Python Exception handling using Try-Except-Finally

Types of errors
• There are basically two types of errors:
– syntax errors
– run time errors (program state errors)
>>> excuse = 'I'm sick'
SyntaxError: invalid syntax
>>> print(hour + ':' + minute + ':' + second)
Traceback (most recent call last):
File "<pyshell#113>", line 1, in <module>
print(hour + ':' + minute + ':' + second)
TypeError: unsupported operand type(s) for +: 'int' and 'str’
>>> infile = open('sample.txt')
infile = open('sample.txt')
IOError: [Errno 2] No such file or directory: 'sample.txt’

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try() is used in Error and Exception Handling
There are two kinds of errors :
•Syntax Error : Also known as Parsing Errors, most basic. Arise
when the Python parser is unable to understand a line of code.
•Exception : Errors which are detected during execution. eg –
ZeroDivisionError.
try and except in Python
List of Exception Errors :
•IOError : if file can’t be opened
•KeyboardInterrupt : when an unrequired key is pressed by the user
•ValueError : when built-in function receives a wrong argument
•EOFError : if End-Of-File is hit without reading any data
•ImportError : if it is unable to find the module

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Introduction to Computing Using Python
Syntax errors
>>> (3+4]
>>> if x == 5
>>> print 'hello'
>>> lst = [4;5;6]
>>> for i in range(10):
print(i)
SyntaxError: expected an indented block
Syntax errors are errors that are due to the incorrect format
of a Python statement
• They occur while the statement is being translated to
machine language and before it is being executed.

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Erroneous state errors
>>> 3/0
File "<pyshell#56>", line 1, in
<module>
3/0
ZeroDivisionError: division by
zero
The program execution gets into an erroneous state
>>> lst
<module>
lst
NameError: name 'lst' is not defined
>>> lst = [12, 13, 14]
>>> lst[3]
<module>
lst[3]
IndexError: list index out of
range
>>> lst * lst
<module>
lst * lst
TypeError: can't multiply sequence by
non-int of type 'list’
>>> int('4.5')
int('4.5')
ValueError: invalid literal for int() with base 10: '4.5'

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when the program execution gets into an erroneous state, an exception
object is created
• This object has a type that is related to the type of error
• The object contains information about the error
• The default behavior is to print this information and interrupt the execution of
the statement that “caused” the error
The reason behind the term “exception” is that when an error occurs and an
exception object is created, the normal execution flow of the program is
interrupted and execution switches to the exceptional control flow

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The idea of the try-except clause is to handle exceptions (errors at runtime).
The syntax of the try-except block is:
try:
<do something>
except Exception:
<handle the error>
The idea of the try-except block is this:
•try: the code with the exception(s) to catch. If an exception is raised, it jumps
straight into the except block.
•except: this code is only executed if an exception occurred in the try block. The
except block is required with a try block, even if it contains only the pass
statement.
It may be combined with the else and finally keywords.
•else: Code in the else block is only executed if no exceptions were raised in the
try block.
•finally: The code in the finally block is always executed, regardless of if a an
exception was raised or not.

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Exception types
Some of the built-in exception classes:
Exception Explanation
KeyboardInterrupt Raised when user hits Ctrl-C, the interrupt key
OverflowError Raised when a floating-point expression evaluates to a value that is too large
ZeroDivisionError Raised when attempting to divide by 0
IOError Raised when an I/O operation fails for an I/O-related reason
IndexError Raised when a sequence index is outside the range of valid indexes
NameError Raised when attempting to evaluate an unassigned identifier (name)
TypeError Raised when an operation of function is applied to an object of the wrong type
ValueError Raised when operation/function has an argument of the right type but incorrect value

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Exceptional control flow
>>> f(2)
>>> f(2)
Start f
>>> f(2)
Start f
Start g
>>> f(2)
Start f
Start g
Start h
n = 2
f(2)
n = 1
g(1)
n = 0
h(0)
n = 2
print('Start f')
f(2)
n = 2
print('Start f')
g(n-1)
f(2)
n = 2
print('Start f')
g(n-1)
print(n)
f(2)
n = 1
print('Start g')
g(1)
n = 1
print('Start g')
h(n-1)
g(1)
n = 1
print('Start g')
h(n-1)
print(n)
g(1)
n = 0
print('Start h')
print(1/n)
h(0)
n = 0
print('Start h')
h(0)
n = 0
print('Start h')
print(1/n)
print(n)
h(0)
>>> f(2)
Start f
Start g
Start h
f(2)
File "/Users/me/ch7/stack.py", line 13, in f
g(n-1)
File "/Users/me/ch7/stack.py", line 8, in g
h(n-1)
File "/Users/me/ch7/stack.py", line 3, in h
print(1/n)
ZeroDivisionError: division by zero
>>>
Normal control flow
Exceptional control flow
The default behavior is to interrupt the execution of
each “active” statement and print the error information
contained in the exception object.
1. def h(n):
2. print('Start h')
3. print(1/n)
4. print(n)
5.
6. def g(n):
7. print('Start g')
8. h(n-1)
9. print(n )
10.
11. def f(n):
12. print('Start f')
13. g(n-1)
14. print(n)

