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 :
| Exception Name | Description |
|---|
BaseException | The base class for all built-in exceptions. |
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Exception | The base class for all non-exit exceptions. |
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ArithmeticError | Base class for all errors related to arithmetic operations. |
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ZeroDivisionError | Raised when a division or modulo operation is performed with zero as the divisor. |
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OverflowError | Raised when a numerical operation exceeds the maximum limit of a data type. |
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FloatingPointError | Raised when a floating-point operation fails. |
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AssertionError | Raised when an assert statement fails. |
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AttributeError | Raised when an attribute reference or assignment fails. |
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IndexError | Raised when a sequence subscript is out of range. |
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KeyError | Raised when a dictionary key is not found. |
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MemoryError | Raised when an operation runs out of memory. |
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NameError | Raised when a local or global name is not found. |
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OSError | Raised when a system-related operation (like file I/O) fails. |
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TypeError | Raised when an operation or function is applied to an object of inappropriate type. |
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ValueError | Raised when a function receives an argument of the right type but inappropriate value. |
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ImportError | Raised when an import statement has issues. |
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ModuleNotFoundError | Raised when a module cannot be found. |
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IOError | Raised when an I/O operation (like reading or writing to a file) fails. |
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FileNotFoundError | Raised when a file or directory is requested but cannot be found. |
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StopIteration | Raised when the next() function is called and there are no more items in an iterator. |
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KeyboardInterrupt | Raised when the user presses Ctrl+C or interrupts the program’s execution. |
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SystemExit | Raised when the sys.exit() function is called to exit the program. |
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NotImplementedError | Raised when an abstract method that needs to be implemented is called. |
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RuntimeError | Raised when a general error occurs in the program. |
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RecursionError | Raised when the maximum recursion depth is exceeded. |
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SyntaxError | Raised when there is an error in the syntax of the code. |
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IndentationError | Raised when there is an indentation error in the code. |
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TabError | Raised when the indentation consists of inconsistent use of tabs and spaces. |
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UnicodeError | Raised when a Unicode-related encoding or decoding error occurs. |
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Let’s understand each exception in detail:
1. BaseException
The base class for all built-in exceptions. Example:
Python try: raise BaseException("This is a BaseException") except BaseException as e: print(e) OutputThis 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:
Python try: raise Exception("This is a generic exception") except Exception as e: print(e) OutputThis 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:
Python try: raise ArithmeticError("Arithmetic error occurred") except ArithmeticError as e: print(e) OutputArithmetic 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:
Python try: result = 10 / 0 except ZeroDivisionError as e: print(e)
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:
Python import math try: result = math.exp(1000) # Exponential function with a large argument except OverflowError as e: print(e)
Explanation: The exponential function with a large argument causes an OverflowError.
6. FloatingPointError
Raised when a floating-point operation fails. Example:
Python 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:
Python try: assert 1 == 2, "Assertion failed" except AssertionError as e: print(e)
Explanation: This code fails an assertion, raising an AssertionError.
8. AttributeError
Raised when an attribute reference or assignment fails. Example:
Python class MyClass: pass obj = MyClass() try: obj.some_attribute except AttributeError as e: print(e)
Output'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:
Python my_list = [1, 2, 3] try: element = my_list[5] except IndexError as e: print(e)
Outputlist 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:
Python d = {"key1": "value1"} try: val = d["key2"] except KeyError as e: print(e) 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:
Python 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:
Python try: print(var) except NameError as e: print(e)
Outputname '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:
Python try: open("non_existent_file.txt") except OSError as e: print(e) Output[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:
Python try: result = '2' + 2 except TypeError as e: print(e)
Outputcan 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:
Python try: res = int("abc") except ValueError as e: print(e) Outputinvalid 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:
Python try: import mod except ImportError as e: print(e)
OutputNo 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:
Python try: import module except ModuleNotFoundError as e: print(e)
OutputNo 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:
Python try: open("non_existent_file.txt") except IOError as e: print(e) Output[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:
Python try: open("non_existent_file.txt") except FileNotFoundError as e: print(e) Output[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:
Python my_iter = iter([1, 2, 3]) try: while True: print(next(my_iter)) except StopIteration as e: print("End of iterator") Output1 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:
Python 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:
Python import sys try: sys.exit() except SystemExit as e: print("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:
Python 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) OutputThis 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:
Python try: raise RuntimeError("A runtime error occurred") except RuntimeError as e: print(e) OutputA runtime error occurred
Explanation: This manually raises a RuntimeError and catches it.
25. RecursionError
Raised when the maximum recursion depth is exceeded. Example:
Python try: def recursive_function(): recursive_function() recursive_function() except RecursionError as e: print(e)
Outputmaximum 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:
Python try: eval('x === 2') except SyntaxError as e: print(e) Outputinvalid 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:
Python 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:
Python 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:
Python try: 'æ'.encode('ascii') except UnicodeError as e: print(e) Output'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.
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