Exceptions and Errors in Python
Tags: python, programming
Published: 2022-12-19 09:15:55

In Python, an error indicates a problem that occurs while the program is executing, while an exception is a signal that indicates an exceptional condition that has occurred.

Errors are typically caused by problems with the code itself, such as syntax errors, name errors, or type errors. These are usually detected by the interpreter before the program starts running, and the interpreter will display an error message indicating what went wrong.

Here's an example of a syntax error in Python:

```
def foo():
    print('Hello')
    return

foo()

```
If you try to run this code, you'll see an error message like this:

```
File "example.py", line 6
    foo()
    ^
IndentationError: expected an indented block
```
Exceptions, on the other hand, are events that occur during the execution of the program and can be handled by the code. Exceptions are used to signal a variety of conditions, such as division by zero, file not found, or invalid input. When an exception occurs, the interpreter raises (or "throws") an exception object.

Here's an example of how you can catch and handle an exception in Python:

```
try:
    x = int(input('Enter a number: '))
    print(1 / x)
except ZeroDivisionError:
    print('You cannot divide by zero!')

```
If the user enters a number other than zero, the code will execute without any problems. If the user enters zero, however, the interpreter will raise a ZeroDivisionError exception, which will be caught by the except clause and the appropriate message will be printed.

In summary, errors are problems that occur when the code is incorrect and cannot be executed, while exceptions are events that occur during execution and can be handled by the code.

## When do we need to use exceptions?
Exceptions are used in Python to signal a variety of exceptional conditions that can occur during the execution of a program. Some common examples of when exceptions might be used include:

- Handling runtime errors: Exceptions can be used to handle runtime errors, such as division by zero or out-of-bounds array access, which would otherwise cause the program to crash.
- Validating user input: Exceptions can be used to validate user input and ensure that it meets certain criteria. For example, you might use an exception to ensure that a user has entered a valid email address or phone number.
- Dealing with external resources: Exceptions can be used to handle errors that occur when interacting with external resources, such as reading from or writing to a file, or connecting to a network server.
- Handling unexpected conditions: Exceptions can be used to handle unexpected conditions that may arise during the execution of a program. For example, you might use an exception to handle the case where a user attempts to access a resource that does not exist.

Overall, exceptions are a useful tool for handling errors and exceptional conditions in a clean and robust way. They allow you to write code that is more resilient to failure and can gracefully
handle a variety of error scenarios.

## How to correctly handle an exception
To handle an exception in a program, you can use a try-except block. The try block contains the code that may throw an exception, and the except block contains the code that will handle the exception if it occurs. Here's an example:

```
try:
   # code that may throw an exception
except ExceptionType:
   # code to handle the exception

```
You can specify a particular type of exception to handle by replacing ExceptionType with the name of the exception class. For example, to handle a ZeroDivisionError, you can use:

```
try:
   # code that may throw a ZeroDivisionError
except ZeroDivisionError:
   # code to handle the ZeroDivisionError

```
You can also handle multiple exception types by using multiple except blocks. For example:

```
try:
   # code that may throw an exception
except ZeroDivisionError:
   # code to handle a ZeroDivisionError
except ValueError:
   # code to handle a ValueError

```

If you want to handle all exception types, you can use the Exception class as the exception type:

```
try:
   # code that may throw an exception
except Exception:
   # code to handle any exception

```
It's generally a good idea to be specific about which exception types you want to handle, rather than catching all exceptions, because this can help you identify and fix errors in your code.

Finally, you can use the finally block to include code that should always be executed, whether or not an exception occurs. For example:

```
try:
   # code that may throw an exception
except ExceptionType:
   # code to handle the exception
finally:

   # code that should always be executed

```

## What’s the purpose of catching exceptions
The purpose of catching exceptions is to handle errors that may occur during the execution of a program. When an error, or exception, occurs in a program, it can cause the program to crash or produce incorrect results. By catching exceptions, you can prevent these issues and gracefully handle the errors in your program.

**There are several benefits to catching exceptions:**

1. Improved stability: Catching exceptions can help prevent your program from crashing or producing incorrect results when an error occurs. This can improve the stability of your program and make it more reliable.
2. Improved user experience: If your program crashes or produces incorrect results, it can be frustrating for the user. By catching exceptions, you can provide a better user experience by handling errors in a way that is more user-friendly.
3. Debugging: Catching exceptions can also help you identify and fix errors in your program. When you catch an exception, you can print out a message or log the error to help you understand what went wrong and how to fix it.

In summary, the purpose of catching exceptions is to handle errors that may occur during the execution of a program, improve the stability and reliability of the program, and provide a better user experience.

## When do we need to implement a clean-up action after an exception

You may need to implement a clean-up action after an exception if your program has performed some action that needs to be undone or resources that need to be released.

For example, consider a program that opens a file, reads from it, and then closes the file. If an exception occurs while reading from the file, the file may not be closed properly. In this case, you would want to include a clean-up action in the except block to ensure that the file is closed properly.

You can use the finally block to include code that should always be executed, whether or not an exception occurs. The finally block is useful for performing clean-up actions, such as releasing resources or closing open files.

Here's an example of how you can use the finally block to perform a clean-up action after an exception:

```
try:
   # code that may throw an exception
except ExceptionType:
   # code to handle the exception
finally:
   # clean-up action

```
In this example, the clean-up action will be executed whether or not an exception occurs. This ensures that the clean-up action is always performed, even if an exception occurs.