programming with python ppt

Python Programming
Language
By: Priyanka Pradhan
Priyanka Pradhan 1

Books include:
• Learning Python by Mark Lutz
• Python Essential Reference by David
Beazley
• Python Cookbook, ed. by Martelli,
Ravenscroft and Ascher
• (online at
http://code.activestate.com/recipes/langs/pyt
hon/)
• http://wiki.python.org/moin/PythonBooksPriyanka Pradhan 2

4 Major Versions of Python
• “Python” or “CPython” is written in C/C++
- Version 2.7 came out in mid-2010
- Version 3.1.2 came out in early 2010
• “Jython” is written in Java for the JVM
• “IronPython” is written in C# for the .Net
environment
Priyanka Pradhan 3

Contd…
• Created in 1989 by Guido Van Rossum
• Python 1.0 released in 1994
• Python 2.7 is the recommended version
• 3.0 adoption will take a few years
Priyanka Pradhan 4

Development Environments
IDE
1. PyDev with Eclipse
2. Komodo
3. Emacs
4. Vim
5. TextMate
6. Gedit
7. Idle
8. PIDA (Linux)(VIM Based)
9. NotePad++ (Windows)
10.Pycharm
Priyanka Pradhan 5

Web Frameworks
• Django
• Flask
• Pylons
• TurboGears
• Zope
• Grok
Priyanka Pradhan 6

Introduction
• Multi-purpose (Web, GUI, Scripting,
etc.)
• Object Oriented
• Interpreted
• Strongly typed and Dynamically typed
• Focus on readability and productivity
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Python features
• no compiling or linking
• rapid development cycle
• no type declarations
• simpler, shorter, more flexible
• automatic memory management
• garbage collection
• high-level data types and operations
Priyanka Pradhan 8

Contd..
• fast development
• object-oriented programming
• code structuring and reuse, C++
• embedding and extending in C
• mixed language systems
• classes, modules, exceptions,multithreading
• "programming-in-the-large" support
Priyanka Pradhan 9

Uses of Python
• shell tools
– system admin tools, command line programs
• extension-language work
• rapid prototyping and development
• language-based modules
– instead of special-purpose parsers
• graphical user interfaces
• database access
• distributed programming
• Internet scripting Priyanka Pradhan 10

Who Uses Python
• Google
• PBS
• NASA
• Library of Congress
• the ONION
• ...the list goes on...
Priyanka Pradhan 11

Python structure
• modules: Python source files or C extensions
– import, top-level via from, reload
• statements
– control flow
– create objects
– indentation matters – instead of {}
• objects
– everything is an object
– automatically reclaimed when no longer needed
Priyanka Pradhan 12

Indentation
• Most languages don’t care about
indentation
• Most humans do
• We tend to group similar things together
Priyanka Pradhan 13

Hello World
>>> 'hello world!'
'hello world!'
•Open a terminal window and type “python”
•If on Windows open a Python IDE like IDLE
•At the prompt type ‘hello world!’
Priyanka Pradhan 14

Python Overview
• Programs are composed of modules
• Modules contain statements
• Statements contain expressions
• Expressions create and process objects
Priyanka Pradhan 15

The Python Interpreter
•Python is an interpreted
language
•The interpreter provides an
interactive environment to
play with the language
•Results of expressions are
printed on the screen
>>> 3 + 7
10
>>> 3 < 15
True
>>> 'print me'
'print me'
>>> print 'print me'
print me
>>>
Priyanka Pradhan 16

The print Statement
>>> print 'hello'
hello
>>> print 'hello', 'there'
hello there
•Elements separated by
commas print with a space
between them
•A comma at the end of the
statement (print ‘hello’,) will
not print a newline character
Priyanka Pradhan 17

Documentation
>>> 'this will print'
'this will print'
>>> #'this will not'
>>>
The ‘#’ starts a line comment
Priyanka Pradhan 18

Variables
• Are not declared, just assigned
• The variable is created the first time you assign it
a value
• Are references to objects
• Type information is with the object, not the
reference
• Everything in Python is an object
Priyanka Pradhan 19

Everything is an object
• Everything means
everything, including
functions and classes
(more on this later!)
• Data type is a property
of the object and not of
the variable
>>> x = 7
>>> x
7
>>> x = 'hello'
>>> x
'hello'
>>>
Priyanka Pradhan 20

