John Nduli Linux enthusiast, Anime + Manga Fan, Audiophile, Tinkerer
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March 12, 2019

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Pagination Review of Django's and Pelican's Implementation

I wrote this article earlier on my blog here.

Pagination is the process of dividing up a document into discrete pages (wikipedia). Django and Pelican have similar pagination implementations. This boils down to having a class that has accepts a sliceable object as one of its params, and returns a page containing list of items depending on number of items per page. The django paginator docs describe the object list as a list, tuple, QuerySet, or other sliceable object with a count() or len() method.

paginator = Paginator(object_list=list, per_page=10)

To get the contents of a particular page one calls the get_page method:

paginator.get_page(2)

What this does is to first check if the number provided is valid using the validate_number method. A number is valid if its an integer, it's greater than 0 and is less that the total number of pages in the paginator (if the alloy_empty_first_page parameter was set to True, 1 will be valid even if the total number of pages is 0).

If the number was valid the page method is called. This creates the required start and end slices i.e.start:end on the object list, and returns a Page object that only contains these elements.

The code below shows some boiler plate that can achieve this:

class Paginator:

   def __init__(self, object_list, per_page,
                allow_empty_first_page=True):
       self.object_list = object_list
       self.per_page = int(per_page)
       self.orphans = int(orphans)
       self.allow_empty_first_page = allow_empty_first_page

   def validate_number(self, number):
       # Make sure number is integer, greater than 1 and less than
       # the total number of pages

  def get_page(self, number):
       # if number is valid, returns a page object.
       # This is done by getting range of items required
       # e.g. 11:20, and returning an object that one can iterate
       # through (called a Page object)

  def num_pages(self):
     # Returns the total number of pages

  def count(self):
     # Returns total number of objects

To loop over the page object with a for, the Page class implements the getitem for this. Pelican's implementation does not have this method though. It also has some useful methods like:

  • has_next
  • has_previous
  • has_other_pages
  • next_page_number
  • previous_page_number

With these one can effectively get the contents of a page by looping through the paginator (in django only) and have various logic on which page numbers to display.

The example provided in the django docs here shows a really good usage of the various methods.

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