[Python-checkins] r67123 - python/trunk/Doc/tutorial/controlflow.rst
georg.brandl
python-checkins at python.org
Thu Nov 6 19:49:16 CET 2008
Author: georg.brandl
Date: Thu Nov 6 19:49:15 2008
New Revision: 67123
Log:
#4247: add "pass" examples to tutorial.
Modified:
python/trunk/Doc/tutorial/controlflow.rst
Modified: python/trunk/Doc/tutorial/controlflow.rst
==============================================================================
--- python/trunk/Doc/tutorial/controlflow.rst (original)
+++ python/trunk/Doc/tutorial/controlflow.rst Thu Nov 6 19:49:15 2008
@@ -166,6 +166,39 @@
... pass # Busy-wait for keyboard interrupt (Ctrl+C)
...
+This is commonly used for creating minimal classes like with exceptions, or
+for skipping unwanted exceptions::
+
+ >>> class ParserError(Exception):
+ ... pass
+ ...
+ >>> try:
+ ... import audioop
+ ... except ImportError:
+ ... pass
+ ...
+
+Another place it can be used is as a place-holder for a function or
+conditional body when you are working on new code, allowing you to keep
+thinking at a more abstract level. However, as :keyword:`pass` is silently
+ignored, a better choice may be to raise a :exc:`NotImplementedError`
+exception::
+
+ >>> def initlog(*args):
+ ... raise NotImplementedError # Open logfile if not already open
+ ... if not logfp:
+ ... raise NotImplementedError # Set up dummy log back-end
+ ... raise NotImplementedError('Call log initialization handler')
+ ...
+
+If :keyword:`pass` were used here and you later ran tests, they may fail
+without indicating why. Using :exc:`NotImplementedError` causes this code
+to raise an exception, allowing you to tell exactly where code that you
+need to complete is. Note the two call styles of the exceptions above.
+The comment style is useful in that when you remove the exception you can
+easily leave the comment, which ideally would be a good description for
+the block of code the exception is a placeholder for. The call-style
+will raise a more useful exception however.
.. _tut-functions:
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