Python 3.3 Tutorial Notes - 6：Error and Exception

1. Handle exception: try-except. An except clause may name multiple exceptions as a parenthesized tuple.

>>> while True:

...      try:

...           x = int(input("Please enter a number: "))

...           break

...      except ValueError:

...           print("Oops! That was no valid number. Try again...")

...      except (RuntimeError, TypeError, NameError):

...           print("Error!!")

...       raise

2. "else" part contains the code for if the exception does not happen in "try" part.

3. Arguments of an exception: args

>>> try:

...      raise Exception(’spam’, ’eggs’)

... except Exception as inst:

...      print(type(inst))           # the exception instance

...      print(inst.args)           # arguments stored in .args

...      print(inst)                     # __str__ allows args to be printed directly,

...                                              # but may be overridden in exception subclasses

...      x, y = inst.args           # unpack args

...      print(’x =’, x)

...      print(’y =’, y)

...

4. User can define exception by inherit from Exception class.

5. Most exceptions are defined with names that end in “Error,” similar to the naming of the standard exceptions.

6. "finally", user defined clean-up action

7. The with statement allows objects like files to be used in a way that ensures they are always cleaned up promptly and correctly.