Exception handling in python

Exception Handling in Python
February 13, 2018
1 Exception
• An indication of a special event that occurs during a programs exceution.
• Indicates that, although the event can occur, the event occurs infrequently.
• Leads to Resource Leak
– Aborting a program component could leae a file or a network connection in a state in
which other programs are not able to acquire the resource.
• When a Python script encounters a situation that it can’t cope with, it raises an exception.
• An exception is a Python object that represents an error
• Examples of Exceptions
1. Division by Zero
2. Addition of wo incompatible types
3. Accessing a file that is nonexistent.
4. Accessing a nonexistent index of a sequence.
5. Deleting a table in a disconnected database seerver.
6. Withdraing money greater than the available amount.
7. etc ...
1.1 Exception Handling
• When no exceptions occur, Exception Handling code incurs little or no performance penal-
ties.
• Programs that implement Exception handling operate more effiently than programs that
perform Error Handling throughout the program logic.
• Exception handling enables the programmer to remove error-handling code from the "main
line" of the programs execution.
• This improves program clarity and enhances modifiability
1

• Advantages of Exception Handling
1. Handle the error
– catching the exception
– resolving the exception
2. Continue processing as if no error had occured
3. Programs are more clear, robust and more fault-tolerant
1.2 Exception class
• Exception are objects of classes that inherit from class Exception.
• Programmer deﬁned exception classes should derive directly or indirectly from class Excep-
tion.
1.2.1 Class Hierarchy of all built-in Exceptions
BaseException
+-- SystemExit
+-- KeyboardInterrupt
+-- GeneratorExit
+-- Exception
+-- StopIteration
+-- StopAsyncIteration
+-- ArithmeticError
| +-- FloatingPointError
| +-- OverflowError
| +-- ZeroDivisionError
+-- AssertionError
+-- AttributeError
+-- BufferError
+-- EOFError
+-- ImportError
| +-- ModuleNotFoundError
+-- LookupError
| +-- IndexError
| +-- KeyError
+-- MemoryError
+-- NameError
| +-- UnboundLocalError
+-- OSError
| +-- BlockingIOError
| +-- ChildProcessError
| +-- ConnectionError
| | +-- BrokenPipeError
| | +-- ConnectionAbortedError
| | +-- ConnectionRefusedError
2

| | +-- ConnectionResetError
| +-- FileExistsError
| +-- FileNotFoundError
| +-- InterruptedError
| +-- IsADirectoryError
| +-- NotADirectoryError
| +-- PermissionError
| +-- ProcessLookupError
| +-- TimeoutError
+-- ReferenceError
+-- RuntimeError
| +-- NotImplementedError
| +-- RecursionError
+-- SyntaxError
| +-- IndentationError
| +-- TabError
+-- SystemError
+-- TypeError
+-- ValueError
| +-- UnicodeError
| +-- UnicodeDecodeError
| +-- UnicodeEncodeError
| +-- UnicodeTranslateError
+-- Warning
+-- DeprecationWarning
+-- PendingDeprecationWarning
+-- RuntimeWarning
+-- SyntaxWarning
+-- UserWarning
+-- FutureWarning
+-- ImportWarning
+-- UnicodeWarning
+-- BytesWarning
+-- ResourceWarning
In [1]: print(1/0)
print("Cool code")
---------------------------------------------------------------------------
ZeroDivisionError Traceback (most recent call last)
<ipython-input-1-fcea2cfbf768> in <module>()
----> 1 print(1/0)
2 print("Cool code")
3

ZeroDivisionError: division by zero
In [16]: x = 10
if x > 0:
print "greater"
File "<ipython-input-16-f27b142f89d4>", line 3
print "greater"
ˆ
SyntaxError: Missing parentheses in call to 'print'
1.3 Syntax : try - except - else clause
In [15]: try:
print(1/0)
except:
pass
print("Cool Code")
Cool Code
In [14]: try:
x = 1
y = "Hai"
quotient = x/y
except TypeError:
print("Type error found!!")
except ZeroDivisionError:
4

print("Division by zero!!")
else:
print(quotient)
Type error found!!
In [13]: try:
x = 1
y = 0
quotient = x/y
except TypeError:
else:
print(quotient)
Division by zero!!
In [12]: try:
x = 1
y = 0
quotient = x/y
except TypeError:
else:
print(quotient)
print("Finally done!!")
Division by zero!!
Finally done!!
1.4 finally clause
• Programs frequently request and release resources dynamically.
• Programs that obtain certain types of resources (such as files) sometines must return re-
sources explicitly to the system to avoid resource leaks.
• The finally clause is an ideal location to place resource deallocation code for resources ac-
quired.
In [11]: try:
fh = open("testfile","w")
try:
fh.write("This is my test file for exception handling!!")
5

