Python Built-in Exceptions

In Python, exceptions are events that can alter the flow of control in a program. These errors can arise during program execution and need to be handled appropriately. Python provides a set of built-in exceptions, each designed to signal a specific type of error and help you debug more effectively.

These built-in exceptions can be viewed using the local() built-in functions as follows :

>>> locals()['__builtins__']

This returns a dictionary of built-in exceptions, functions and attributes.

Examples of Built in Exceptions

Let's understand each exception in detail:

1. BaseException

The BaseException class is the root of Python’s exception hierarchy. All other exceptions directly or indirectly inherit from it. While it is rarely used directly in code, it is important because it forms the foundation of Python’s error-handling system.

Example: This example manually raises a BaseException and catches it to show how the root exception works.

try:
    raise BaseException("This is a BaseException")
except BaseException as e:
    print(e)

This is a BaseException

Explanation: Here, we forcefully raise a BaseException. Since we catch it in the except block, message is printed instead of the program crashing.

2. Exception

The Exception class is the base for all non-exit exceptions. You will often catch Exception in general error-handling code when you are not targeting a specific error type.

Example: This code raises a generic Exception and handles it inside the except block.

try:
    raise Exception("This is a generic exception")
except Exception as e:
    print(e)

This is a generic exception

Explanation: We manually raise an Exception with a message. The error is caught and message is displayed instead of halting the program.

3. ArithmeticError

The ArithmeticError class is the base for all errors related to mathematical operations. You don’t usually raise it directly, but it provides a way to catch all math-related errors in one block.

Example: This example raises an ArithmeticError manually to demonstrate how it works.

try:
    raise ArithmeticError("Arithmetic error occurred")
except ArithmeticError as e:
    print(e)

Arithmetic error occurred

Explanation: We raise an ArithmeticError with a custom message. The program catches it and prints the error message, preventing a crash.

4. ZeroDivisionError

A ZeroDivisionError occurs when you attempt to divide a number by zero. Since division by zero is undefined in mathematics, Python raises this exception to signal the error.

Example: This code attempts to divide 10 by 0, which triggers a ZeroDivisionError.

try:
    result = 10 / 0
except ZeroDivisionError as e:
    print(e)

division by zero

Explanation: The operation 10 / 0 is invalid. Python raises a ZeroDivisionError, which is then caught in the except block. Instead of crashing, program prints the error message.

5. OverflowError

An OverflowError occurs when the result of a numerical operation is too large for Python to represent. While Python handles large integers well, certain floating-point operations (like very large exponentials) can still cause this error.

Example: This example uses the math.exp() function with a very large input, which causes an overflow.

import math
try:
    result = math.exp(1000)  # Exponential function with a large argument
except OverflowError as e:
    print(e)

math range error

Explanation: The exponential function with input 1000 produces a number too large to handle. Python raises an OverflowError, which is caught and displayed as an error message.

6. FloatingPointError

The FloatingPointError occurs when a floating-point calculation fails. By default, Python handles most floating-point issues silently (like dividing by zero results in inf or nan). However, you can explicitly enable floating-point error reporting with libraries like NumPy.

Example: This example enables error reporting in NumPy and performs a division by zero to trigger FloatingPointError.

import numpy as np
np.seterr(all='raise')

try:
    np.divide(1, 0)
except FloatingPointError as e:
    print("FloatingPointError caught:", e)

FloatingPointError caught: divide by zero encountered in divide

Explanation: Normally, NumPy would return inf for 1/0. But since we enabled error raising, the division explicitly throws a FloatingPointError, which we catch and print.

7. AssertionError

An AssertionError is raised when the assert statement fails. The assert keyword is often used for debugging or testing assumptions in code.

Example: This example checks if 1 == 2 using assert. Since the condition is false, it raises an AssertionError.

try:
    assert 1 == 2, "Assertion failed"
except AssertionError as e:
    print(e)

Assertion failed

Explanation: The assert statement expects 1 == 2 to be true, but it isn’t. Therefore, Python raises an AssertionError with the message "Assertion failed".

8. AttributeError

An AttributeError occurs when you try to access or assign an attribute that does not exist for an object.

Example: This example tries to access a non-existent attribute in a class instance.

class MyClass:
    pass

obj = MyClass()

try:
    obj.some_attribute
except AttributeError as e:
    print(e)

'MyClass' object has no attribute 'some_attribute'

Explanation: The object obj is created from MyClass, which has no attributes. Attempting to access some_attribute raises an AttributeError.

9. IndexError

An IndexError happens when you try to access a list (or any sequence) element with an index that is out of range.

Example: This example tries to access the 6th element of a list that only has 3 elements.

my_list = [1, 2, 3]

try:
    element = my_list[5]
except IndexError as e:
    print(e)

list index out of range

Explanation: The list has only indices 0, 1, 2. When we try to access index 5, Python raises an IndexError.

10. KeyError

A KeyError occurs when you try to access a dictionary key that doesn’t exist.

Example: This example tries to access the key "key2" in a dictionary that only has "key1".

d = {"key1": "value1"}

try:
    val = d["key2"]
except KeyError as e:
    print(e)

'key2'

Explanation: The dictionary does not contain "key2". Attempting to retrieve it raises a KeyError, which is caught and printed.

11. MemoryError

A MemoryError occurs when Python cannot allocate enough memory for an operation. This usually happens when trying to create extremely large data structures.

Example: This example tries to create a very large list, which may exceed memory limits.

try:
    li = [1] * (10**10)
except MemoryError as e:
    print(e)

Explanation: Attempting to create a list with 10**10 elements requires more memory than available. Python raises a MemoryError.

12. NameError

A NameError occurs when you use a variable or function name that has not been defined.

Example: This example tries to print a variable that was never declared.

try:
    print(var)
except NameError as e:
    print(e)

name 'var' is not defined

Explanation: Since var was never defined in the code, Python raises a NameError.

13. OSError (and related errors)

Raised when a system-related operation (like file I/O, opening files, or interacting with the OS) fails. In Python 3:

• IOError is just an alias for OSError (they are the same).
• FileNotFoundError is a subclass of OSError, specifically raised when a file or directory does not exist.

Example: This example attempts to open a missing file, which triggers FileNotFoundError (a subclass of OSError).

try:
    open("non_existent_file.txt")  # File does not exist
except FileNotFoundError as e:     # More specific
    print("FileNotFoundError caught:", e)
except OSError as e:               # General OS-related error
    print("OSError caught:", e)

FileNotFoundError caught: [Errno 2] No such file or directory: 'non_existent_file.txt'

Explanation: The missing file raises FileNotFoundError, which is a subclass of OSError. Since we catch it specifically, the error is displayed clearly.

More Built-in Exceptions

Apart from the exceptions we explored with examples above, Python provides several other built-in exceptions. These usually occur in specific situations. Below is a summary table of these exceptions and what they represent: