Wednesday 20 October 2010

Implementing filters using callables

It’s been pointed out that for implementing logging filters, you shouldn’t have to go to the lengths of creating a Filter class just to define its one filter method. Of course, there are cases where you will need to use Filter classes, but there are also cases where it would be beneficial to just use a function. To cater for this, changes have been made to Python 3.2. Once this enters beta, you will be able to pass a function or other callable to the  addFilter and removeFilter methods of Handler and Logger, and this will be called with the LogRecord as its single argument to decide whether the event is to be logged.

The previous functionality did not actually require you to create Filter subclasses – you could pass any instance that had a filter method with the appropriate semantics. The Filter class was an illustration of how to do filtering as well as providing a not uncommon use case – filtering out everything other than a specific sub-tree in the logger namespace.

However, with this change, you’re not forced to call your filtering method filter, so you could e.g. use different filtering methods in a single class, or different functions in a module.

The development documentation has been updated to reflect this change.

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