1, 2, 'Hi there']

 Tuples are similar to lists
 Sequence of items
 Key difference is they are immutable
 Often used in place of simple structures
 Automatic unpacking
 >>> point = 2,3
>>> x, y = point
>>> x
2

 Tuples are particularly useful to return
multiple values from a function
 >>> x, y = GetPoint()
 As Python has no concept of byref
parameters, this technique is used widely

 Dictionaries hold key-value pairs
 Often called maps or hashes. Implemented
using hash-tables
 Keys may be any immutable object, values
may be any object
 Declared using braces
 >>> d={}
>>> d[0] = "Hi there"
>>> d["foo"] = 1

 Dictionaries (cont.)
 >>> len(d)
2
>>> d[0]
'Hi there'
>>> d = {0 : "Hi there", 1 :
"Hello"}
>>> len(d)
2

Blocks
 Blocks are delimited by indentation
 Colon used to start a block
 Tabs or spaces may be used
 Maxing tabs and spaces works, but is
discouraged
 >>> if 1:
... print "True"
...
True
>>>

Blocks
 Many hate this when they first see it
 Most Python programmers come to love it
 Humans use indentation when reading
code to determine block structure
 Ever been bitten by the C code?:
 if (1)
printf("True");
CallSomething();

Looping
 The for statement loops over sequences
 >>> for ch in "Hello":
... print ch
...
H
e
l
l
o
>>>

Looping
 Built-in function range() used to build
sequences of integers
 >>> for i in range(3):
print i
...
...
0
1
2
>>>

Looping
 while statement for more traditional
loops
 >>> i = 0
>>> while i < 2:
...
...
print
i = i
i
+ 1
...
0
1
>>>

Functions
 Functions are defined with the def
statement:
 >>> def foo(bar):
... return bar
>>>
 This defines a trivial function named foo
that takes a single parameter bar

Functions
 A function definition simply places a
function object in the namespace
 >>> foo
<function foo at fac680>
>>>
 And the function object can obviously be
called:
 >>> foo(3)
3
>>>

Classes
 Classes are defined using the class
statement
 >>> class Foo:
self.member = 1
def GetMember(self):
return self.member
... def init (self):
...
...
...
...
>>>

Classes
 A few things are worth pointing out in the
previous example:
 The constructor has a special name
init , while a destructor (not shown)
uses del
 The self parameter is the instance (ie, the
this in C++). In Python, the self parameter
is explicit (c.f. C++, where it is implicit)
 The name self is not required - simply a
convention

Classes
 Like functions, a class statement simply
adds a class object to the namespace
 >>> Foo
<class main .Foo at 1000960>
>>>
 Classes are instantiated using call syntax
 >>> f=Foo()
>>> f.GetMember()
1

Modules
 Most of Python’s power comes from
modules
 Modules can be implemented either in
Python, or in C/C++
 import statement makes a module
available
 >>> import string
>>> string.join( ["Hi", "there"] )
'Hi there'
>>>

Exceptions
 Python uses exceptions for errors
 try / except block can handle exceptions
 >>> try:
... 1/0
... except ZeroDivisionError:
... print "Eeek"
...
Eeek
>>>

Exceptions
 try / finally block can guarantee
execute of code even in the face of
exceptions
 >>> try:
... 1/0
... finally:
... print "Doing this anyway"
...
File "<interactive
Doing this anyway
Traceback (innermost last):
input>", line 2, in ?
ZeroDivisionError: integer division or modulo
>>>

Threads
 Number of ways to implement threads
 Highest level interface modelled after Java
 >>> class DemoThread(threading.Thread):
def run(self):
for i in range(3):
time.sleep(3)
print i
...
...
...
...
...
>>> t = DemoThread()
>>> t.start()
>>> t.join()
0
1 <etc>

Standard Library
 Python comes standard with a set of modules,
known as the “standard library”
 Incredibly rich and diverse functionality available
from the standard library
 All common internet protocols, sockets, CGI, OS
services, GUI services (via Tcl/Tk), database,
Berkeley style databases, calendar, Python
parser, file globbing/searching, debugger,
profiler, threading and synchronisation,
persistency, etc

External library
 Many modules are available externally
covering almost every piece of
functionality you could ever desire
 Imaging, numerical analysis, OS specific
functionality, SQL databases, Fortran
interfaces, XML, Corba, COM, Win32 API,
comedilib, serial, parallel, opengl, opencv,
wxpython, gtk, qt, tkinter etc
 Way too many to give the list any justice

More Information on Python
 Comes with extensive documentation,
including tutorials and library reference
 Also a number of Python books available
 Visit www.python.org for more details
 Can find python tutorial and reference manual

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