Python (basic)

Python
(Basic)
Sabyasachi Moitra
Ph.D. Scholar(Techno India University)

Introduction
CompilingandInterpreting
FirstPythonProgram
OVERVIEW

Introduction
Developed by Guido van Rossum in the early 1990s.
Features:
• High-level powerful programming language
• Interpreted language
• Object-oriented
• Portable
• Easy to learn & use
• Open source
• Derived from many other programming languages
3

Introduction (2)
Uses:
• Internet Scripting
• Image Processing
• Database Programming
• Artificial Intelligence
…..
4

Compiling and Interpreting
• Many languages compile (translate) the program into a
machine understandable form.
• Python is directly interpreted into machine instructions.
5
compile execute
----------> ---------->
Source code Intermediate code Output
interpret
---------->
Source code Output

Compiling and Interpreting (2)
Compiler Interpreter
Scans the entire program and
translates it as a whole into machine
code.
Translates program one statement at a
time.
It takes large amount of time to analyze
the source code but the overall
execution time is comparatively faster.
It takes less amount of time to analyze
the source code but the overall
execution time is slower.
Generates intermediate object code
which further requires linking, hence
requires more memory.
No intermediate object code is
generated, hence are memory
efficient.
It generates the error message only
after scanning the whole program.
Hence debugging is comparatively
hard.
Continues translating the program until
the first error is met, in which case it
stops. Hence debugging is easy.
Programming language like C, C++ use
compilers.
Programming language like Python,
Ruby use interpreters.
6

First Python Program
(Hello.py)
7

InteractiveModeProgrammingVSScriptModeProgramming
Identifiers
ReservedWords
LinesandIndentation
SomemoreBasicSyntax
BASIC SYNTAX

Interactive Mode Programming
VS Script Mode Programming
Interactive Mode Programming Script Mode Programming
Invoking the interpreter without
passing a script file as a parameter
brings up the Python prompt. Type a
python statement at the prompt and
press Enter.
Invoking the interpreter with a script
parameter, & begins execution of the
script and continues until the script is
finished. When the script is finished,
the interpreter is no longer active
(assuming that user have Python
interpreter set in PATH variable).
C:Usersmoitra>python
Python 2.7.13 (v2.7.13:a06454b1afa1,
Dec 17 2016, 20:42:59) [MSC v.1500
32 bit (
Intel)] on win32
Type "help", "copyright", "credits" or
"license" for more information.
>>> print("Hello World")
Hello World
G:2.
DOCUMENTSPythonPrograms>pytho
n test.py
Hello World
10

Identifiers
• A Python identifier is a name used to identify a variable,
function, class, module or other object.
• An identifier starts with a letter A to Z or a to z or an
underscore (_) followed by zero or more letters, underscores
and digits (0 to 9).
• Python does not allow punctuation characters such as @, $,
and % within identifiers.
• Python is a case sensitive programming language.
Manpower and manpower are two different identifiers in
Python.
11

Reserved Words
and exec not
assert finally or
break for pass
class from print
continue global raise
def if return
del import try
elif in while
else is with
except lambda yield
and exec not
12

Lines and Indentation
• Python provides no braces to indicate blocks of code for class
and function definitions or flow control. Blocks of code are
denoted by line indentation.
• The number of spaces in the indentation is variable, but all
statements within the block must be indented the same
amount.
13
Correct Erroneous
if True:
print "True"
else:
print "False"
if True:
print "Answer"
print "True"
else:
print "Answer"
print "False"

Basic Syntax (5)
Example
Multi-Line Statements total = item_one +
item_two +
item_three
Comments in Python print "Hello World" # comment
Multiple Statements on a Single Line x = 'foo'; print(x + 'n')
…..
14

StandardDataTypes
OtherOperations
SomeTypeConversions
VARIABLE TYPES

Standard Data Types
Standard Data Type Example
Numbers
counter = 100
miles = 1000.0
String name = "John"
List tinylist = [123, 'john']
Tuple tinytuple = (123, 'john')
Dictionary
tinydict = {'name': 'john','code':6734,
'dept': 'sales'}
16

Other Operations
Operation Example
Multiple Assignment
a = b = c = 1
a,b,c = 1,2,"john"
Delete del var1[,var2[,var3[....,varN]]]]
17
Some Type Conversions
Function Description
int(x) Converts x to an integer number
str(x) Converts x to a string representation
list(s) Converts s to a list
tuple(s) Converts s to a tuple
dict(d) Creates a dictionary d
…..

