In Python, the raise statement is used to explicitly raise an exception. This allows you to create custom exceptions and control the flow of your program when unexpected conditions occur.

Syntax

raise ExceptionType("Optional error message")

ExceptionType is the type of exception you want to raise, and “Optional error message” is an optional string that provides more information about the error.

Examples

Raising a Built-in Exception

def divide(x, y):
    if y == 0:
        raise ZeroDivisionError("Cannot divide by zero!")
    return x / y

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

In this example, the divide function raises a ZeroDivisionError if the divisor is zero. The try block attempts to call the function, and the except block catches the raised exception.

Raising a Custom Exception

class InvalidInputError(Exception):
    pass

def validate_input(input_str):
    if not input_str.isalpha():
        raise InvalidInputError("Input must contain only letters.")
    return True

try:
    validate_input("123")
except InvalidInputError as e:
    print(e)

Here, we define a custom exception InvalidInputError and raise it in the validate_input function if the input contains non-letter characters. The try block attempts to validate the input, and the except block catches the raised exception.

Built-in Exceptions

Here are some python built-in exceptions.

Exception	Description
SyntaxError	Raised when the Python parser detects a syntax error.
IndentationError	Raised when there is incorrect indentation.
NameError	Raised when a local or global name is not found.
TypeError	Raised when an operation or function is applied to an object of inappropriate type.
ValueError	Raised when a function receives an argument of correct type but with an inappropriate value.
ZeroDivisionError	Raised when the second operand of a division or modulo operation is zero.
FileNotFoundError	Raised when a file or directory is requested but cannot be found.
IndexError	Raised when a sequence subscript is out of range.
KeyError	Raised when a dictionary key is not found.
AttributeError	Raised when attribute reference or assignment fails.
ImportError	Raised when an import statement fails to find the module definition.
MemoryError	Raised when the interpreter runs out of memory.
KeyboardInterrupt	Raised when the user presses Ctrl+C while the program is running.
StopIteration	Raised by the next() function to signal that there are no further items produced by the iterator.
OverflowError	Raised when the result of an arithmetic operation is too large to be represented.
RuntimeError	Raised when an error is detected that doesn’t fall into any of the other categories.
OSError	Base class for I/O-related errors.