try...except...else...finally

Python can throw - pardon, raise - Exceptions:

>>> try:
...     a = 1 / 0
... except Exception as e:
...     print 'oops: %s' % e
... else:
...     print 'no problem here'
... finally:
...     print 'done'
... 
oops: integer division or modulo by zero
done

If the exception is raised, it is caught by the except clause, which is executed, while the else clause is not. If no exception is raised, the except clause is not executed, but the else one is. The finally clause is always executed.

There can be multiple except clauses for different possible exceptions:

>>> try:
...     raise TypeError()
... except ValueError:
...     print 'value error'
... except Exception:
...     print 'generic error'
... 
generic error

The else and finally clauses are optional.

Here is a list of built-in Python exceptions + HTTP (defined by web2py)

BaseException
 +-- SystemExit
 +-- KeyboardInterrupt
 +-- GeneratorExit
 +-- Exception
      +-- HTTP (defined by web2py)
      +-- StopIteration
      +-- StandardError
      |    +-- BufferError
      |    +-- ArithmeticError
      |    |    +-- FloatingPointError
      |    |    +-- OverflowError
      |    |    +-- ZeroDivisionError
      |    +-- AssertionError
      |    +-- AttributeError
      |    +-- EnvironmentError
      |    |    +-- IOError
      |    |    +-- OSError
      |    |         +-- WindowsError (Windows)
      |    |         +-- VMSError (VMS)
      |    +-- EOFError
      |    +-- ImportError
      |    +-- LookupError
      |    |    +-- IndexError
      |    |    +-- KeyError
      |    +-- MemoryError
      |    +-- NameError
      |    |    +-- UnboundLocalError
      |    +-- ReferenceError
      |    +-- RuntimeError
      |    |    +-- NotImplementedError
      |    +-- SyntaxError
      |    |    +-- IndentationError
      |    |         +-- TabError
      |    +-- SystemError
      |    +-- TypeError
      |    +-- ValueError
      |         +-- UnicodeError
      |              +-- UnicodeDecodeError
      |              +-- UnicodeEncodeError
      |              +-- UnicodeTranslateError
      +-- Warning
           +-- DeprecationWarning
           +-- PendingDeprecationWarning
           +-- RuntimeWarning
           +-- SyntaxWarning
           +-- UserWarning
           +-- FutureWarning
           +-- ImportWarning
           +-- UnicodeWarning
           +-- BytesWarning

For a detailed description of each of them, refer to the official Python documentation.

web2py exposes only one new exception, called HTTP. When raised, it causes the program to return an HTTP error page (for more on this refer to Chapter 4).

Any object can be raised as an exception, but it is good practice to raise objects that extend one of the built-in exception classes.