TypesofOperator
BASIC OPERATORS

Types of Operator
Type Operators
Arithmetic Operators
+, -, *, /, %, ** (exponent), // (floor
division)
Relational Operators ==, !=, >, <, >=, <=
Assignment Operators =, +=, -=, *=, /=, %=, **=, //=
Logical Operators and, or, not
Bitwise Operators &, |, ~, ^, <<, >>
Membership Operators in, not in
Identity Operators is, not is
19

Addition of two nos.
add.py
#addition of two numbers
a=raw_input("Enter 1st no.: ")
b=raw_input("Enter 2nd no.: ")
c=int(a)+int(b)
print('The sum of {0} and {1} is {2}'.format(a, b, c))
Output
G:2. DOCUMENTSPythonPrograms>python add.py
Enter 1st no.: 1
Enter 2nd no.: 2
The sum of 1 and 2 is 3
20

Simpleifstatement
if…elsestatement
nestedifstatement
elif Statement
DECISION MAKING

Simple if statement
simple_if.py
#even no. check
a=input("Enter a no: ")
if(a%2==0):
print('{0} is even'.format(a))
Output
22
if even if not even
G:2.
DOCUMENTSPythonPrograms
>python simple_if.py
Enter a no: 10
10 is even
G:2.
>python simple_if.py
Enter a no: 11

if…else statement
if_else.py
#even-odd no. check
a=input("Enter a no: ")
if(a%2==0):
print('{0} is even'.format(a))
else:
print('{0} is odd'.format(a))
Output
23
if even if not even
G:2.
>python if_else.py
Enter a no: 10
10 is even
G:2.
>python if_else.py
Enter a no: 11
11 is odd

nested if statement (nested_if.py)
24

elif Statement (else_if_ladder.py)
26

while Loop
forLoop
nestedLoop
LOOPING

while Loop
while_loop.py
i=1
while(i<=5):
print(i)
i=i+1
Output
G:2. DOCUMENTSPythonPrograms>python while_loop.py
1
2
3
4
5
29

for Loop
for_loop.py
for i in range(1,6):
print(i)
Output
G:2. DOCUMENTSPythonPrograms>python for_loop.py
1
2
3
4
5
30

nested Loops (prime_factor.py)
31

NumericalTypes
NumericFunctions
MathematicalConstants
PYTHON NUMBERS

Python Numbers (1)
• Python supports four different numerical types:
• Python also includes some predefined numeric functions:
34
int long float complex
10 51924361L 15.20 -.6545+26J
Type Function Description Example
Mathematical
pow(x, y) The value of
x**y
import math
print
"math.pow(2, 4)
: ", math.pow(2,
4)
math.pow(2, 4) : 16.0
abs(x) The absolute
value of x
print "abs(-45)
: ", abs(-45)
abs(-45) : 45
…..

Python Numbers (2)
Random Number
choice(seq) A random item
from a list, tuple,
or string
import random
print "choice([1, 2,
3, 5, 9]) : ",
random.choice([1, 2,
3, 5, 9])
choice([1, 2, 3, 5, 9]) : 2
shuffle(lst) Randomizes the
items of a list in
place. Returns
None
import random
list = [20, 16, 10,
5];
random.shuffle(list)
print "Reshuffled
list : ", list
Reshuffled list : [16, 5, 10, 20]
…..
35

Python Numbers (3)
Trigonometric
sin (x) Return the sine of
x radians
import math
print "sin(3) : ",
math.sin(3)
sin(3) : 0.14112000806
cox (x) Return the cosine
of x radians
import math
print “cos(3) : ",
math.cos(3)
sin(3) : 0.14112000806
…..
36

Python Numbers (4)
• Python also supports two mathematical constants:
pi
e
37

Example
Built-inStringMethods
PYTHON STRINGS

Python Strings (1)
str = 'Hello World!'
print str # Prints complete string
print str[0] # Prints first character of the string
print str[2:5] # Prints characters starting from 3rd to 5th
print str[2:] # Prints string starting from 3rd character
print str * 2 # Prints string two times
print str + "TEST" # Prints concatenated string
print "Updated String :- ", str[:6] + 'Python'
Output
Hello World!
H
llo
llo World!
Hello World!Hello World!
Hello World!TEST
Updated String :- Hello Python
39

Python Strings (2)
40
Python includes some built-in methods to manipulate strings:
String Method Description Example
capitalize() Capitalizes first letter of
string
str = "this
is string
example....wo
w!!!";
print
"str.capitali
ze() : ",
str.capitaliz
e()
str.capitalize() : This
is string
example....wow!!!
len() Returns the length of the
string
str = "this
is string
example....wo
w!!!";
print "Length
of the
string: ",
len(str)
Length of the
string: 32
…..