print("File Created in the working directory!!")
finally:
fh.close()
except IOError:
print("Error : can't find the file or read data")
print("Done!!")
File Created in the working directory!!
Done!!
In [22]: class Jeep:
def __init__(self,name):
self.name = name
class Van:
def __init__(self,value):
self.value = value
j = Jeep("ABC")
v = Van("XYZ")
try:
if v.value != j.name:
raise Exception
except Exception:
print("Names are not matching!!")
finally:
del j
del v
print("Both objects are deleted!!")
#print(v, j)
Names are not matching!!
Both objects are deleted!!
1.5 Raising / Throwing an Exception
• raise keyword
• Catching and handling exceptions enables a program to know when an error has occured,
then to take actions to minimize the consequences of that error.
• specify an argument or arguments that initailize the exception object.
– Syntax : raise [ExceptionName]
In [9]: x = int(input("Enter level :"))
try:
6

if (x >10 or x < 1):
raise ValueError
else:
print("Wait while the game is loading....")
except ValueError:
print("The game consists of 10 levels only ranging from 1 to 10")
Enter level :-1
The game consists of 10 levels only ranging from 1 to 10
In [35]: x = 2
y = 8
z = y / x
print("quotient: ",z)
while(1):
try:
s = int(input("enter the number :"))
z = y / s
print("Quotient: ",z)
except ValueError as arg:
print("Invalid Input!!", arg)
except ArithmeticError as arg:
print("Invalid Input!!", arg)
else:
print("Exit while loop")
break
print("Done")
quotient: 4.0
enter the number :hello
Invalid Input!! invalid literal for int() with base 10: 'hello'
enter the number :0
Invalid Input!! division by zero
enter the number :2
Quotient: 4.0
Exit while loop
Done
1.6 User defined Exceptions
• User can create exception classes to handle exceptions specific to a program.
• User Defined Exception classes must be derived from bass class, Exception
In [38]: class MyException(Exception):
def __init__(self):
self.argument = "I am a User Defined Exception"
try:
7

print("Example of a User Defined Exception")
print()
raise MyException
except MyException:
print(MyException().argument)
Example of a User Defined Exception
I am a User Defined Exception
In [42]: import math
class NegativeNumberError(ArithmeticError):
def __init__(self):
self.argument = "Square root of Negative Number not defined!!"
def square_root(number):
if number < 0:
raise NegativeNumberError
return math.sqrt(number)
while(1):
try:
user_value = float(input("enter the value :"))
print(square_root(user_value))
except ValueError:
print("Oops!! Value Error found!!")
except NegativeNumberError as arg:
print(NegativeNumberError().argument)
else:
print("Exit while loop")
break
enter the value :-1
Square root of Negative Number not defined!!
enter the value :hello
Oops!! Value Error found!!
enter the value :2
1.4142135623730951
Exit while loop
In [43]: class CarException(Exception):
def __init__(self):
self.argument = "Found an invalid Car name!!"
pass
class Car:
def __init__(self,value):
8

self.value = value
car_list = [Car("Maruti 800"), Car("Scorpio"),Car("Van")]
for c in car_list:
try:
print(c.value, end = ' ')
if c.value == "Van":
raise CarException
except CarException:
print(": ",CarException().argument)
else:
print(" : Valid Car name!!!")
print("Done!!")
Maruti 800 : Valid Car name!!!
Scorpio : Valid Car name!!!
Van : Found an invalid Car name!!
Done!!
1.7 Assertion
• Assertion is a sanity-check that you can turn on/off when you are done with your testing of
the program
• It’s like a raise-if statement (or to be more accurate, a raise-if-not statement).
• An expression is tested, and if the result comes up false an exception is raised.
• Programs often place assertions
• at the start of a function to check for valid input,
• and after a function call to check for valid output
• Syntax:
• assert Expression [, Arguments]
In [61]: def apply_discount(product,discount):
price = int(product['price'] *(1.0-discount))
try:
assert 0 <= price<= product['price']
except AssertionError:
print("Assertion Error")
else:
return price/100
# Sample product: Nice Shoes for Rs. 149.00
shoes = {'name':'Fancy Shoes','price':14900}
9

# 25% off --> 111.75
apply_discount(shoes,0.25)
Out[61]: 111.75
In [62]: def apply_discount(product,discount):
price = int(product['price'] *(1.0-discount))
try:
assert 0 <= price<= product['price']
print("Assertion Error")
else:
return price/100
# Sample product: Nice Shoes for Rs. 149.00
shoes = {'name':'Fancy Shoes','price':14900}
# 200% discount
apply_discount(shoes,2.0)
Assertion Error
In [54]: def set_bearing(new_bearing):
bearing = new_bearing
try:
assert(0 <= bearing <=90)
print("AssertionError")
return bearing
print(set_bearing(45))
45
In [55]: def compute_perimeter(parcel):
perimeter = 0
for p in parcel:
#assert(p >= 0)
perimeter += p
try:
10

assert(perimeter > 1)
print("AssertionError")
return perimeter
parcel = [2,3,-4]
print(compute_perimeter(parcel))
AssertionError
1
11

Exception handling in python

More Related Content

What's hot (20)

Similar to Exception handling in python (20)

More from Lifna C.S (7)

Recently uploaded (20)

Exception handling in python