Friday, April 17, 2009

The Django paginator

Most, if not all, modern web applications need pagination in one form or another. Pagination is the act of transforming a large data set into pages of a more manageable size. Google gives us a perfect example case of pagination. Google constantly deals with enormous data sets. Users who perform searches using google would promptly switch to a different search engine if there were no pagination provided. On the other side of the coin, pagination also provides more manageable data sets for the server code to deal with. Instead of the client saying "give me this entire large data set" the client says "give me page one of this large data set, page size being ten". From the developer perspective, pagination isn't always the most enjoyable task. They are error prone if not implemented correctly and can pose challenges when different query constraints come into play. An additional challenge with pagination is the fact that different web application frameworks use slightly different approaches to their pagination implementation. Another approach to implementing pagination could be to implement the functionality directly into the ORM. This would obviously only work with database query results but this is probably the most common use for pagination. However, if an ORM like SQLAlchemy for instance, were to implement pagination functionality, it could be used by developers inside a web application framework while still being functional outside the framework. The Django Python web application framework offers a good pagination implementation. It is not restricted to database query results and is easy for developers to understand and use. There is also much room in the implementation for extended functionality if so desired. To two classes used to implement the pagination functionality in Django are Paginator and Page as illustrated below.

The main class used by the Django web application framework is the Paginator class. The Paginator class deals directly with the data set in question. The constructor accepts an object_list as a required parameter. The constructor also accepts a per_page parameter that specifies the page size. This parameter is also required. Once the Paginator class has been instantiated with a data set, it can be used to generate Page instances. This is done by invoking the method, specifying the desired page number. The Paginator class also defines several managed properties that can be used to query the state of the Paginator instance. Managed properties in Python are simply methods that may be invoked as attributes. The count attribute will return the number of objects in the data set. The num_pages attribute will return the number of pages that the data set contains, based on the specified page size. Finally, the page_range attribute will return a list containing each page number.

There is room for extended functionality in the Paginator class. Mainly, there could be some operators overloaded to make Paginator instances behave more like new-style Python instances. The Paginator class could define a __len__() method. This way, developers could invoke the builtin len() function on Paginator instances to retrieve the number of pages. An __iter__() method could also be defined for the Paginator class. This would allow instances of this class to be used in iterations. Each iteration would yield a new Page instance. Finally, a __getitem__() method would allow Paginator instances to behave like Python lists. The desired page number could be specified as an index.

The Page class complements the Paginator class in that it represents a specific page within the data set managed by the Paginator instance. The has_next(), has_previous() and has_other_pages() methods of Page instances are useful in determining if the page has any neighbouring pages. The start_index() method will return the index in the original data set owned by the Paginator instance that created the Page instance. The end_index() will return the end index in the original data set.

Like the Paginator class, there is also room for extended functionality here to make Page instances behave more like new-style Python instances. The Page class could define a __len__() method that could return the page size. The Page class could also define an __iter__() method that could enable Page instances to be used in iterations. Finally, the __getitem__() method, if it were defined, could return the specified object from the original object list.

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