Example
BasicListOperations
FunctionsVSMethods
Built-inListFunctions
Built-inListMethods
PYTHON LISTS

Python Lists (1)
list = [ 'abcd', 786 , 2.23, 'john', 70.2 ]
tinylist = [123, 'john']
print list # Prints complete list
print list[0] # Prints first element of the list
print list[-1] # Prints last element of the list
print list[1:3] # Prints elements starting from 2nd till 3rd
print list[2:] # Prints elements starting from 3rd element
print tinylist * 2 # Prints list two times
print list + tinylist # Prints concatenated lists
list[0] = 'efgh'; # updating list
print list
del list[2]; # deleting list element
print list 42

Python Lists (2)
Output
['abcd', 786, 2.23, 'john', 70.2]
abcd
70.2
[786, 2.23]
[2.23, 'john', 70.2]
[123, 'john', 123, 'john']
['abcd', 786, 2.23, 'john', 70.2, 123, 'john']
['efgh', 786, 2.23, 'john', 70.2]
['efgh', 786, 'john', 70.2]
43

Python Lists (3)
Other Basic List Operations
44
Expression Result Description
len([1, 2, 3]) 3 Length
3 in [1, 2, 3] True Membership
for x in [1, 2, 3]:
print x,
1 2 3 Iteration

Functions VS Methods
Function Method
A function is a piece of code that is
called by name.
A method is a piece of code that is
called by name that is associated with
an object.
It can pass data to operate on (i.e., the
parameters) and can optionally return
data (the return value).
It is able to operate on data that is
contained within the class.
All data that is passed to a function is
explicitly passed.
It is implicitly passed for the object for
which it was called.
def sum(num1, num2):
return num1 + num2
class Dog:
def my_method(self):
print "I am a Dog"
dog = Dog()
dog.my_method() # Prints "I am a
Dog"
45

Python Lists (4)
Some Built-in List Functions
46
Function Description Example
max(list) Returns item from
the list with max
value.
list = [456, 700, 200]
print "Max value
element : ", max(list)
Max value element : 700
min(list) Returns item from
the list with min
value.
list = [456, 700, 200]
print "Min value
element : ", min(list)
Min value element : 200
…..

Python Lists (5)
Some Built-in List Methods
47
Method Description Example
list.append(obj) Appends a
passed obj into the
existing list.
aList = [123, 'xyz',
'zara', 'abc'];
aList.append( 2009 );
print "Updated List :
", aList
Updated List : [123, 'xyz', 'zara',
'abc', 2009]
list.count(obj) Returns count of
how many
times obj occurs in
list.
aList = [123, 'xyz',
'zara', 'abc', 123];
print "Count for 123 :
", aList.count(123)
print "Count for zara :
", aList.count('zara')
Count for 123 : 2
Count for zara : 1
…..

Example
BasicTupleOperations
Built-inTupleFunctions
ListsVSTuples
PYTHON TUPLES

Python Tuples (1)
tuple = ( 'abcd', 786 , 2.23, 'john', 70.2 )
tinytuple = (123, 'john')
print tuple # Prints complete tuple
print tuple[0] # Prints first element of the tuple
print tuple[-1] # Prints last element of the tuple
print tuple[1:3] # Prints elements starting from 2nd till 3rd
print tuple[2:] # Prints elements starting from 3rd element
print tinytuple * 2 # Prints tuple two times
print tuple + tinytuple # Prints concatenated tuples
49

Python Tuples (2)
Output
('abcd', 786, 2.23, 'john', 70.2)
abcd
70.2
(786, 2.23)
(2.23, 'john', 70.2)
(123, 'john', 123, 'john')
('abcd', 786, 2.23, 'john', 70.2, 123, 'john')
50

Python Tuples (3)
Other Basic Tuple Operations
51
Expression Result Description
len((1, 2, 3)) 3 Length
3 in (1, 2, 3) True Membership
for x in (1, 2, 3):
print x,
1 2 3 Iteration

Python Tuples (4)
Some Built-in Tuple Functions
52
max(tuple) Returns item from
the tuple with max
value.
tuple = (456, 700, 200)
print "Max value
element : ", max(tuple)
Max value element : 700
min(tuple) Returns item from
the tuple with min
value.
tuple = (456, 700, 200)
print "Min value
element : ", min(tuple)
Min value element : 200
…..