Basic operations
• Assignment:
– size = 40
– a = b = c = 3
• Numbers
– integer, float
– complex numbers: 1j+3, abs(z)
• Strings
– 'hello world', 'it's hot'
– "bye world"
Priyanka Pradhan 21

String operations
• concatenate with + or neighbours
– word = 'Help' + x
– word = 'Help' 'a'
• subscripting of strings
– 'Hello'[2]  'l'
– slice: 'Hello'[1:2]  'el'
– word[-1]  last character
– len(word)  5
– immutable: cannot assign to subscriptPriyanka Pradhan 22

Numbers: Integers
• Integer – the equivalent of
a C long
• Long Integer – an
unbounded integer value.
>>> 132224
132224
>>> 132323 ** 2
17509376329L
>>>
Priyanka Pradhan 23

Numbers: Floating Point
• int(x) converts x to an
integer
• float(x) converts x to a
floating point
• The interpreter shows
a lot of digits
>>> 1.23232
1.2323200000000001
>>> print 1.23232
1.23232
>>> 1.3E7
13000000.0
>>> int(2.0)
2
>>> float(2)
2.0
Priyanka Pradhan 24

Numbers: Complex
• Built into Python
• Same operations are
supported as integer and
float
>>> x = 3 + 2j
>>> y = -1j
>>> x + y
(3+1j)
>>> x * y
(2-3j)
Priyanka Pradhan 25

Numbers are immutable
>>> x = 4.5
>>> y = x
>>> y += 3
>>> x
4.5
>>> y
7.5
x 4.5
y
x 4.5
y 7.5
Priyanka Pradhan 26

String Literals
• 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'
Priyanka Pradhan 27

String Literals: Many Kinds
• Can use single or double quotes, and three double
quotes for a multi-line string
>>> 'I am a string'
'I am a string'
>>> "So am I!"
'So am I!'
>>> s = """And me too!
though I am much longer
than the others :)"""
'And me too!nthough I am much longernthan the others :)‘
>>> print s
And me too!
though I am much longer
than the others :)‘ Priyanka Pradhan 28

Substrings and Methods
>>> s = '012345'
>>> s[3]
'3'
>>> s[1:4]
'123'
>>> s[2:]
'2345'
>>> s[:4]
'0123'
>>> s[-2]
'4'
• len(String) – returns the
number of characters in the
String
• str(Object) – returns a
String representation of the
Object
>>> len(x)
6
>>> str(10.3)
'10.3'
Priyanka Pradhan 29

String Formatting
• Similar to C’s printf
• <formatted string> % <elements to insert>
• 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'
>>>
Priyanka Pradhan 30

Do nothing
• pass does nothing
• syntactic filler
while 1:
pass
Priyanka Pradhan 31

Operators
• Arithmetic
Priyanka Pradhan 32

String Manipulation
Priyanka Pradhan 33

Priyanka Pradhan 34
Logical Comparison

Identity Comparison
Priyanka Pradhan 35

Arithmetic Comparison
Priyanka Pradhan 36

Class Declaration
Priyanka Pradhan 37

Class Attributes
• Attributes assigned at class declaration
should always be immutable
Priyanka Pradhan 38

Class Methods
Priyanka Pradhan 39

Class Instantiation & Attribute
Access
Priyanka Pradhan 40

Class Inheritance
Priyanka Pradhan 41

Error Handling
Priyanka Pradhan 43

Lists
• Ordered collection of data
• Data can be of different
types
• Lists are mutable
• Issues with shared
references and mutability
• Same subset operations as
Strings
>>> x = [1,'hello', (3 + 2j)]
>>> x
[1, 'hello', (3+2j)]
>>> x[2]
(3+2j)
>>> x[0:2]
[1, 'hello']
Priyanka Pradhan 44

List
• lists can be heterogeneous
– a = ['spam', 'eggs', 100, 1234, 2*2]
• Lists can be indexed and sliced:
– a[0]  spam
– a[:2]  ['spam', 'eggs']
• Lists can be manipulated
– a[2] = a[2] + 23
– a[0:2] = [1,12]
– a[0:0] = []
– len(a)  5 Priyanka Pradhan 45

List methods
• append(x)
• extend(L)
– append all items in list (like Tcl lappend)
• insert(i,x)
• remove(x)
• pop([i]), pop()
– create stack (FIFO), or queue (LIFO)  pop(0)
• index(x)
– return the index for value x
Priyanka Pradhan 46

