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Diffstat (limited to 'Doc/tutorial')
| -rw-r--r-- | Doc/tutorial/controlflow.rst | 35 |
1 files changed, 35 insertions, 0 deletions
diff --git a/Doc/tutorial/controlflow.rst b/Doc/tutorial/controlflow.rst index 5fdddcb..2a55cf1 100644 --- a/Doc/tutorial/controlflow.rst +++ b/Doc/tutorial/controlflow.rst @@ -196,6 +196,41 @@ required syntactically but the program requires no action. For example:: ... pass # Busy-wait for keyboard interrupt (Ctrl+C) ... +This is commonly used for creating minimal classes such as exceptions, or +for ignoring unwanted exceptions:: + + >>> class ParserError(Exception): + ... pass + ... + >>> try: + ... import audioop + ... except ImportError: + ... pass + ... + +Another place :keyword:`pass` 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, telling you exactly where the incomplete code +is. Note the two calling styles of the exceptions above. +The first style, with no message but with an accompanying comment, +lets you easily leave the comment when you remove the exception, +which ideally would be a good description for +the block of code the exception is a placeholder for. However, the +third example, providing a message for the exception, will produce +a more useful traceback. .. _tut-functions: |