Python programming : Exceptions

Errors

Categories of Errors

Compile-time

Runtime

Logical

Errors
Compile-Time
What? These are syntactical errors found in the code, due to which program
fails to compile
Example Missing a colon in the statements llike if, while, for, def etc
Program Output
x = 1
if x == 1
print("Colon missing")
py 1.0_compile_time_error.py
File "1.0_compile_time_error.py", line 5
if x == 1
^
SyntaxError: invalid syntax
x = 1
#Indentation Error
if x == 1:
print("Hai")
print("Hello")
py 1.1_compile_time_error.py
File "1.1_compile_time_error.py", line 8
print("Hello")
^
IndentationError: unexpected indent

Errors
Runtime - 1
What? When PVM cannot execute the byte code, it flags runtime error
Example Insufficient memory to store something or inability of the PVM to
execute some statement come under runtime errors
Program Output
def combine(a, b):
print(a + b)
#Call the combine function
combine("Hai", 25)
py 2.0_runtime_errors.py
Traceback (most recent call last):
File "2.0_runtime_errors.py", line 7, in <module>
combine("Hai", 25)
File "2.0_runtime_errors.py", line 4, in combine
print(a + b)
TypeError: can only concatenate str (not "int") to str
"""
Conclusion:
1. Compiler will not check the datatypes.
2.Type checking is done by PVM during run-time.
"""

Errors
Runtime - 2
What? When PVM cannot execute the byte code, it flags runtime error
Example Insufficient memory to store something or inability of the PVM to
execute some statement come under runtime errors
Program Output
#Accessing the item beyond the array
bounds
lst = ["A", "B", "C"]
print(lst[3])
py 2.1_runtime_errors.py
File "2.1_runtime_errors.py", line 5, in <module>
print(lst[3])
IndexError: list index out of range

Errors
Logical-1
What? These errors depicts flaws in the logic of the program
Example Usage of wrong formulas
Program Output
def increment(sal):
sal = sal * 15 / 100
return sal
#Call the increment()
sal = increment(5000.00)
print("New Salary: %.2f" % sal)
py 3.0_logical_errors.py
New Salary: 750.00

Errors
Logical-2
What? These errors depicts flaws in the logic of the program
Example Usage of wrong formulas
Program Output
#1. Open the file
f = open("myfile", "w")
#Accept a, b, store the result of a/b into the file
a, b = [int(x) for x in input("Enter two number: ").split()]
c = a / b
#Write the result into the file
f.write("Writing %d into myfile" % c)
#Close the file
f.close()
print("File closed")
py 4_effect_of_exception.py
Enter two number: 10 0
File "4_effect_of_exception.py", line 8, in
<module>
c = a / b
ZeroDivisionError: division by zero

Errors
Common

When there is an error in a program, due to its sudden termination, the following things
can be suspected

The important data in the files or databases used in the program may be lost

The software may be corrupted

The program abruptly terminates giving error message to the user making the user
losing trust in the software

Exceptions
Introduction

An exception is a runtime error which can be handled by the programmer

The programmer can guess an error and he can do something to eliminate the harm caused by
that error called an ‘Exception’
BaseException
Exception
StandardError Warning
ArthmeticError
AssertionError
SyntaxError
TypeError
EOFError
RuntimeError
ImportError
NameError
DeprecationWarning
RuntimeWarning
ImportantWarning

Exceptions
Exception Handling

The purpose of handling errors is to make program robust
Step-1 try:
statements
#To handle the ZeroDivisionError Exception
try:
a, b = [int(x) for x in input("Enter two numbers: ").split()]
c = a / b
Step-2 except exeptionname:
statements
except ZeroDivisionError:
print("Divide by Zero Error")
print("Don't enter zero as input")
Step-3 finally:
statements
finally:
f.close()
print("Myfile closed")

Exceptions
Program
#To handle the ZeroDivisionError Exception
#An Exception handling Example
try:
a, b = [int(x) for x in input("Enter two numbers: ").split()]
c = a / b
print("Divide by Zero Error")
print("Don't enter zero as input")
finally:
f.close()
print("Myfile closed")
Output:
py 5_exception_handling.py
Enter two numbers: 10 0
Divide by Zero Error
Don't enter zero as input
Myfile closed

Exceptions
Exception Handling Syntax
try:
statements
except Exception1:
handler1
except Exception2:
handler2
else:
statements
finally:
statements

Exceptions
Exception Handling: Noteworthy
- A single try block can contain several except blocks.
- Multiple except blocks can be used to handle multiple exceptions.
- We cannot have except block without the try block.
- We can write try block without any except block.
- Else and finally are not compulsory.
- When there is no exception, else block is executed after the try block.
- Finally block is always executed.