Lists VS Tuples
Lists Tuples
Lists are enclosed in brackets ( [ ] ). Tuples are enclosed in parentheses ( ( )
).
Lists' elements and size can be
changed.
Tuples' elements and size cannot be
changed.
Support item deletion.
Example
list = [123, 'john']
print list
del list[1]
print list
[123, 'john']
[123]
Doesn't support item deletion.
Example
tuple = (123, 'john')
print tuple
del tuple[1]
print tuple
(123, 'john')
Traceback (most recent call last):
File "main.py", line 6, in
del tuple[1]
TypeError: 'tuple' object doesn't support item deletion
53

Introduction
Example
PropertiesofDictionaryKeys
Built-inDictionaryFunctions
Built-inDictionaryMethods
PYTHON DICTIONARY

Python Dictionary (1)
• Python's dictionaries are kind of hash table type.
• They work like associative arrays or hashes found in PHP and
Perl respectively.
• It consists of key-value pairs.
• A dictionary key can be almost any Python type, but are
usually numbers or strings, & values can be any arbitrary
Python object.
• Dictionaries are enclosed by curly braces ({ }) and values can
be assigned and accessed using square braces ([]).
55

dict = {}
dict['one'] = "This is one"
dict[2] = "This is two"
tinydict = {'name': 'john','code':6734,'dept':'sales'}
print dict['one'] # Prints value for 'one' key
print dict[2] # Prints value for 2 key
print tinydict # Prints complete dictionary
print tinydict.keys() # Prints all the keys
print tinydict.values() # Prints all the values
tinydict['name']="Peter" # update existing entry
tinydict['org']="abcd" # Add new entry
print tinydict
del dict[2]; # remove entry with key 2
print "dict: ", dict
dict.clear(); # remove all entries in dict
del dict ; # delete entire dictionary
56

Output
This is one
This is two
{'dept': 'sales', 'code': 6734, 'name': 'john'}
['dept', 'code', 'name']
['sales', 6734, 'john']
{'dept': 'sales', 'code': 6734, 'name': 'Peter', 'org': 'abcd'}
dict: {'one': 'This is one'}
dict: {}
dict:
57

Properties of Dictionary Keys
• More than one entry per key not allowed, i.e., no duplicate
key is allowed. When duplicate keys encountered during
assignment, the last assignment wins.
• Keys must be immutable. Which means you can use strings,
numbers or tuples as dictionary keys but something like ['key']
is not allowed.
58

Some Built-in Dictionary Functions
59
len(dict) Gives the total
length of the
dictionary.
dict = {'Name': 'Zara',
'Age': 7};
print "Length : %d" %
len (dict)
Length : 2
str(dict) Produces a
printable string
representation of a
dictionary.
'Age': 7};
print "Equivalent
String : %s" % str
(dict)
Equivalent String : {'Age': 7,
'Name': 'Zara'}
…..

Some Built-in Dictionary Methods
60
Method Description Example
dict.clear() Removes all
elements of
dictionary dict.
'Age': 7};print "Start
Len : %d" % len(dict)
dict.clear()
print "End Len : %d" %
len(dict)
Start Len : 2
End Len : 0
dict.copy() Returns a shallow
copy of
dictionary dict.
dict1 = {'Name':
'Zara', 'Age': 7};
dict2 = dict1.copy()
print "New Dictinary :
%s" % str(dict2)
New Dictinary : {'Age': 7, 'Name':
'Zara'}
…..

