What are Exceptions?

An exception is an event that disrupts the normal flow of a program’s execution. When an exception occurs, Python creates an exception object that contains information about what went wrong.

Exceptions vs Syntax Errors

Syntax Errors: These are detected by Python before the program runs. They occur when the code doesn’t follow Python’s syntax rules.

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# Syntax Error - missing colon
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def greet(name)
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    print(f"Hello, {name}!")

Exceptions: These occur during program execution when something goes wrong, even though the syntax is correct.

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# This code has correct syntax but will raise an exception
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x = 10 / 0  # ZeroDivisionError

Common Built-in Exceptions

Python provides many built-in exception types. Here are some of the most common ones:

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result = 10 / 0  # ZeroDivisionError: division by zero

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x = int("abc")  # ValueError: invalid literal for int() with base 10: 'abc'

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result = "hello" + 5  # TypeError: can only concatenate str (not "int") to str

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my_list = [1, 2, 3]
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print(my_list[5])  # IndexError: list index out of range

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my_dict = {"name": "Alice", "age": 30}
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print(my_dict["city"])  # KeyError: 'city'

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my_list = [1, 2, 3]
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my_list.appendd(4)  # AttributeError: 'list' object has no attribute 'appendd'

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with open("nonexistent.txt", "r") as f:
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    content = f.read()  # FileNotFoundError: [Errno 2] No such file or directory: 'nonexistent.txt'

Exception Hierarchy

All exceptions in Python inherit from the BaseException class. The most common base class for user-defined exceptions is Exception, which itself inherits from BaseException.

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BaseException
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    ├── Exception
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    │   ├── ArithmeticError
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    │   │   └── ZeroDivisionError
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    │   ├── LookupError
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    │   │   ├── IndexError
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    │   │   └── KeyError
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    │   ├── ValueError
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    │   ├── TypeError
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    │   └── ...
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    └── ...

Why Handle Exceptions?

Handling exceptions allows your program to:

1. Continue running even when errors occur
2. Provide meaningful error messages to users
3. Clean up resources (close files, connections, etc.)
4. Log errors for debugging
5. Gracefully degrade functionality when something goes wrong

Exercises

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# ZeroDivisionError
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try:
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    result = 10 / 0
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except ZeroDivisionError:
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    print("ZeroDivisionError occurred")
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# ValueError
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try:
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    x = int("abc")
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except ValueError:
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    print("ValueError occurred")
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# TypeError
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try:
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    result = "hello" + 5
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except TypeError:
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    print("TypeError occurred")
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# IndexError
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try:
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    my_list = [1, 2, 3]
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    print(my_list[5])
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except IndexError:
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    print("IndexError occurred")
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# KeyError
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try:
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    my_dict = {"name": "Alice"}
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    print(my_dict["age"])
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except KeyError:
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    print("KeyError occurred")

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def safe_get_item(my_list, index):
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    try:
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        return my_list[index]
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    except IndexError:
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        return None
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n = int(input())
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my_list = []
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for i in range(n):
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    my_list.append(int(input()))
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index = int(input())
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result = safe_get_item(my_list, index)
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print(result)