Python Exception Chaining

It is useful sometimes to catch an exception and then re-raise it or raise a different one. This can be done using the raise ... from statement.

try:
    # Code that might raise an exception
except ExceptionType as e:
    # Code that handles the exception
    raise ExceptionType2("Message") from e

The above code will raise an exception of type ExceptionType2 with the message Message and the original exception will be stored in the __cause__ attribute of the new exception.

By default, the nested raised exception gets chained to the new exception so in our case the from e part is optional. But if you want to raise a new exception and not chain the nested one, you can use the from None statement.

try:
    # Code that might raise an exception
except ExceptionType as e:
    # Code that handles the exception
    raise ExceptionType2("Message") from None

Note: It is recommended to always chain exceptions, unless you have a good reason not to.

Note 2: Re-raising an exception inside an except statement is usually a bad practice. But it can be useful sometimes, for example when you want to add more information to the exception.