25-exception-handling.md

Python Exception Handling

Video link: https://youtu.be/brICUKrzVR0

In this video, we learned about exceptions in detail and also about handling them in Python using try...except statement.

Programs in the Video

• Exceptions
• Handling Exceptions
• Handling Specific Exception
• Python try...finally

---

Exceptions

Let's write a program that will give us an error.

numerator = 10
denominator = 0

print(numerator/denominator)

Output

Traceback (most recent call last):
  File "<string>", line 1, in <module>
ZeroDivisionError: division by zero

Even though our code was correct syntax wise, it's not allowed to divide a number by 0 in Python. This is an exception. In this case, we are getting the ZeroDivisionError exception.

There are numerous other built-in exceptions:

Exception	Cause of Error
FileNotFoundError	When a file that doesn't exist is accessed
IndexError	When the index of a sequence is out of range
FloatingPointError	When a floating point operation fails

---

Handling Exceptions

When our program encounters an exception, our code ends abruptly with an error message. Most of the time, rather than showing this default message, we may want to show a custom message that's more helpful or run a different set of code.

This is known as exception handling. It's the process of responding to exceptions in a custom way during the execution of a program.

In Python, we use the try..except block to handle exceptions. Its syntax is:

try:
    # code that may cause exception
except:
    # code to run when exception occurs

• Inside the try block, we write the code that may raise an exception.
• If an exception occurs, the control of the program jumps to the except block.
• If exceptions don't occur, the except block is skipped.

Let's see an example:

try:
    numerator = int(input("Enter numerator: "))
    denominator = int(input("Enter denominator: "))

    result = numerator/denominator

    print(result)
except:
    print("Denominator cannot be 0. Try again.")

print("Program ends")

Output

Enter numerator: 3
Enter denominator: 0
Denominator cannot be 0. Try again.
Program ends

Enter numerator: 3
Enter denominator: 4
0.75
Program ends

---

Handling Specific Exception

In our previous example, the except block handles all types of exception. If we want to make it handle only certain type of exception we can:

try:
    numerator = int(input("Enter numerator: "))
    denominator = int(input("Enter denominator: "))

    result = numerator/denominator

    print(result)
except ZeroDivisionError:
    print("Denominator cannot be 0. Try again.")

print("Program ends")

Output

Enter numerator: 3
Enter denominator: 0
Denominator cannot be 0. Try again.
Program ends

Now the except block is handling only the ZeroDivisionError.

This is mostly useful if the code inside try block can raise more than one exception.

For example:

try:
    numerator = int(input("Enter numerator: "))
    denominator = int(input("Enter denominator: "))

    result = numerator/denominator

    print(result)
    
    my_list = [1, 2, 3]
    i = int(input("Enter index: "))
    print(my_list[i])
except ZeroDivisionError:
    print("Denominator cannot be 0. Try again.")
except IndexError:
    print("Index is wrong.")

print("Program ends")

Output

Enter numerator: 6
Enter denominator: 5
1.2
Enter index: 4
Index is wrong.
Program ends

Here, there is no ZeroDivisionError, so except ZeroDivisionError block was skipped. If there was ZeroDivisionError:

Enter numerator: 3
Enter denominator: 0
Denominator cannot be 0. Try again.
Program ends

---

Python try...finally

A try statement can also have an optional finally block which is executed regardless of whether an exception occurs or not.

Its syntax looks something like this:

try:
    print(1/0)
except:
    print("Wrong denominator")
finally:
    print("Always printed")

Output

Wrong denominator
Always printed

The code gives the ZeroDivisionError so the except block is executed. Finally, the code inside the finally block is also executed.

However, if an exception doesn't occur in the try block, the except block is skipped but finally block is still executed at the end.

Note: The finally block is usually used to perform cleanup actions that need to be executed under all circumstances. Suppose, we are working with an external file in our program. We need to close this file at the end even if there was an error while writing to it. In this case, we put the close_file() function inside the finally block.