What are chained exceptions?

Python 3 introduced a very interesting feature called 'Exception chaining' to effectively deal with exceptions.

Imagine a situation where you are already handling an exception and your code incidentally triggers an additional exception. Should your code lose the information about the previous exception? Of course not. So the information should be available to the code following the erroneous code. This is an example of implicit chaining.

Another case pops up when we knowingly wish to handle an exception and translate it to another type of exception. Such a situation is typical when you have a good reason for the unifying behavior of one piece of code to act similarly to another piece of code, like a legacy code. In this situation it would also be nice to keep the details of the former exception. This is an example of explicit chaining.

This chaining concept introduces two attributes of exception instances:

• the __context__ attribute, which is inherent for implicitly chained exceptions;
• the __cause__ attribute, which is inherent for explicitly chained exceptions.

Those attributes help the programmer to keep a reference to the original exception object in a handy and consistent way for later processing like logging, etc.