SomeExamples
PYTHON DATE & TIME

Python Date & Time (1)
import time; # This is required to include time module.
ticks = time.time() # returns the current system time in ticks
# since 12:00am, January 1, 1970
print "Number of ticks since 12:00am, January 1, 1970:", ticks
Output
Number of ticks since 12:00am, January 1, 1970: 1498320786.91
62

Python Date & Time (2)
import calendar
cal = calendar.month(2017, 6)
print "Here is the calendar:"
print cal
Output
Here is the calendar:
June 2017
Mo Tu We Th Fr Sa Su
1 2 3 4
5 6 7 8 9 10 11
12 13 14 15 16 17 18
19 20 21 22 23 24 25
26 27 28 29 30
63

WhatisaFunction?
Example
PassbyReference VSPass byValue
TypesofFunctionArguments
ThereturnStatement
GlobalVariableVSLocalVariable
PYTHON FUNCTION

What is a Function?
A function is a block of organized, reusable code that is used to
perform a single, related action.
Syntax
def function_name( parameters ):
#function_body
65

Example
#python function
def test(str):
print (str)
test("Hello World!!!")
Output
Hello World!!!
66

Pass by Reference VS Pass by
Value
Pass by Reference Pass by Value
# Function definition is here
def changeme( mylist ):
# This changes a passed list into
this function"
mylist.append([1,2,3,4]);
print "Values inside the
function: ", mylist
return
# Now you can call changeme function
mylist = [10,20,30];
changeme( mylist );
print "Values outside the function:
", mylist
Output
Values inside the function: [10, 20, 30, [1, 2, 3, 4]]
Values outside the function: [10, 20, 30, [1, 2, 3, 4]]
def changeme( mylist ):
# This changes a passed list into
this function"
mylist = [1,2,3,4]; # This would
# assign new reference in mylist
print "Values inside the
function: ", mylist
return
# Now you can call changeme function
mylist = [10,20,30];
changeme( mylist );
print "Values outside the function:
", mylist
Output
Values inside the function: [1, 2, 3, 4]
Values outside the function: [10, 20, 30]
67

Types of Function Arguments
Function Argument Description Example
Required Arguments The arguments passed to
a function in correct
positional order. The
number of arguments in
the function call should
match exactly with the
function definition.
def test(str):
print (str)
test()
File "python_func.py", line 4, in <module>
test()
TypeError: test() takes exactly 1 argument
(0 given)
Keyword Arguments When use keyword
arguments in a function
call, the caller identifies
the arguments by the
parameter name. Thus,
allowing the arguments
to place in any order.
def printinfo( name, age ):
print "Name: ", name
print "Age ", age
printinfo( age=50,
name="miki" )
Name: miki
Age 50
68

(2)
Default Arguments An argument that
assumes a default value
if a value is not provided
in the function call for
that argument.
def printinfo( name, age = 35
):
print "Name: ", name
print "Age ", age
printinfo( age=50,
name="miki" )
printinfo( name="miki" )
Name: miki
Age 50
Name: miki
Age 35
69

(3)
Variable-length
Arguments
We may need to process
a function for more
arguments than
specified while defining
the function. These
arguments are called
variable-length
arguments and are not
named in the function
definition
def printinfo( arg1,
*vartuple ):
print "Output is: "
print arg1
for var in vartuple:
print var
printinfo( 10 )
printinfo( 70, 60, 50 )
Output is:
10
Output is:
70
60
50
NOTE:
The asterisk (*) placed before the variable
vartuple holds the values of all non-
keyword variable arguments.
70

The return Statement
def add(x,y):
sum=x+y
return sum
total=add(10,20)
print total
Output
30
71

Global Variable VS Local
Variable
72
total = 0; # This is global variable
def sum( arg1, arg2 ):
# Add both the parameters and return them
total = arg1 + arg2; # Here total is local variable
print "Inside the function local total : ", total
return total;
# Now you can call sum function
sum( 10, 20 );
print "Outside the function global total : ", total
Output
Inside the function local total : 30
Outside the function global total : 0

What is Recursion?
• When function is called within the same function, it is known
as recursion.
• The function which calls the same function, is known as
recursive function.
74

WhatisaModule?
importStatement
from…importStatement
NamespacesandScoping
SomeFunctions
PythonPackages
PYTHON MODULES

What is a Module?
• A module is a file consisting of Python code.
• A module can define functions, classes and variables. It can
also include runnable code.
• A module allows you to logically organize your Python code.
• A module is a Python object with arbitrarily named attributes
that you can bind and reference.
77
Example
tmodule.py
def print_func( par ):
print "Hello : ", par

import Statement
# Import module tmodule
import tmodule
# Now you can call defined function that
# module as follows
tmodule.print_func("Zara")
Output
Hello : Zara
78

from…import Statement
Python's from statement allows you to import specific attributes
from a module into the current namespace.
79
Syntax Example
from modname import
name1[, name2[, ...
nameN]]
tmodule.py
def print_func( par ):
print "Hello : ", par
def print_func_2(str) :
print "Good Morning
",str
from_import_module.py
from tmodule import print_func_2
print_func_2("Zara")
Output
Good Morning Zara

