【CS 61a study notes 8】Exceptions

Handling errors

Exception

A built-in mechanism in a programming language to declare and respond to exceptional conditions.

Exceptions can be handled by the program, preventing the interpreter from halting.Unhandled exceptions will cause Python to halt execution and print a stack trace.

Mastering exceptions

• Exceptions are objects ,which have classes with constructor.
• They enable non-local continuations of control:
  • If f calls g and g calls h ,exceptions can shift control from h to f without waiting for g to return

Raising Exceptions

Assert Statements

Assert statement raise an exception of type AssertionError
__ assert <expression> , <string>__
Assertions are designed to be used liberally . They can be ignored to increase efficiency by running Python with the ‘-o’ flag. “o” stands for optimized.
Whether assertions are enabled is governed by a bool _debug_ ._debug_ is False in ‘python -o’ environment.

Raise Statements

raise <expression>
<expression> must evaluate to a sub