Contd…
• count(x)
– how many times x appears in list
• sort()
– sort items in place
• reverse()
– reverse list
Priyanka Pradhan 47

Lists: Modifying Content
• x[i] = a reassigns the
ith element to the
value a
• Since x and y point to
the same list object,
both are changed
• The method append
also modifies the list
>>> x = [1,2,3]
>>> y = x
>>> x[1] = 15
>>> x
[1, 15, 3]
>>> y
[1, 15, 3]
>>> x.append(12)
>>> y
[1, 15, 3, 12]
Priyanka Pradhan 48

Lists: Modifying Contents
• The method
append modifies
the list and returns
None
• List addition (+)
returns a new list
>>> x = [1,2,3]
>>> y = x
>>> z = x.append(12)
>>> z == None
True
>>> y
[1, 2, 3, 12]
>>> x = x + [9,10]
>>> x
[1, 2, 3, 12, 9, 10]
>>> y
[1, 2, 3, 12]
>>>Priyanka Pradhan 49

Strings share many features with
lists
>>> smiles = "C(=N)(N)N.C(=O)(O)O"
>>> smiles[0]
'C'
>>> smiles[1]
'('
>>> smiles[-1]
'O'
>>> smiles[1:5]
'(=N)'
>>> smiles[10:-4]
'C(=O)' Priyanka Pradhan 50

String Methods: find, split
smiles = "C(=N)(N)N.C(=O)(O)O"smiles = "C(=N)(N)N.C(=O)(O)O"
>>> smiles.find("(O)")
15
>>> smiles.find(".")
9
>>> smiles.find(".", 10)
-1
>>> smiles.split(".")
['C(=N)(N)N', 'C(=O)(O)O']
>>>
Priyanka Pradhan 51

String operators: in, not in
if "Br" in “Brother”:
print "contains brother“
email_address = “clin”
if "@" not in email_address:
email_address += "@brandeis.edu“
Priyanka Pradhan 52

String Method: “strip”, “rstrip”, “lstrip” are
ways to
remove whitespace or selected characters
>>> line = " # This is a comment line n"
>>> line.strip()
'# This is a comment line'
>>> line.rstrip()
' # This is a comment line'
>>> line.rstrip("n")
' # This is a comment line '
>>>
Priyanka Pradhan 53

More String methods
email.startswith(“c") endswith(“u”)
True/FalseTrue/False
>>> "%s@brandeis.edu" % "clin"
'clin@brandeis.edu''clin@brandeis.edu'
>>> names = [“Ben", “Chen", “Yaqin"]
>>> ", ".join(names)
‘‘Ben, Chen, Yaqin‘Ben, Chen, Yaqin‘
>>> “chen".upper()
‘‘CHEN'CHEN'
Priyanka Pradhan 54

“” is for special characters
n -> newline
t -> tab
-> backslash
...
But Windows uses backslash for directories!
filename = "M:nickel_projectreactive.smi" # DANGER!
filename = "M:nickel_projectreactive.smi" # Better!
filename = "M:/nickel_project/reactive.smi" # Usually works
Priyanka Pradhan 55

Tuples
• Tuples are immutable
versions of lists
• One strange point is the
format to make a tuple with
one element:
‘,’ is needed to differentiate
from the mathematical
expression (2)
>>> x = (1,2,3)
>>> x[1:]
(2, 3)
>>> y = (2,)
>>> y
(2,)
>>>
Priyanka Pradhan 56

Tuples and sequences
• lists, strings, tuples: examples of sequence
type
• tuple = values separated by commas
>>> t = 123, 543, 'bar'
>>> t[0]
123
>>> t
(123, 543, 'bar')
Priyanka Pradhan 57

Contd…
• Tuples may be nested
>>> u = t, (1,2)
>>> u
((123, 542, 'bar'), (1,2))
• Empty tuples: ()
>>> empty = ()
>>> len(empty)
0 Priyanka Pradhan 58

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]
Priyanka Pradhan 59

Contd..
• no particular order
• delete elements with del
>>> del tel['foo']
• keys() method  unsorted list of keys
>>> tel.keys()
['cs', 'lennox', 'hgs']
• use has_key() to check for existence
>>> tel.has_key('foo')
0 Priyanka Pradhan 60

