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The raise statement is a control flow mechanism used to explicitly trigger an exception during program execution. It interrupts normal execution flow and transfers control to the nearest enclosing exception handler (except block) that matches the raised exception type. If no appropriate handler is found, the interpreter terminates the program and prints a traceback.

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Syntax

raise [exception_expression [from cause_expression]]
The exception_expression must evaluate to either an instance of a class deriving from BaseException, or a class deriving from BaseException.

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Execution Mechanics

The raise statement operates in four distinct forms based on the provided arguments:

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1. Raising an Exception Instance

When provided with an instantiated exception object, raise throws that specific instance. This allows for the inclusion of custom error messages or attributes. 
raise ValueError("Invalid parameter type")

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2. Raising an Exception Class

If exception_expression is an exception class rather than an instance, the Python interpreter implicitly instantiates the class by calling its constructor with no arguments. 
raise TypeError

# This is functionally identical to: raise TypeError()

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3. Re-raising an Active Exception (Bare raise)

When raise is executed without any arguments, it re-raises the active exception currently being handled anywhere in the call stack. This can occur directly inside an except block or within a function invoked during exception handling. This preserves the original exception object, including its exact traceback and type, allowing an intermediate handler to inspect the exception before passing it up the call stack. 
try:
    1 / 0
except ZeroDivisionError:
    # Performs operations, then propagates the exact same exception
    raise
Note: If a bare raise is executed where no exception is currently active, Python raises a RuntimeError.

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4. Exception Chaining (raise ... from ...)

The from clause enables explicit exception chaining. The cause_expression must evaluate to an exception instance, an exception class (which the interpreter will implicitly instantiate), or None. When an exception is raised using from, the raised exception’s __cause__ attribute is set to the evaluated cause_expression. The interpreter uses this to format a traceback showing both exceptions, indicating that one directly caused the other. 
try:
    my_dict = {}
    my_dict["missing"]
except KeyError as e:
    raise RuntimeError("Processing failed") from e
If from None is used, it explicitly disables automatic exception chaining. By default, raising a new exception while another exception is being handled sets the new exception’s __context__ attribute to the active exception. from None suppresses this behavior, preventing the original exception from appearing in the traceback. 
try:
    my_dict = {}
    my_dict["missing"]
except KeyError:
    raise RuntimeError("Processing failed") from None
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