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Python Exceptions

Exceptions are errors detected during execution. When Python encounters an error it cannot handle, it raises an exception that interrupts normal program flow.

What Are Exceptions?

# Exceptions occur when something goes wrong at runtime
# (as opposed to syntax errors, which are caught before running)

# Common built-in exceptions:
print(10 / 0)           # ZeroDivisionError
print(int("hello"))     # ValueError
print(my_var)           # NameError (undefined variable)
print([1, 2][5])        # IndexError
print({"a": 1}["b"])    # KeyError
print("hello" + 5)      # TypeError
open("missing.txt")     # FileNotFoundError

Exception Hierarchy

# All exceptions inherit from BaseException
# Most user-facing exceptions inherit from Exception

# BaseException
# ├── SystemExit
# ├── KeyboardInterrupt
# ├── GeneratorExit
# └── Exception
#     ├── ValueError
#     ├── TypeError
#     ├── KeyError
#     ├── IndexError
#     ├── FileNotFoundError (subclass of OSError)
#     ├── ZeroDivisionError (subclass of ArithmeticError)
#     ├── AttributeError
#     ├── ImportError
#     │   └── ModuleNotFoundError
#     ├── RuntimeError
#     │   └── RecursionError
#     └── StopIteration

# You can inspect the hierarchy:
print(ValueError.__mro__)
# (ValueError, Exception, BaseException, object)

Basic Exception Handling

# try/except catches exceptions and prevents crashes
try:
    number = int(input("Enter a number: "))
    result = 100 / number
    print(f"Result: {result}")
except ValueError:
    print("That is not a valid number!")
except ZeroDivisionError:
    print("Cannot divide by zero!")

# Catch the exception object for details
try:
    data = [1, 2, 3]
    print(data[10])
except IndexError as e:
    print(f"Error: {e}")        # list index out of range
    print(f"Type: {type(e)}")   # <class 'IndexError'>

Common Exception Types and When They Occur

Raising Exceptions

# Use raise to throw an exception intentionally
def set_age(age):
    if age < 0:
        raise ValueError("Age cannot be negative")
    if age > 150:
        raise ValueError("Age seems unrealistic")
    return age

try:
    set_age(-5)
except ValueError as e:
    print(e)  # Age cannot be negative

# Re-raise an exception after logging
try:
    result = risky_operation()
except Exception as e:
    print(f"Error occurred: {e}")
    raise  # re-raises the same exception

• Exceptions are runtime errors — different from syntax errors which prevent execution.
• Use try/except to handle exceptions gracefully instead of crashing.
• Catch specific exceptions (not bare except:) to avoid hiding bugs.
• Use raise to signal errors in your own functions with descriptive messages.
• All exceptions form a hierarchy — catching a parent catches all its children.