Dictionaries: Add/Modify
>>> 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]}
• Entries can be changed by assigning to that entry
• Assigning to a key that does not exist adds an entry
Priyanka Pradhan 61

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'}
Priyanka Pradhan 62

Copying Dictionaries and Lists
• The built-in list
function will
copy a list
• The dictionary
has a method
called copy
>>> l1 = [1]
>>> l2 = list(l1)
>>> l1[0] = 22
>>> l1
[22]
>>> l2
[1]
>>> d = {1 : 10}
>>> d2 = d.copy()
>>> d[1] = 22
>>> d
{1: 22}
>>> d2
{1: 10}
Priyanka Pradhan 63

Dictionary Methods
Priyanka Pradhan 64

Data Type Summary
• Lists, Tuples, and Dictionaries can store any type
(including other lists, tuples, and dictionaries!)
• Only lists and dictionaries are mutable
• All variables are references
Priyanka Pradhan 65

Contd…
• Integers: 2323, 3234L
• Floating Point: 32.3, 3.1E2
• Complex: 3 + 2j, 1j
• Lists: l = [ 1,2,3]
• Tuples: t = (1,2,3)
• Dictionaries: d = {‘hello’ : ‘there’, 2 : 15}
Priyanka Pradhan 66

Modules
• collection of functions and variables,
typically in scripts
• definitions can be imported
• file name is module name + .py
• e.g., create module fibo.py
def fib(n): # write Fib. series up to n
...
def fib2(n): # return Fib. series up to nPriyanka Pradhan 67

Contd…
• function definition + executable statements
• executed only when module is imported
• modules have private symbol tables
• avoids name clash for global variables
• accessible as module.globalname
• can import into name space:
>>> from fibo import fib, fib2
>>> fib(500)
• can import all names defined by module:
>>> from fibo import *
Priyanka Pradhan 68

Input
• The raw_input(string) method returns a line of
user input as a string
• The parameter is used as a prompt
• The string can be converted by using the
conversion methods int(string), float(string), etc.
Priyanka Pradhan 69

Input: Example
print “enter your name?"
name = raw_input("> ")
print "When were you born?"
birthyear = int(raw_input("> "))
print "Hi %s! You are %d years old!" % (name, 2017 - birthyear)
~: python input.py
What's your name?
> Michael
What year were you born?
>1980
Hi Michael! You are 31 years old!Priyanka Pradhan 70

Booleans
• 0 and None are false
• Everything else is true
• True and False are aliases for 1 and 0 respectively
Priyanka Pradhan 71

Boolean Expressions
• Compound boolean expressions
short circuit
• and and or return one of the
elements in the expression
• Note that when None is returned
the interpreter does not print
anything
>>> True and False
False
>>> False or True
True
>>> 7 and 14
14
>>> None and 2
>>> None or 2
2
Priyanka Pradhan 72

No Braces
• Python uses indentation instead of braces to
determine the scope of expressions
• All lines must be indented the same amount to be part
of the scope (or indented more if part of an inner
scope)
• This forces the programmer to use proper indentation
since the indenting is part of the program!
Priyanka Pradhan 73

If Statements
import math
x = 30
if x <= 15 :
y = x + 15
elif x <= 30 :
y = x + 30
else :
y = x
print ‘y = ‘,
print math.sin(y)
In file ifstatement.py
>>> import ifstatement
y = 0.999911860107
>>>
In interpreter
Priyanka Pradhan 74

While Loops
x = 1
while x < 10 :
print x
x = x + 1
>>> import whileloop
1
2
3
4
5
6
7
8
9
>>>
In whileloop.py
In interpreter
Priyanka Pradhan 75

Loop Control Statements
break Jumps out of the closest
enclosing loop
continue Jumps to the top of the closest
enclosing loop
pass Does nothing, empty statement
placeholder
Priyanka Pradhan 76

The Loop Else Clause
• The optional else clause runs only if the loop exits
normally (not by break)
x = 1
while x < 3 :
print x
x = x + 1
else:
print 'hello'
~: python whileelse.py
1
2
hello
Run from the command line
In whileelse.py
Priyanka Pradhan 77

The Loop Else Clause
x = 1
while x < 5 :
print x
x = x + 1
break
else :
print 'i got here'
~: python whileelse2.py
1
whileelse2.py
Priyanka Pradhan 78