Namespaces and Scoping
• Variables are names (identifiers) that map to objects. A namespace is a
dictionary of variable names (keys) and their corresponding objects
(values).
• A Python statement can access variables in a local namespace and in the
global namespace. If a local and a global variable have the same name,
the local variable shadows the global variable.
• Each function has its own local namespace.
• Python assumes that any variable assigned a value in a function is local.
• Therefore, in order to assign a value to a global variable within a
function, you must first use the global statement.
• The statement global VarName tells Python that VarName is a global
variable. Python stops searching the local namespace for the variable.
80

Example
Money = 2000
def AddMoney():
# Uncomment the following line
# to fix the code:
# global Money
Money = Money + 1
print Money
AddMoney()
print Money
Money = 2000
def AddMoney():
# Uncomment the following line
# to fix the code:
global Money
Money = Money + 1
print Money
AddMoney()
print Money
Output
2000
File "namespace_scoping.py", line 8, in <module>
AddMoney()
File "namespace_scoping.py", line 5, in AddMoney
Money = Money + 1
UnboundLocalError: local variable 'Money' referenced
before assignment
Output
2000
2001
81

Some Functions
dir( ) Returns a sorted list of
strings containing the
names defined by a
module.
import tmodule
print dir(tmodule)
Output
['__builtins__', '__doc__', '__file__',
'__name__', '__package__', 'print_func',
'print_func_2']
locals() Returns all the names
that can be accessed
locally from the function,
in which its called.
total = 0;
def sum( arg1, arg2 ):
total = arg1 + arg2;
print "locals():
",locals()
print "globals():
",globals()
sum( 10, 20 );
Output
locals(): {'arg1': 10, 'arg2': 20, 'total': 30}
globals(): {'__builtins__': <module
'__builtin__' (built-in)>, '__file__': 'loc
al_global.py', '__package__': None, 'sum':
<function sum at 0x017638F0>, '__name
__': '__main__', 'total': 0, '__doc__': None}
globals() Returns all the names
that can be accessed
globally from the
function, in which its
called.
…..
82

Python Packages
A package is a hierarchical file directory structure that defines a
single Python application environment that consists of modules
and subpackages and sub-subpackages, and so on.
G:.
└──phone
__init__.py (required to make Python treat phone as a package)
g3.py
isdn.py
pots.py
83

Example
pots.py
def pots_func():
print "I'm Pots Phone"
isdn.py
def pots_func():
print "I'm ISDN Phone"
g3.py
def pots_func():
print "I'm G3 Phone" 84

Example (contd…)
__init__.py
from pots import pots_func
from isdn import isdn_func
from g3 import g3_func
package.py
import sys
sys.path.append('G:2. DOCUMENTSPythonPrograms')
import phone
phone.pots_func()
phone.isdn_func()
phone.g3_func() 85

Output
I'm Pots Phone
I'm ISDN Phone
I'm G3 Phone
86

Introduction
Example
SomeMoreFileOperationFunctions
PYTHON FILE MANAGEMENT

Introduction
• Until now, we have been reading and writing to the standard
input and output. This works fine as long as the data is small.
• However, many real life problems involve large volumes of
data & in such situations the standard I/O operations poses
two major problems:
- It becomes cumbersome & time consuming to handle large
volumes of data through terminals.
- The entire data is lost either the program is terminated or the
computer is turned off.
• Thus, to have a more flexible approach where data can be
stored on the disks & read whenever necessary without
destroying, the concept of file is employed. 88

Introduction (2)
• Like most other languages, Python supports a number of
functions that have the ability to perform basic file operations:
- Naming a file
- Opening a file (fileobject = open(file_name [, access_mode][, buffering]))
- Reading a file (string = fileobject.read([count]))
- Writing to a file (fileobject.write(string))
- Closing a file (fileobject.close())
access_mode = r, rb, r+, rb+, w, wb, w+, wb+, a, ab,a+, ab+
buffering = <1, 0, >1
count = number of bytes to be read from the opened file
string = content to be written into the opened file 89

Example
# Open a file in write mode
fo = open("foo.txt", "wb")
fo.write( "Python is a great language.nYeah its
great!!n");
# Close opened file
fo.close()
# Open the file in read mode
fo = open("foo.txt", "rb")
string = fo.read()
print string
# Close opened file
fo.close() 90

Output
Python is a great language.
Yeah its great!!
91

Some More File Operation
Functions
92
Function Name Description
tell() Gives the current position within the
file
seek(offset[, from]) Changes the current file position. The
offset argument indicates the number
of bytes to be moved. The from
argument specifies the reference
position from where the bytes are to
be moved.
…..

