Used when there is some code behind a return or raise statement, which will never be accessed.
In a method or function, processing of commands stops after a return or raise statement, and control is given back to the code that called the function/method. Any code in a function placed after such a statement will never be accessed.
If we have this function:
def function(value): return True print value
The code print value is placed after the end of the function (the return statement) and thus will never be executed.