For Loops
~: python forloop1.py
1
7
13
2
for x in [1,7,13,2] :
print xforloop1.py
~: python forloop2.py
0
1
2
3
4
for x in range(5) :
print xforloop2.py
range(N) generates a list of numbers [0,1, …, n-1]Priyanka Pradhan 79

For Loops
• For loops also may have the optional else clause
for x in range(5):
print x
break
else :
print 'i got here'
~: python elseforloop.py
1
elseforloop.py
Priyanka Pradhan 80

Function Basics
def max(x,y) :
if x < y :
return x
else :
return y
>>> import functionbasics
>>> max(3,5)
5
>>> max('hello', 'there')
'there'
>>> max(3, 'hello')
'hello'
functionbasics.py
Priyanka Pradhan 81

Functions are first class objects
• Can be assigned to a variable
• Can be passed as a parameter
• Can be returned from a function
• Functions are treated like any other variable in
Python, the def statement simply assigns a
function to a variable
Priyanka Pradhan 82

Function names are like any variable
• Functions are objects
• The same reference
rules hold for them as
for other objects
>>> x = 10
>>> x
10
>>> def x () :
... print 'hello'
>>> x
<function x at 0x619f0>
>>> x()
hello
>>> x = 'blah'
>>> x
'blah'
Priyanka Pradhan 83

Functions as Parameters
def foo(f, a) :
return f(a)
def bar(x) :
return x * x
>>> from funcasparam import *
>>> foo(bar, 3)
9
Note that the function foo takes two
parameters and applies the first as a
function with the second as its
parameter
funcasparam.py
Priyanka Pradhan 84

Higher-Order Functions
map(func,seq) – for all i, applies func(seq[i]) and returns the
corresponding sequence of the calculated results.
def double(x):
return 2*x
>>> from highorder import *
>>> lst = range(10)
>>> lst
[0,1,2,3,4,5,6,7,8,9]
>>> map(double,lst)
[0,2,4,6,8,10,12,14,16,18]
highorder.py
Priyanka Pradhan 85

filter(boolfunc,seq) – returns a sequence containing all those items
in seq for which boolfunc is True.
def even(x):
return ((x%2 == 0)
>>> lst = range(10)
>>> lst
[0,1,2,3,4,5,6,7,8,9]
>>> filter(even,lst)
[0,2,4,6,8]
highorder.py
Priyanka Pradhan 86

reduce(func,seq) – applies func to the items of seq, from left to
right, two-at-time, to reduce the seq to a single value.
def plus(x,y):
return (x + y)
>>> lst = [‘h’,’e’,’l’,’l’,’o’]
>>> reduce(plus,lst)
‘hello’
highorder.py
Priyanka Pradhan 87

Functions Inside Functions
• Since they are like any other object, you can have
functions inside functions
def foo (x,y) :
def bar (z) :
return z * 2
return bar(x) + y
>>> from funcinfunc import *
>>> foo(2,3)
7
funcinfunc.py
Priyanka Pradhan 88

Functions Returning Functions
def foo (x) :
def bar(y) :
return x + y
return bar
# main
f = foo(3)
print f
print f(2)
~: python funcreturnfunc.py
<function bar at 0x612b0>
5
funcreturnfunc.py
Priyanka Pradhan 89

Parameters: Defaults
• Parameters can be
assigned default
values
• They are overridden if
a parameter is given
for them
• The type of the default
doesn’t limit the type
of a parameter
>>> def foo(x = 3) :
... print x
...
>>> foo()
3
>>> foo(10)
10
>>> foo('hello')
hello
Priyanka Pradhan 90

Parameters: Named
• Call by name
• Any positional
arguments must
come before
named ones in a
call
>>> def foo (a,b,c) :
... print a, b, c
...
>>> foo(c = 10, a = 2, b = 14)
2 14 10
>>> foo(3, c = 2, b = 19)
3 19 2
Priyanka Pradhan 91

Anonymous Functions
• A lambda expression
returns a function
object
• The body can only
be a simple
expression, not
complex statements
>>> f = lambda x,y : x + y
>>> f(2,3)
5
>>> lst = ['one', lambda x : x * x, 3]
>>> lst[1](4)
16
Priyanka Pradhan 92

Modules
• The highest level structure of Python
• Each file with the py suffix is a module
• Each module has its own namespace
Priyanka Pradhan 93

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