Python Errors & Exceptions

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Errors & Exceptions


There are two major kinds of errors:

  1. Syntax Errors
  2. Exceptions

Syntax Errors

Syntax errors are parsing errors which occur when the code is not adhering to Python Syntax.

Code

if True print("Hello")
PYTHON

Output

SyntaxError: invalid syntax

When there is a syntax error, the program will not execute even if that part of code is not used.

Code

print("Hello")
def greet():
print("World"
PYTHON

Output

SyntaxError: unexpected EOF while parsing

Notice that in the above code, the syntax error is inside the

greet
function, which is not used in rest of the code.

Exceptions


Even when a statement or expression is syntactically correct, it may cause an error when an attempt is made to execute it.

Errors detected during execution are called exceptions.

Example Scenario


We wrote a program to download a Video over the Internet.

  • Internet is disconnected during the download

  • We do not have space left on the device to download the video

Example 1

Division Example

Input given by the user is not within expected values.

Code

def divide(a, b):
return a / b
divide(5, 0)
PYTHON

Output

ZeroDivisionError: division by zero

Example 2

Input given by the user is not within expected values.

Code

def divide(a, b):
return a / b
divide("5", "10")
PYTHON

Output

TypeError: unsupported operand type(s) for /: 'str' and 'str'

Example 3

Consider the following code, which is used to update the quantity of items in store.

Code

class Store:
def __init__(self):
self.items = {
"milk" : 20, "bread" : 30, }
def add_item(self, name, quantity):
self.items[name] += quantity
s = Store()
s.add_item('biscuits', 10)
PYTHON

Output

KeyError: 'biscuits'

Working With Exceptions


What happens when your code runs into an exception during execution?

The application/program crashes.

End-User Applications


When you develop applications that are directly used by end-users, you need to handle different possible exceptions in your code so that the application will not crash.

Reusable Modules


When you develop modules that are used by other developers, you should raise exceptions for different scenarios so that other developers can handle them.

Money Transfer App Scenario


Let’s consider we are creating an app that allows users to transfer money between them.

Bank Account Class


Example 1

Code

class BankAccount:
def __init__(self, account_number):
self.account_number = str(account_number)
self.balance = 0
def get_balance(self):
return self.balance
def withdraw(self, amount):
if self.balance >= amount:
self.balance -= amount
else:
print("Insufficient Funds")
def deposit(self, amount):
self.balance += amount
def transfer_amount(acc_1, acc_2, amount):
acc_1.withdraw(amount)
acc_2.deposit(amount)
user_1 = BankAccount("001")
user_2 = BankAccount("002")
user_1.deposit(250)
user_2.deposit(100)
print("User 1 Balance: {}/-".format(user_1.get_balance()))
print("User 2 Balance: {}/-".format(user_2.get_balance()))
transfer_amount(user_1, user_2, 50)
print("Transferring 50/- from User 1 to User 2")
print("User 1 Balance: {}/-".format(user_1.get_balance()))
print("User 2 Balance: {}/-".format(user_2.get_balance()))
PYTHON
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Output

User 1 Balance: 250/-
User 2 Balance: 100/-
Transferring 50/- from User 1 to User 2
User 1 Balance: 200/-
User 2 Balance: 150/-

Example 2

Code

class BankAccount:
def __init__(self, account_number):
self.account_number = str(account_number)
self.balance = 0
def get_balance(self):
return self.balance
def withdraw(self, amount):
if self.balance >= amount:
self.balance -= amount
else:
print("Insufficient Funds")
def deposit(self, amount):
self.balance += amount
def transfer_amount(acc_1, acc_2, amount):
acc_1.withdraw(amount)
acc_2.deposit(amount)
user_1 = BankAccount("001")
user_2 = BankAccount("002")
user_1.deposit(25)
user_2.deposit(100)
print("User 1 Balance: {}/-".format(user_1.get_balance()))
print("User 2 Balance: {}/-".format(user_2.get_balance()))
transfer_amount(user_1, user_2, 50)
print("Transferring 50/- from User 1 to User 2")
print("User 1 Balance: {}/-".format(user_1.get_balance()))
print("User 2 Balance: {}/-".format(user_2.get_balance()))
PYTHON
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Output

User 1 Balance: 25/-
User 2 Balance: 100/-
Insufficient Funds
Transferring 50/- from User 1 to User 2
User 1 Balance: 25/-
User 2 Balance: 150/-

Raising Exceptions


When your code enters an unexpected state, raise an exception to communicate it.

Built-in Exceptions


Different exception classes which are raised in different scenarios.

You can use the built-in exception classes with raise keyword to raise an exception in the program.

Code

We can pass message as argument .

raise ValueError("Unexpected Value!!")
PYTHON

Output

ValueError:Unexpected Value!!

Bank Account Class

Example 1

Code

class BankAccount:
def __init__(self, account_number):
self.account_number = str(account_number)
self.balance = 0
def get_balance(self):
return self.balance
def withdraw(self, amount):
if self.balance >= amount:
self.balance -= amount
else:
raise ValueError("Insufficient Funds")
def deposit(self, amount):
self.balance += amount
def transfer_amount(acc_1, acc_2, amount):
acc_1.withdraw(amount)
acc_2.deposit(amount)
user_1 = BankAccount("001")
user_2 = BankAccount("002")
user_1.deposit(25)
user_2.deposit(100)
print("User 1 Balance: {}/-".format(user_1.get_balance()))
print("User 2 Balance: {}/-".format(user_2.get_balance()))
transfer_amount(user_1, user_2, 50)
print("Transferring 50/- from User 1 to User 2")
print("User 1 Balance: {}/-".format(user_1.get_balance()))
print("User 2 Balance: {}/-".format(user_2.get_balance()))
PYTHON
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Output

User 1 Balance: 25/-
User 2 Balance: 100/-
ValueError: Insufficient Funds

Handling Exceptions


Python provides a way to catch the exceptions that were raised so that they can be properly handled.

