Learn Python Programming

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.