WhatisException?
HandlinganException
try.....except.....else
try.....finally
ArgumentofanException
User-Defined Exceptions
PYTHON EXCEPTION
HANDLING

What is Exception?
• An exception is an event, which occurs during the execution of
a program that disrupts the normal flow of the program's
instructions.
• When a Python script encounters a situation that it cannot
cope with, it raises an exception.
• An exception is a Python object that represents an error.
• When a Python script raises an exception, it must either
handle the exception immediately otherwise it terminates and
quits.
95

Handling an Exception
• If you have some suspicious code that may raise an exception,
you can defend your program by placing the suspicious code in
a try: block.
• After the try: block, include an except: statement, followed by
a block of code which handles the problem as elegantly as
possible.
• After the except clause(s), you can include an else-clause. The
code in the else-block executes if the code in the try: block
does not raise an exception.
96

try.....except.....else (Syntax)
try:
You do your operations here;
......................
except ExceptionI:
If there is ExceptionI, then execute this block.
except ExceptionII:
If there is ExceptionII, then execute this block.
......................
except ExceptionN:
If there is ExceptionN, then execute this block.
else:
If there is no exception then execute this block.
97

try.....except.....else (Example)
Example I Example II
try:
fh = open("testfile", "w")
fh.write("This is my test file
for exception handling!!")
except IOError:
print "Error: can't find file
or read data"
else:
print "Written content in the
file successfully"
fh.close()
try:
fh = open("testfile_2", "r")
string = fh.read()
except IOError:
or read data"
else:
print string
fh.close()
Output
Written content in the file successfully
Output
Error: can't find file or read data
98

try.....finally
• You can use a finally: block along with a try: block.
• The finally block is a place to put any code that must execute, whether the try-block raised
an exception or not.
Syntax
try:
......................
[except ExceptionI:
If there is ExceptionI, then execute this block.
except ExceptionII:
If there is ExceptionII, then execute this block.
......................
except ExceptionN:
If there is ExceptionN, then execute this block.
else:
If there is no exception then execute this block.]
finally:
This would always be executed.
......................
99

Example
Example I Example II
try:
fh = open("testfile", "w")
fh.write("This is my test file
for exception handling!!")
except IOError:
or read data"
else:
print "Written content in the
file successfully"
finally:
print "Closing the file"
fh.close()
try:
fh = open("testfile_2", "r")
string = fh.read()
except IOError:
or read data"
else:
print string
finally:
print "Closing the file"
fh.close()
Output
Written content in the file successfully
Closing the file
Output
Error: can't find file or read data
Closing the file
100

Argument of an Exception
• An exception can have an argument, which is a value that
gives additional information about the problem.
• This argument receives the value of the exception mostly
containing the cause of the exception.
Syntax
try:
......................
except ExceptionType, Argument:
You can print value of Argument here...
101

Example
# Define a function here.
def temp_convert(var):
try:
return int(var)
except ValueError, Argument:
print "The argument does not contain
numbersn", Argument
# Call above function here.
temp_convert("xyz");
Output
The argument does not contain numbers
invalid literal for int() with base 10: 'xyz'
102

User-Defined Exceptions
Python also allows you to create your own exceptions by
deriving classes from the standard built-in exceptions.
103
Example
# user-defined exception
class LevelError(ValueError):
def __init__(self, arg):
self.arg = arg
try:
level=input("Enter level: ")
if(level<1):
# Raise an exception with argument
raise LevelError("Level less than 1")
except LevelError, le:
# Catch exception
print 'Error: ', le.arg
else:
print level
Output
Enter level: 0
Error: Level less than 1

References
• Courtesy of YouTube – ProgrammingKnowledge. URL:
https://www.youtube.com/user/ProgrammingKnowledge,
Python Tutorial for Beginners (For Absolute Beginners)
• Courtesy of TutorialsPoint – Python Tutorial. URL:
http://www.tutorialspoint.com/python
• Courtesy of W3Resource – Python Tutorial. URL:
http://www.w3resource.com/python/python-tutorial.php
104

Python (basic)

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