Python - Try Except

It is highly recommended to handle exceptions.

The code which may raise an exception is called Risky Code.

We have to place the risky code inside the try block, and the corresponding handling code should be placed inside the except block.

The try-except statement is used to handle runtime exceptions.

The program first executes the statements inside the try block.

• If no exception occurs, the except block is skipped.
• If an exception occurs, Python checks whether the exception matches the specified exception type. If it matches, the corresponding handling code is executed.

The try block contains the code that may raise an exception.

Only those statements which may raise an exception should be placed inside the try block.

The except block contains the exception handling code.

If an exception occurs inside the try block and it matches the specified exception type, then the statements inside the except block are executed.

The code inside the except block is also called Alternative Code.

The risky statement raises a ZeroDivisionError.

Since the exception type matches the except ZeroDivisionError block, Python executes the alternative code instead of terminating the program immediately.

Program Starts
        │
        ▼
Execute try Block
        │
        ▼
Exception Raised?
   ┌────┴────┐
   │         │
 No         Yes
   │         │
   ▼         ▼
Continue   Find Matching
Program    except Block
                │
         ┌──────┴──────┐
         │             │
         ▼             ▼
      Match         No Match
         │             │
         ▼             ▼
 Execute Handling   Abnormal
      Code         Termination
         │
         ▼
Continue Remaining Program
      

• Handles Runtime Exceptions.
• Prevents abnormal program termination.
• Provides an alternative solution.
• Allows the remaining part of the program to execute normally.
• Helps achieve Graceful Termination.

If we do not use the try-except block, then whenever an exception occurs, the Python Interpreter terminates the program abnormally.

Since there is no exception handling code available, the remaining statements of the program are not executed. This is called Abnormal Termination or Non-Graceful Termination.

Abnormal Termination

(Non-Graceful Termination)

stmt-1 executes successfully.

print(10/0) raises a ZeroDivisionError. Since no handling code is available, the interpreter immediately terminates the program.

stmt-3 is never executed.

If the risky code is placed inside the try block and the handling code is placed inside the except block, then Python can handle the exception and continue executing the remaining program.

This is called Graceful Termination.

Normal Termination

(Graceful Termination)

stmt-1 executes successfully.

The risky statement inside the try block raises a ZeroDivisionError. Instead of terminating the program immediately, Python searches for a matching except block.

The raised exception matches ZeroDivisionError, so print(10/2) executes as an alternative solution and prints 5.0.

Since the exception has been handled successfully, stmt-3 executes normally.

The execution of a try-except block depends on whether an exception occurs inside the try block and whether a matching except block is available.

              Program Starts
                     │
                     ▼
               Execute try Block
                     │
         ┌───────────┴───────────┐
         │                       │
         ▼                       ▼
 No Exception            Exception Raised
         │                       │
         ▼                       ▼
Skip except Block      Search Matching except
         │                       │
         │              ┌────────┴────────┐
         │              │                 │
         ▼              ▼                 ▼
     stmt-5       Match Found      No Match
         │              │                 │
         ▼              ▼                 ▼
 Normal Termination  Execute stmt-4  Abnormal Termination
                            │
                            ▼
                         Execute stmt-5
                            │
                            ▼
                     Normal Termination
      

1. If no exception occurs, the except block is skipped.
2. If a matching except block exists, the exception is handled and the remaining program continues.
3. If no matching except block exists, abnormal termination occurs.
4. If another exception occurs inside the except block, abnormal termination occurs.

We can print the exception information by using the exception object inside the except block.

Instead of simply handling the exception, we can also display the actual exception description generated by Python.

• ExceptionType represents the type of exception to be handled.
• msg is the exception object, which stores the description of the raised exception.
• We can print this object to display the exact error message.

• Every raised exception has an associated exception object.
• The syntax except ExceptionType as msg stores the exception object inside msg.
• The exception object can be printed directly to show the error description.