Built-in Exceptions Reference¶

Comprehensive reference for Python's built-in exceptions.

Exception Hierarchy¶

BaseException
├── SystemExit
├── KeyboardInterrupt
├── GeneratorExit
└── Exception
    ├── StopIteration
    ├── ArithmeticError
    │   ├── FloatingPointError
    │   ├── OverflowError
    │   └── ZeroDivisionError
    ├── AssertionError
    ├── AttributeError
    ├── BufferError
    ├── EOFError
    ├── ImportError
    │   └── ModuleNotFoundError
    ├── LookupError
    │   ├── IndexError
    │   └── KeyError
    ├── MemoryError
    ├── NameError
    │   └── UnboundLocalError
    ├── OSError
    │   ├── FileNotFoundError
    │   ├── PermissionError
    │   └── TimeoutError
    ├── ReferenceError
    ├── RuntimeError
    ├── SyntaxError
    │   └── IndentationError
    ├── SystemError
    ├── TypeError
    ├── ValueError
    └── Warning

Common Exceptions¶

SyntaxError¶

Raised when Python encounters invalid syntax.

# Invalid syntax
if x = 5:  # SyntaxError: invalid syntax
    pass

# Caught at parse time, before execution

IndentationError¶

Raised when indentation is incorrect.

if x > 5:
print("x is greater than 5")  # IndentationError: expected an indented block

NameError¶

Raised when a variable or function name is not found.

print(undefined_variable)  # NameError: name 'undefined_variable' is not defined

TypeError¶

Raised when an operation is applied to an object of inappropriate type.

"hello" + 5  # TypeError: can only concatenate str (not "int") to str
len(5)  # TypeError: object of type 'int' has no len()

ValueError¶

Raised when an operation receives an argument of correct type but inappropriate value.

int("abc")  # ValueError: invalid literal for int() with base 10: 'abc'
int("10.5")  # ValueError: invalid literal for int() with base 10: '10.5'

IndexError¶

Raised when a sequence index is out of range.

lst = [1, 2, 3]
lst[10]  # IndexError: list index out of range

KeyError¶

Raised when a dictionary key is not found.

d = {"a": 1}
d["b"]  # KeyError: 'b'

AttributeError¶

Raised when an attribute reference or assignment fails.

x = 5
x.append(1)  # AttributeError: 'int' object has no attribute 'append'

FileNotFoundError¶

Raised when a file or directory is requested but doesn't exist.

open("nonexistent.txt")  # FileNotFoundError: [Errno 2] No such file or directory

ZeroDivisionError¶

Raised when division by zero is attempted.

10 / 0  # ZeroDivisionError: division by zero

ImportError / ModuleNotFoundError¶

Raised when an import fails.

import nonexistent_module  # ModuleNotFoundError: No module named 'nonexistent_module'
from os import nonexistent  # ImportError: cannot import name 'nonexistent'

RuntimeError¶

Raised when an error is detected that doesn't fall into any other category.

# Rarely raised directly; usually indicates a problem in the Python runtime

AssertionError¶

Raised when an assert statement fails.

assert 1 == 2  # AssertionError
assert False, "This is false"  # AssertionError: This is false

StopIteration¶

Raised by an iterator to signal that there are no more items.

# Usually handled automatically by for loops
it = iter([1, 2, 3])
next(it)  # 1
next(it)  # 2
next(it)  # 3
next(it)  # StopIteration

How to Read Exception Names¶

Exception names follow a pattern:

Error suffix: Indicates something went wrong (e.g., ValueError, TypeError)
Exception suffix: More general (e.g., ImportError)
Name structure: Usually describes what went wrong
NameError: A name (variable/function) was not found
TypeError: Wrong type for an operation
ValueError: Wrong value for an operation
IndexError: Index out of range
KeyError: Dictionary key not found

When to Catch Exceptions¶

Catch Specific Exceptions¶

try:
    x = int(user_input)
except ValueError:
    print("Please enter a valid number")

Catch Multiple Exceptions¶

try:
    result = 10 / x
except (ValueError, ZeroDivisionError):
    print("Invalid input or division by zero")

Catch and Inspect¶

try:
    open("file.txt")
except FileNotFoundError as e:
    print(f"File not found: {e}")

Catch All (Use Sparingly)¶

try:
    # code
except Exception as e:
    print(f"An error occurred: {e}")

Finally Block¶

try:
    f = open("file.txt")
    content = f.read()
except FileNotFoundError:
    print("File not found")
finally:
    f.close()  # Always runs

Best Practices¶

Catch specific exceptions: Avoid bare except: or except Exception:
Use context managers: with statements handle cleanup automatically
Don't catch and ignore: At least log or print the error
Raise with context: Include the original exception when re-raising
Use custom exceptions: For application-specific errors, create custom exception classes

class InvalidAgeError(ValueError):
    """Raised when age is invalid."""
    pass

def set_age(age):
    if age < 0:
        raise InvalidAgeError("Age cannot be negative")
    return age