  • Exceptions can be handled with try-except block.

  • Whenever an exception occurs at some line in try block, the execution stops at that line and jumps to except block.

    try:
    # Write code that
    # might cause exceptions.
    except:
    # The code to be run when
    # there is an exception.
    PYTHON

Transfer Amount

Example 1

Code

class BankAccount:
def __init__(self, account_number):
self.account_number = str(account_number)
self.balance = 0
def get_balance(self):
return self.balance
def withdraw(self, amount):
if self.balance >= amount:
self.balance -= amount
else:
raise ValueError("Insufficient Funds")
def deposit(self, amount):
self.balance += amount
def transfer_amount(acc_1, acc_2, amount):
try:
acc_1.withdraw(amount)
acc_2.deposit(amount)
return True
except:
return False
user_1 = BankAccount("001")
user_2 = BankAccount("002")
user_1.deposit(25)
user_2.deposit(100)
print("User 1 Balance: {}/-".format(user_1.get_balance()))
print("User 2 Balance: {}/-".format(user_2.get_balance()))
print(transfer_amount(user_1, user_2, 50))
print("Transferring 50/- from User 1 to User 2")
print("User 1 Balance: {}/-".format(user_1.get_balance()))
print("User 2 Balance: {}/-".format(user_2.get_balance()))
PYTHON
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Output

User 1 Balance: 25/-
User 2 Balance: 100/-
False
Transferring 50/- from User 1 to User 2
User 1 Balance: 25/-
User 2 Balance: 100/-

Summary

Reusable Modules

  • While developing reusable modules, we need to raise Exceptions to stop our code from being used in a bad way.

End-User Applications

  • While developing end-user applications, we need to handle Exceptions so that application will not crash when used.

Handling Specific Exceptions

We can specifically mention the name of exception to catch all exceptions of that specific type.

Syntax

try:
# Write code that
# might cause exceptions.
except Exception:
# The code to be run when
# there is an exception.
PYTHON

Example 1

Code

try:
a = int(input())
b = int(input())
c = a/b
print(c)
except ZeroDivisionError:
print("Denominator can't be 0")
except:
print("Unhandled Exception")
PYTHON

Input

5
0

Output

Denominator can't be 0

Example 2

Code

Input given by the user is not within expected values.

try:
a = int(input())
b = int(input())
c = a/b
print(c)
except ZeroDivisionError:
print("Denominator can't be 0")
except:
print("Unhandled Exception")
PYTHON

Input

12
a

Output

Unhandled Exception

We can also access the handled exception in an object.

Syntax

try:
# Write code that
# might cause exceptions.
except Exception as e:
# The code to be run when
# there is an exception.
PYTHON

Code

class BankAccount:
def __init__(self, account_number):
self.account_number = str(account_number)
self.balance = 0
def get_balance(self):
return self.balance
def withdraw(self, amount):
if self.balance >= amount:
self.balance -= amount
else:
raise ValueError("Insufficient Funds")
def deposit(self, amount):
self.balance += amount
def transfer_amount(acc_1, acc_2, amount):
try:
acc_1.withdraw(amount)
acc_2.deposit(amount)
return True
except ValueError as e:
print(str(e))
print(type(e))
print(e.args)
return False
user_1 = BankAccount("001")
user_2 = BankAccount("002")
user_1.deposit(25)
user_2.deposit(100)
print("User 1 Balance: {}/-".format(user_1.get_balance()))
print("User 2 Balance: {}/-".format(user_2.get_balance()))
print(transfer_amount(user_1, user_2, 50))
print("Transferring 50/- from User 1 to User 2")
print("User 1 Balance: {}/-".format(user_1.get_balance()))
print("User 2 Balance: {}/-".format(user_2.get_balance()))
PYTHON
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Output

User 1 Balance: 25/-
User 2 Balance: 100/-
Insufficient Funds
<class 'ValueError'>
('Insufficient Funds',)
False
Transferring 50/- from User 1 to User 2
User 1 Balance: 25/-
User 2 Balance: 100/-

Handling Multiple Exceptions

We can write multiple exception blocks to handle different types of exceptions differently.

Syntax

try:
# Write code that
# might cause exceptions.
except Exception1:
# The code to be run when
# there is an exception.
except Exception2:
# The code to be run when
# there is an exception.
PYTHON

Example 1

Code

try:
a = int(input())
b = int(input())
c = a/b
print(c)
except ZeroDivisionError:
print("Denominator can't be 0")
except ValueError:
print("Input should be an integer")
except:
print("Something went wrong")
PYTHON
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Input

5
0

Output

Denominator can't be 0

Example 2

Code

try:
a = int(input())
b = int(input())
c = a/b
print(c)
except ZeroDivisionError:
print("Denominator can't be 0")
except ValueError:
print("Input should be an integer")
except:
print("Something went wrong")
PYTHON
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Input

12
a

Output

Input should be an integer
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