Exceptions
Types: Program-1
#To handle the syntax error given by eval() function
#Example for Synatx error
try:
date = eval(input("Enter the date: "))
except SyntaxError:
print("Invalid Date")
else:
print("You entered: ", date)
Output:
Run-1:
Enter the date: 5, 12, 2018
You entered: (5, 12, 2018)
Run-2:
Enter the date: 5d, 12m, 2018y
Invalid Date

Exceptions
Types: Program-2
#To handle the IOError by open() function
#Example for IOError
try:
name = input("Enter the filename: ")
f = open(name, "r")
except IOError:
print("File not found: ", name)
else:
n = len(f.readlines())
print(name, "has", n, "Lines")
f.close()
If the entered file is not exists, it will raise an IOError

Exceptions
Types: Program-3
#Example for two exceptions
#A function to find the total and average of list elements
def avg(list):
tot = 0
for x in list:
tot += x
avg = tot / len(list)
return tot.avg
#Call avg() and pass the list
try:
t, a = avg([1, 2, 3, 4, 5, 'a'])
#t, a = avg([]) #Will give ZeroDivisionError
print("Total = {}, Average = {}". format(t, a))
except TypeError:
print("Type Error: Pls provide the numbers")
print("ZeroDivisionError, Pls do not give empty list")
Output:
Run-1:
Type Error: Pls provide the numbers
Run-2:
ZeroDivisionError, Pls do not give empty list

Exceptions
Except Block: Various formats
Format-1 except Exceptionclass:
Format-2 except Exceptionclass as obj:
Format-3 except (Exceptionclass1, Exceptionclass2, ...):
Format-4 except:

Exceptions
Types: Program-3A
#Example for two exceptions
#A function to find the total and average of list elements
def avg(list):
tot = 0
for x in list:
tot += x
avg = tot / len(list)
return tot.avg
#Call avg() and pass the list
try:
t, a = avg([1, 2, 3, 4, 5, 'a'])
#t, a = avg([]) #Will give ZeroDivisionError
print("Total = {}, Average = {}". format(t, a))
except (TypeError, ZeroDivisionError):
print("Type Error / ZeroDivisionError”)
Output:
Run-1:
Type Error / ZeroDivisionError
Run-2:
Type Error / ZeroDivisionError

Exceptions
The assert Statement

It is useful to ensure that a given condition is True, It is not True, it raises
AssertionError.

Syntax:
assert condition, message

Exceptions
The assert Statement: Programs
Program - 1 Program - 2
#Handling AssertionError
try:
x = int(input("Enter the number between 5 and 10: "))
assert x >= 5 and x <= 10
print("The number entered: ", x)
except AssertionError:
print("The condition is not fulfilled")
#Handling AssertionError
try:
x = int(input("Enter the number between 5 and 10: "))
assert x >= 5 and x <= 10, "Your input is INVALID"
print("The number entered: ", x)
except AssertionError as Obj:
print(Obj)

Exceptions
User-Defined Exceptions
Step-1 class MyException(Exception):
def __init__(self, arg):
self.msg = arg
Step-2 raise MyException("Message")
Step-3 try:
#code
except MyException as me:
print(me)

Exceptions
User-Defined Exceptions: Program
#To create our own exceptions and raise it when needed
class MyException(Exception):
def __init__(self, arg):
self.msg = arg
def check(dict):
for k, v in dict.items():
print("Name = {:15s} Balance = {:10.2f}" . format(k, v)) if (v < 2000.00):
raise MyException("Less Bal Amount" + k)
bank = {"Raj": 5000.00, "Vani": 8900.50, "Ajay": 1990.00}
try:
check(bank)
except MyException as me:
print(me)

Python programming : Exceptions

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