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Exceptions

Introduction to Exceptions

In Python, exceptions are events that occur during the execution of a program that disrupts the normal flow of the program's instructions. Handling exceptions is crucial for building robust and error-tolerant applications.

What are Exceptions?

Exceptions are errors that are detected during execution. They might be caused by invalid input, logical errors, resource constraints, etc. Python has numerous built-in exceptions like ValueError, TypeError, IndexError, etc., and allows the creation of custom exceptions.

When to use exceptions

  • Invalid User Input: When a function receives input that's outside the expected or allowable range.

  • File and I/O Operations: Handling issues like file not found, read/write errors, or network communication failures.

  • Resource Unavailability: When an external resource (like a database or a web service) is unavailable or unresponsive.

  • Value Constraints in Data: Catching violations of data integrity or value constraints (e.g., division by zero, invalid date values).

  • Illegal State Transitions: In state-driven applications or state machines, handling transitions to illegal or undefined states.

  • Integrity Checks in Data Processing: When processing data, checking for and handling corrupt, missing, or invalid data.

  • API or Protocol Contract Violations: When a function or module's API contract is violated (e.g., incorrect parameters, unexpected data types).

  • Exception Propagation: Propagating errors from lower levels of the application to higher levels where they can be handled appropriately.

  • Hardware and System-Level Errors: Handling errors related to hardware or system-level issues, like memory allocation failures.

  • Interrupting Long-Running Processes: Interrupting or stopping long-running operations in response to specific conditions (e.g., user cancellation, timeout).

Basic Exception Handling

Try and Except Blocks

The primary mechanism for handling exceptions in Python is the try and except statement.

Example:

try:
    # Code that may cause an exception
    result = 10 / 0
except ZeroDivisionError:
    # Code to handle the exception
    print("Cannot divide by zero")

Catching Multiple Exceptions

You can catch multiple exceptions in a single except block or use multiple except blocks.

Example:

try:
    # Code that may cause an exception
except (TypeError, ValueError):
    # Handle multiple exceptions
    print("Type or Value Error occurred")
except Exception as e:
    # General exception block
    print(f"An error occurred: {e}")

The Else Clause

An else block can be used to execute code if no exception was raised in the try block.

Example:

try:
    result = 10 / 5
except ZeroDivisionError:
    print("Cannot divide by zero")
else:
    print("Division successful")

The Finally Clause

A finally block is executed no matter whether an exception was raised or not, often used for clean-up actions.

Example:

try:
    file = open('file.txt', 'r')
except FileNotFoundError:
    print("File not found")
finally:
    file.close()

Advanced Exception Handling

Raising Exceptions

You can raise exceptions using the raise statement, either re-raising a caught exception or raising a new one.

Example:

if some_condition:
    raise ValueError("A value error happened!")

Creating Custom Exceptions

Custom exceptions can be defined by subclassing Exception or any other built-in exception class.

Example:

class CustomError(Exception):
    pass

raise CustomError("This is a custom error message")

Exception Chaining

Exception chaining occurs when an exception is raised while handling another exception. Use from to chain exceptions.

Example:

try:
    # some code
except SomeException as e:
    raise NewException("An error occurred") from e

Assertions

Assertions are a debugging aid that test a condition as an internal self-check in your program. They are not intended to handle expected runtime errors, but rather to identify issues in code logic during the development phase.

Example:

assert condition, "Error message if condition is False"

When Asserts Can Be Beneficial:

  • Debugging and Development: assert statements are excellent for catching unexpected conditions during the development phase. They can quickly point out flaws in logic or incorrect assumptions.

  • Self-Documenting Code: They can serve as a form of documentation, clearly stating the assumptions made at certain points in the code.

  • Unit Testing: In test environments, assert statements are commonly used to verify that a piece of code behaves as expected.

Having zero assert statements in production code is a practice followed by many developers, especially in critical applications where reliability is paramount. However, in development, testing, and non-critical code, assert can be a useful tool.