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 meant to signal a particular type of error.

We can catch exceptions using try and except blocks, allowing your program to continue running even if an error occurs. 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.

Python Built-in Exceptions

Here’s a table listing all the major Python built-in exceptions along with a brief :

Let's understand each exception in detail:

1. BaseException

The base class for all built-in exceptions. Example:

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

This is a BaseException

Explanation: This code manually raises a BaseException and catches it, printing the exception message.

2. Exception

The base class for all non-exit exceptions. Example:

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

This is a generic exception

Explanation: This raises a generic Exception and handles it, printing the message.

3. ArithmeticError

Base class for all errors related to arithmetic operations. Example:

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

Arithmetic error occurred

Explanation: This raises an ArithmeticError and catches it, displaying the error message.

4. ZeroDivisionError

Raised when a division or modulo operation is performed with zero as the divisor. Example:

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

division by zero

Explanation: This code attempts to divide by zero, causing a ZeroDivisionError.

5. OverflowError

Raised when a numerical operation exceeds the maximum limit of a data type. Example:

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 a large argument causes an OverflowError.

6. FloatingPointError

Raised when a floating-point operation fails. Example:

import sys
import math

sys.float_info.max = 1.79e+308  # Maximum float value

try:
    math.sqrt(-1.0)  # This doesn't raise a FloatingPointError by default
except FloatingPointError as e:
    print(e)

Explanation: Floating-point errors are rare in Python; this code demonstrates the typical use case.

7. AssertionError

Raised when an assert statement fails. Example:

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

Assertion failed

Explanation: This code fails an assertion, raising an AssertionError.

8. AttributeError

Raised when an attribute reference or assignment fails. Example:

class MyClass:
    pass

obj = MyClass()

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

'MyClass' object has no attribute 'some_attribute'

Explanation: This code tries to access a non-existent attribute, causing an AttributeError.

9. IndexError

Raised when a sequence subscript is out of range. Example:

my_list = [1, 2, 3]

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

list index out of range

Explanation: This code tries to access an out-of-range index in a list, raising an IndexError.

10. KeyError

Raised when a dictionary key is not found. Example:

d = {"key1": "value1"}

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

'key2'

Explanation: This code attempts to access a non-existent dictionary key, causing a KeyError.

11. MemoryError

Raised when an operation runs out of memory. Example:

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

Explanation: This code tries to create a very large list, causing a MemoryError.

12. NameError

Raised when a local or global name is not found. Example:

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

name 'var' is not defined

Explanation: This code attempts to use an undefined variable, raising a NameError.

13. OSError

Raised when a system-related operation (like file I/O) fails. Example:

try:
    open("non_existent_file.txt")
except OSError as e:
    print(e)

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

Explanation: This code tries to open a non-existent file, causing an OSError.

14. TypeError

Raised when an operation or function is applied to an object of inappropriate type. Example:

try:
    result = '2' + 2
except TypeError as e:
    print(e)

can only concatenate str (not "int") to str

Explanation: This code tries to add a string and an integer, causing a TypeError.

15. ValueError

Raised when a function receives an argument of the right type but inappropriate value. Example:

try:
    res = int("abc")
except ValueError as e:
    print(e)

invalid literal for int() with base 10: 'abc'

Explanation: This code tries to convert a non-numeric string to an integer, raising a ValueError.

16. ImportError

Raised when an import statement has issues. Example:

try:
    import mod
except ImportError as e:
    print(e)

No module named 'mod'

Explanation: This code attempts to import a non-existent module, causing an ImportError.

17. ModuleNotFoundError

Raised when a module cannot be found. Example:

try:
    import module
except ModuleNotFoundError as e:
    print(e)

No module named 'module'

Explanation: Similar to ImportError, this code tries to import a module that doesn't exist.

18. IOError

Raised when an I/O operation (like reading or writing to a file) fails. Example:

try:
    open("non_existent_file.txt")
except IOError as e:
    print(e)

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

Explanation: This is another example of file-related errors, causing an IOError.

19. FileNotFoundError

Raised when a file or directory is requested but cannot be found. Example:

try:
    open("non_existent_file.txt")
except FileNotFoundError as e:
    print(e)

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

Explanation: This specifically catches the FileNotFoundError when trying to open a missing file.

20. StopIteration

Raised when the next() function is called and there are no more items in an iterator. Example:

my_iter = iter([1, 2, 3])

try:
    while True:
        print(next(my_iter))
except StopIteration as e:
    print("End of iterator")

1
2
3
End of iterator

Explanation: This code iterates through a list and raises StopIteration when the list is exhausted.

21. KeyboardInterrupt

Raised when the user presses Ctrl+C or interrupts the program’s execution. Example:

try:
    while True:
        pass
except KeyboardInterrupt as e:
    print("Program interrupted by user")

Explanation: This code runs an infinite loop until the user interrupts it with Ctrl+C.

22. SystemExit

Raised when the sys.exit() function is called to exit the program. Example:

import sys

try:
    sys.exit()
except SystemExit as e:
    print("System exit called")

System exit called

Explanation: This code calls sys.exit(), raising a SystemExit exception.

23. NotImplementedError

Raised when an abstract method that needs to be implemented is called. Example:

class BaseClass:
    def some_method(self):
        raise NotImplementedError("This method should be overridden")

try:
    obj = BaseClass()
    obj.some_method()
except NotImplementedError as e:
    print(e)

This method should be overridden

Explanation: This code demonstrates a base class method that should be overridden in subclasses, raising NotImplementedError if called.

24. RuntimeError

Raised when a general error occurs in the program. Example:

try:
    raise RuntimeError("A runtime error occurred")
except RuntimeError as e:
    print(e)

A runtime error occurred

Explanation: This manually raises a RuntimeError and catches it.

25. RecursionError

Raised when the maximum recursion depth is exceeded. Example:

try:
    def recursive_function():
        recursive_function()

    recursive_function()
except RecursionError as e:
    print(e)

maximum recursion depth exceeded

Explanation: This code defines a recursive function that calls itself indefinitely, causing a RecursionError.

26. SyntaxError

Raised when there is an error in the syntax of the code. Example:

try:
    eval('x === 2')
except SyntaxError as e:
    print(e)

invalid syntax (<string>, line 1)

Explanation: This code uses eval() to evaluate an incorrect syntax, raising a SyntaxError.

27. IndentationError

Raised when there is an indentation error in the code. Example:

try:
    eval('def func():\n  print("Hello")\n print("World")')
except IndentationError as e:
    print(e)

Explanation: This code uses eval() to evaluate code with incorrect indentation, causing an IndentationError.

28. TabError

Raised when the indentation consists of inconsistent use of tabs and spaces. Example:

try:
    eval('def func():\n\tprint("Hello")\n    print("World")')
except TabError as e:
    print(e)

Explanation: This code uses eval() to evaluate code with mixed tabs and spaces, causing a TabError.

29. UnicodeError

Raised when a Unicode-related encoding or decoding error occurs. Example:

try:
    'æ'.encode('ascii')
except UnicodeError as e:
    print(e)

'ascii' codec can't encode character '\xe6' in position 0: ordinal not in range(128)

Explanation: This code tries to encode a non-ASCII character using the ASCII codec, raising a UnicodeError.