• The raising of an exception interrupts normal program flow and transfers control to an exception handler.

• Exception handlers are defined using the try … except construct.

• The try blocks define a context in which exceptions can be detected.

• Corresponding the except blocks define handlers for specific types of exceptions.

• Python uses exceptions pervasively and many built-in language features depend on them.

• The except blocks can capture an exception object, which is often of a standard type such as ValueError, KeyError or IndexError.

• Programmer errors such as IndentationError and SyntaxError should not normally be handled.

• Exceptional conditions can be signaled using the raise keyword which accepts a single parameter of an exception object.

• Raise without an argument within an except block re-raises the exception which is currently being processed.

• We tend not to routinely check for TypeErrors. To do so would negate the flexibility afforded to us by Python's dynamic type system.

• The exception objects can be converted to strings using the str() constructor for the purposes of printing message payloads.

• The exceptions thrown by a function form part of it's API and should be appropriately documented.

• When raising exceptions prefer to use the most appropriate built-in exception type.

• Clean-up and restorative actions can be performed using the try … finally construct which may optionally be used in conjunction with except blocks.

Along the way we saw that:

• The output of the print() function can be redirected to stderr using the optional file argument.

• Python supports the logical operators and and or for combining boolean expressions.

• Return codes are too easily ignored.

• Platform specific actions can be implemented using an Easier to Ask Forgiveness than Permission approach facilitated by intercepting ImportErrors and providing alternative implementations.