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Catching and handling exceptions
It is possible to override the default behavior (print error information
and “crash”) when an exception is raised, using try/except
statements
strAge = input('Enter your age: ')
intAge = int(strAge)
print('You are {} years old.'.format(intAge))
try:
strAge = input('Enter your age: ')
print('You are {} years old.'.format(intAge))
except:
print('Enter your age using digits 0-9!')
>>> ======================== RESTART ========================
>>>
Enter your age: fifteen
File "/Users/me/age1.py", line 2, in <module>
ValueError: invalid literal for int() with base 10: 'fifteen'
>>>
>>> ========== RESTART ==========
>>>
Enter your age: fifteen
Enter your age using digits 0-9!
>>>
Default behavior:
Custom behavior:
If an exception is raised while
executing the try block, then
the block of the associated
except statement is executed
The except code block is the exception handler

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Format of a try/except statement pair
try:
<indented code block>
except:
<exception handler block>
<non-indented statement>
The format of a try/except pair of statements is:
The exception handler handles any
exception raised in the try block
The except statement is said to
catch the (raised) exception
It is possible to restrict the except statement to catch exceptions of
a specific type only
try:
<indented code block>
except <ExceptionType>:
<exception handler block>
<non-indented statement>

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>>> readAge('age.txt')
Value cannot be converted to integer.
>>>
Format of a try/except statement pair
def readAge(filename):
'converts first line of file filename to an integer and prints it'
try:
infile = open(filename)
strAge = infile.readline()
age = int(strAge)
print('age is', age)
except ValueError:
print('Value cannot be converted to integer.')
1 fifteen
age.txt
>>> readAge('age.txt')
Value cannot be converted to integer.
>>> readAge('age.text')
readAge('age.text')
File "/Users/me/ch7.py", line 12, in readAge
IOError: [Errno 2] No such file or directory: 'age.text'
>>>
default exception
handler prints this

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Multiple exception handlers
def readAge(filename):
'converts first line of file filename to an integer and prints it'
try:
strAge = infile.readline()
age = int(strAge)
print('age is',age)
except IOError:
# executed only if an IOError exception is raised
print('Input/Output error.')
except ValueError:
# executed only if a ValueError exception is raised
print('Value cannot be converted to integer.')
except:
# executed if an exception other than IOError or ValueError is raised
print('Other error.')

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Basic Syntax :
try:
// Code
except:
// Code
else: (optional)
// code
finally : (optional)
//code
How try() works?
•First try clause is executed
i.e. the code between try and except clause.
•If there is no exception,
then only try clause will run, except clause is finished.
•If any exception occurred, try clause will be skipped
and except clause will run.
•If any exception occurs,
but the except clause within the code doesn’t handle it,
it is passed on to the outer try statements.
If the exception left unhandled, then the execution
stops.
•A try statement can have more than
one except clause

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# Python code to illustrate
# working of try()
def divide(x, y):
try:
# Floor Division : Gives only
Fractional Part as Answer
result = x // y
print("Yeah ! Your answer is :",
result)
except ZeroDivisionError:
print("Sorry ! You are dividing by
zero ")
# Look at parameters and note the working of Program
divide(3, 2)
Ans. Yeah ! Your answer is : 1

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# Python code to illustrate
# working of try()
def divide(x, y):
try:
# Floor Division : Gives only Fractional Part as Answer
result = x // y
print("Yeah ! Your answer is :", result)
print("Sorry ! You are dividing by zero ")
# Look at parameters and note the working of Program
divide(3, 0)
Ans. Sorry ! You are dividing by zero

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data = 50
try:
data = data/0
print('Cannot divide by 0 ', end = '')
else:
print('Division successful ', end = '')
try:
data = data/5
except:
print('Inside except block ', end = '')
else:
print('VKS', end = '')
a) Cannot divide by 0 VKS
b) Cannot divide by 0
c) Cannot divide by 0 Inside except block VKS
d) Cannot divide by 0 Inside except block
Ans. (a)
Explanation: The else block of code is executed only when there occurs no
exception in try block.

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data = 50
try:
data = data/10
print('Cannot divide by 0 ', end = '')
finally:
print(‘VKS-Learning-Hub ', end = '')
else:
print('Division successful ', end = '')
a) Runtime error
b) Cannot divide by 0 VKS-Learning-Hub
c) VKS-Learning-Hub Division successful
d) VKS-Learning-Hub
Ans. (a)
Explanation: else block following a finally block is not allowed in python. Python
throws syntax error when such format is used.

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value = [1, 2, 3, 4]
data = 0
try:
data = value[4]
except IndexError:
print('VKS', end = '')
except:
print('VKS Learning Hub', end = '')
a) VKS Learning Hub
b) VKS
c) VKS VKS Learning Hub
d) Compilation error
Ans. (b)
Explanation: At a time only one exception is caught, even though the throw exception
in the try block is likely to belong to multiple exception type.

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value = [1, 2, 3, 4]
data = 0
try:
data = value[3]
except IndexError:
print('GFG IndexError ', end = '')
except:
print('VKS Learning Hub IndexError ', end = '')
finally:
print('VKS IndexError ', end = '')
data = 10
try:
data = data/0
print('VKS ZeroDivisionError ', end = '')
finally:
print('VKS ZeroDivisionError ')
a) VKS ZeroDivisionError VKS ZeroDivisionError
b) VKS ZeroDivisionError VKS ZeroDivisionError
c) VKS IndexError VKS ZeroDivisionError VKS ZeroDivisionError
d) VKS IndexError VKS ZeroDivisionError
Ans. (c)
Explanation: finally block of code is always executed whether the exception
occurs or not. If exception occurs, the except block is executed first followed
by finally block.

Python Exception handling using Try-Except-Finally

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