Exceptions
**********

.. highlight:: python

- http://www.python.org/doc/2.4.2/tut/node10.html
- `Good catch all exceptions`_
  Are catch-all exceptions ever OK?

Catch
=====

::

  try:
      self.open()
  except HTTPError as e:
      print http error: ' + str(e)

Chained
-------

python 3 only::

  try:
      v = {}['a']
  except KeyError as e:
      raise ValueError('failed') from e

Unknown
-------

...to catch exceptions of an unknown, try::

  try:
      somecode()
  except Exception as e:
      print e

or::

  try:
      somecode()
  except:
      print sys.exc_info()[0]

Finally
=======

**Note** for python/jython 2.1 see *Note* below...

`Try..Finally`_::

  try:
      f = file('poem.txt')
      for line in f:
          print line,
  finally:
      f.close()

.. note:: `PEP 341: Unified try/except/finally`_
          Until Python 2.5, the try statement came in two flavours.  You could
          use a finally block to ensure that code is always executed, or one or
          more except blocks to catch specific exceptions.  You couldn't
          combine both except blocks and a finally block*

To solve this problem nest a try, except within a try, finally e.g::

  cursor = None
  try:
      try:
          cursor = somecode()
      except Exception as e:
          print e
  finally:
      if cursor != None:
          c.close()

.. _python_exceptions_throw:

Throw
=====

To throw an exception::

  class SyncError(Exception):

      def __init__(self, value):
          Exception.__init__(self)
          self.value = value

      def __str__(self):
          return repr('{}, {}'.format(self.__class__.__name__, self.value))

::

  try:
      raise SyncError(2*2)
  except SyncError as e:
      print 'My exception occurred, value:', e.value

Exceptions
----------

::

  raise NotImplementedError("Village, has_school")

Rethrow
=======

raise_

If no expressions are present, raise re-raises the last exception that was
active in the current scope::

  raise

Stack Trace
===========

::

  import traceback
  try:
      raise Exception("print exception!")
  except:
      print traceback.format_exc()


.. _`Are catch-all exceptions ever OK?`: https://convore.com/python/are-catch-all-exceptions-ever-ok/
.. _`Good catch all exceptions`: http://blog.ianbicking.org/good-catch-all-exceptions.html
.. _`PEP 341: Unified try/except/finally`: http://docs.python.org/whatsnew/pep-341.html
.. _`Try..Finally`: http://www.ibiblio.org/g2swap/byteofpython/read/try-finally.html
.. _raise: http://docs.python.org/ref/raise.html