Python - Built-in Exceptions

---

---

Built-in exceptions are pre-defined error classes in Python that handle errors and exceptional conditions in programs. They are derived from the base class "BaseException" and are part of the standard library.

Standard Built-in Exceptions in Python

Here is a list of Standard Exceptions available in Python −

Sr.No.	Exception Name & Description
1	Exception Base class for all exceptions
2	StopIteration Raised when the next() method of an iterator does not point to any object.
3	SystemExit Raised by the sys.exit() function.
4	StandardError Base class for all built-in exceptions except StopIteration and SystemExit.
5	ArithmeticError Base class for all errors that occur for numeric calculation.
6	OverflowError Raised when a calculation exceeds maximum limit for a numeric type.
7	FloatingPointError Raised when a floating point calculation fails.
8	ZeroDivisonError Raised when division or modulo by zero takes place for all numeric types.
9	AssertionError Raised in case of failure of the Assert statement.
10	AttributeError Raised in case of failure of attribute reference or assignment.
11	EOFError Raised when there is no input from either the raw_input() or input() function and the end of file is reached.
12	ImportError Raised when an import statement fails.
13	KeyboardInterrupt Raised when the user interrupts program execution, usually by pressing Ctrl+C.
14	LookupError Base class for all lookup errors.
15	IndexError Raised when an index is not found in a sequence.
16	KeyError Raised when the specified key is not found in the dictionary.
17	NameError Raised when an identifier is not found in the local or global namespace.
18	UnboundLocalError Raised when trying to access a local variable in a function or method but no value has been assigned to it.
19	EnvironmentError Base class for all exceptions that occur outside the Python environment.
20	IOError Raised when an input/ output operation fails, such as the print statement or the open() function when trying to open a file that does not exist.
21	OSError Raised for operating system-related errors.
22	SyntaxError Raised when there is an error in Python syntax.
23	IndentationError Raised when indentation is not specified properly.
24	SystemError Raised when the interpreter finds an internal problem, but when this error is encountered the Python interpreter does not exit.
25	SystemExit Raised when Python interpreter is quit by using the sys.exit() function. If not handled in the code, causes the interpreter to exit.
26	TypeError Raised when an operation or function is attempted that is invalid for the specified data type.
27	ValueError Raised when the built-in function for a data type has the valid type of arguments, but the arguments have invalid values specified.
28	RuntimeError Raised when a generated error does not fall into any category.
29	NotImplementedError Raised when an abstract method that needs to be implemented in an inherited class is not actually implemented.

Here are some examples of standard exceptions −

IndexError

It is shown when trying to access item at invalid index.

numbers=[10,20,30,40]
for n in range(5):
   print (numbers[n])

It will produce the following output −

10
20
30
40
Traceback (most recent call last):

   print (numbers[n])
IndexError: list index out of range

ModuleNotFoundError

This is displayed when module could not be found.

import notamodule
Traceback (most recent call last):

   import notamodule
ModuleNotFoundError: No module named 'notamodule'

KeyError

It occurs as dictionary key is not found.

D1={'1':"aa", '2':"bb", '3':"cc"}
print ( D1['4'])
Traceback (most recent call last):

   D1['4']
KeyError: '4'

ImportError

It is shown when specified function is not available for import.

from math import cube
Traceback (most recent call last):

   from math import cube
ImportError: cannot import name 'cube'

StopIteration

This error appears when next() function is called after iterator stream exhausts.

.it=iter([1,2,3])
next(it)
next(it)
next(it)
next(it)
Traceback (most recent call last):

   next(it)
StopIteration

TypeError

This is shown when operator or function is applied to an object of inappropriate type.

print ('2'+2)
Traceback (most recent call last):

   '2'+2
TypeError: must be str, not int

ValueError

It is displayed when function's argument is of inappropriate type.

print (int('xyz'))
Traceback (most recent call last):

   int('xyz')
ValueError: invalid literal for int() with base 10: 'xyz'

NameError

This is encountered when object could not be found.

print (age)
Traceback (most recent call last):

   age
NameError: name 'age' is not defined

ZeroDivisionError

It is shown when second operator in division is zero.

x=100/0
Traceback (most recent call last):

   x=100/0
ZeroDivisionError: division by zero

KeyboardInterrupt

When user hits the interrupt key normally Control-C during execution of program.

name=input('enter your name')
enter your name^c
Traceback (most recent call last):

   name=input('enter your name')
KeyboardInterrupt

Hierarchy of Built-in Exceptions

The exceptions in Python are organized in a hierarchical structure, with "BaseException" at the top. Here is a simplified hierarchy −

• BaseException
  • SystemExit
  • KeyboardInterrupt
• Exception
  • ArithmeticError
    • FloatingPointError
    • OverflowError
    • ZeroDivisionError
  • AttributeError
  • EOFError
  • ImportError
  • LookupError
    • IndexError
    • KeyError
  • MemoryError
  • NameError
    • UnboundLocalError
  • OSError
    • FileNotFoundError
  • TypeError
  • ValueError
  • ---(Many others)---

How to Use Built-in Exceptions

As we already know that built-in exceptions in Python are pre-defined classes that handle specific error conditions. Now, here is a detailed guide on how to use them effectively in your Python programs −

Handling Exceptions with try-except Blocks

The primary way to handle exceptions in Python is using "try-except" blocks. This allows you to catch and respond to exceptions that may occur during the execution of your code.

Example

In the following example, the code that may raise an exception is placed inside the "try" block. The "except" block catches the specified exception "ZeroDivisionError" and handles it

try:
   result = 1 / 0
except ZeroDivisionError as e:
   print(f"Caught an exception: {e}")

Following is the output obtained −

Caught an exception: division by zero

Handling Multiple Exceptions

You can handle multiple exceptions by specifying them in a tuple within the "except" block as shown in the example below −

try:
   result = int('abc')
except (ValueError, TypeError) as e:
   print(f"Caught a ValueError or TypeError: {e}")

Output of the above code is as shown below −

Caught a ValueError or TypeError: invalid literal for int() with base 10: 'abc'

Using "else" and "finally" Blocks

The "else" block is executed if the code block in the "try" clause does not raise an exception −

try:
   number = int(input("Enter a number: "))
except ValueError as e:
   print(f"Invalid input: {e}")
else:
   print(f"You entered: {number}")

Output of the above code varies as per the input given −

Enter a number: bn
Invalid input: invalid literal for int() with base 10: 'bn'

The "finally" block is always executed, regardless of whether an exception occurred or not. It's typically used for clean-up actions, such as closing files or releasing resources −

try:
   file = open('example.txt', 'r')
   content = file.read()
except FileNotFoundError as e:
   print(f"File not found: {e}")
finally:
   file.close()
   print("File closed.")

Following is the output of the above code −

File closed.

Explicitly Raising Built-in Exceptions

In Python, you can raise built-in exceptions to indicate errors or exceptional conditions in your code. This allows you to handle specific error scenarios and provide informative error messages to users or developers debugging your application.

Syntax

Following is the basic syntax for raising built-in exception −

raise ExceptionClassName("Error message")

Example

In this example, the "divide" function attempts to divide two numbers "a" and "b". If "b" is zero, it raises a "ZeroDivisionError" with a custom message −

def divide(a, b):
   if b == 0:
      raise ZeroDivisionError("Cannot divide by zero")
   return a / b

try:
   result = divide(10, 0)
except ZeroDivisionError as e:
   print(f"Error: {e}")

The output obtained is as shown below −

Error: Cannot divide by zero