Lernen Basic try-except Blocks | Introduction to Exceptions

Error Handling with try-except Blocks

When you write Python code, unexpected errors — known as exceptions — can occur at any time. These might include trying to divide by zero, accessing a file that does not exist, or using the wrong data type in an operation. If you do not handle these exceptions, your program will stop running and display an error message, which can be confusing for users and disruptive for applications.

To prevent your program from crashing, use a try-except block. This structure lets you "try" a block of code and "except" or catch specific errors if they occur. By handling exceptions, you can provide useful feedback, perform alternate actions, or safely recover from errors. This makes your programs more reliable, user-friendly, and easier to maintain.

You will use try-except blocks to:

Catch and respond to errors without stopping your program;
Provide clear error messages or alternative actions when problems occur;
Ensure your code runs smoothly, even in unexpected situations.


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# Demonstration of a try-except block catching a ZeroDivisionError
numerator = 10
denominator = 0

try:
    result = numerator / denominator
    print("Result:", result)
except ZeroDivisionError:
    print("Error: Division by zero is not allowed.")

Step-by-Step Breakdown of try-except Syntax

A try-except block lets you handle errors without crashing your program. Here is how it works in the code sample:

Place the code that might cause an error inside a try: block;
If the code in the try block runs without any exceptions, the except block is ignored;
If an exception occurs in the try block, Python looks for an except block that matches the exception type;
If a matching except block is found, the code inside it runs instead of stopping the program;
If no matching except block is present, the program will still crash.

In the sample code, dividing by zero raises a ZeroDivisionError. Because the except block is set up to catch ZeroDivisionError, it runs the error message instead of letting the program crash. This keeps your program running and provides a clear, user-friendly error message.


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# Example showing what happens when an exception is not caught by the except clause
numerator = 10
denominator = 0

try:
    result = numerator / denominator
    print("Result:", result)
except ValueError:
    print("A ValueError occurred.")

Why Matching Exception Types Matters

When you use a try-except block, you must specify the correct exception type in the except clause. If the exception type does not match the error that occurs, Python will not catch the exception, and your program will still crash.

Key points to remember:

Each exception in Python has a specific type, such as ZeroDivisionError, ValueError, or TypeError;
The except clause only catches exceptions of the type you specify;
If an exception occurs that does not match any except clause, Python displays an error message and stops the program.

Example: if you write except ValueError: but your code raises a ZeroDivisionError, the except block will not run. Your program will terminate with a traceback for the uncaught exception.

Matching exception types ensures that:

You handle only the errors you expect and understand;
Your error messages are accurate and helpful to users;
Your code does not accidentally hide bugs by catching unrelated exceptions.

Always review your code to identify which exceptions might occur, and write except clauses that match those types. This approach leads to safer, more predictable error handling.

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