Pagination (PageNumber, Cursor, Limit/Offset) in Django - Step-by-Step Execution

Concept Flow - Pagination (PageNumber, Cursor, Limit/Offset)

Start Request

↓

Extract Pagination Params

↓

Limit/Offset

↓

Calculate Slice

↓

PageNumber

↓

Calculate Page Range

↓

Cursor

↓

Use Cursor to Fetch

↓

Query Database with Params

↓

Return Paginated Results

↓

End

The server receives pagination parameters (PageNumber, Cursor, or Limit/Offset), calculates which data slice to fetch, queries the database, and returns the paginated results.

Execution Sample

Django

from django.core.paginator import Paginator

items = list(range(1, 21))
paginator = Paginator(items, 5)
page = paginator.page(2)
print(page.object_list)

This code paginates a list of 20 items into pages of 5 items each and prints the items on page 2.

Execution Table

Step	Action	Input/State	Result/Output
1	Create list of items	items = [1..20]	List with 20 numbers created
2	Create Paginator	Paginator(items, 5)	Paginator with 4 pages (5 items each)
3	Request page 2	page = paginator.page(2)	Page object for page 2 created
4	Get items on page 2	page.object_list	[6, 7, 8, 9, 10]
5	Print items	print(page.object_list)	Outputs: [6, 7, 8, 9, 10]
6	End	No more steps	Pagination complete

💡 Reached requested page 2, output items sliced accordingly

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
items	undefined	[1,2,...,20]	[1,2,...,20]	[1,2,...,20]	[1,2,...,20]	[1,2,...,20]
paginator	undefined	undefined	Paginator object (4 pages)	Paginator object (4 pages)	Paginator object (4 pages)	Paginator object (4 pages)
page	undefined	undefined	undefined	Page 2 object	Page 2 object	Page 2 object
page.object_list	undefined	undefined	undefined	undefined	[6,7,8,9,10]	[6,7,8,9,10]

Key Moments - 3 Insights

Why does page 2 return items starting from 6, not 5?

Because pagination pages start counting items from 1, page 1 has items 1-5, so page 2 starts at item 6 as shown in execution_table row 4.

What happens if I request a page number beyond the last page?

Django's Paginator raises an exception if the page number is too high. This is not shown here but you must handle it to avoid errors.

How does Limit/Offset differ from PageNumber pagination?

Limit/Offset uses a start position and count to slice data directly, while PageNumber uses page count and size to calculate slices, as seen in the concept_flow.

Visual Quiz - 3 Questions

Look at the execution_table, what is the output of page.object_list at step 4?

A [1, 2, 3, 4, 5]

B [6, 7, 8, 9, 10]

C [11, 12, 13, 14, 15]

D [16, 17, 18, 19, 20]

💡 Hint

Check the 'Result/Output' column at step 4 in the execution_table

At which step does the paginator know how many pages exist?

A Step 2

B Step 1

C Step 3

D Step 4

💡 Hint

Look at the 'Action' and 'Result/Output' columns in execution_table step 2

If the page size changes from 5 to 10, how does page 2's object_list change?

A It will contain items [1, 2, ..., 10]

B It will contain items [6, 7, 8, 9, 10]

C It will contain items [11, 12, ..., 20]

D It will be empty

💡 Hint

Refer to variable_tracker and how page size affects slicing in the concept_flow

Concept Snapshot

Django Pagination:
- Use Paginator(items, per_page) to split data
- Access pages with paginator.page(number)
- page.object_list gives items on that page
- PageNumber pagination uses page count
- Limit/Offset slices by start and count
- Cursor pagination uses a pointer for next data slice

Full Transcript

This visual execution shows how Django pagination works using PageNumber style. First, a list of 20 items is created. Then, a Paginator object is made with 5 items per page, resulting in 4 pages. Requesting page 2 returns items 6 to 10. The execution table traces each step from list creation to printing the page items. Variables like items, paginator, page, and page.object_list are tracked through the steps. Key moments clarify why page 2 starts at item 6 and differences between pagination types. The quiz tests understanding of output at steps, paginator state, and effects of changing page size.

Practice

1. Which Django REST Framework pagination style uses a page number to fetch specific pages of data?

easy

A. PageNumberPagination

B. CursorPagination

C. LimitOffsetPagination

D. OffsetPagination

Solution

1. Step 1: Understand pagination styles

   PageNumberPagination uses page numbers like 1, 2, 3 to get data pages.

2. Step 2: Match style to description

   CursorPagination uses a cursor token, LimitOffsetPagination uses limit and offset numbers, so they don't use page numbers.

3. Final Answer:

   PageNumberPagination -> Option A

4. Quick Check:

   Page number style = PageNumberPagination [OK]

Hint: PageNumberPagination uses simple page numbers like 1, 2, 3 [OK]

Common Mistakes:

• Confusing CursorPagination with page numbers
• Thinking LimitOffsetPagination uses page numbers
• Assuming OffsetPagination is a valid DRF style

2. Which of the following is the correct way to set LimitOffsetPagination in Django REST Framework settings?

easy

A. "DEFAULT_PAGINATION_CLASS": "rest_framework.pagination.OffsetPagination"

B. "DEFAULT_PAGINATION_CLASS": "rest_framework.pagination.PageNumberPagination"

C. "DEFAULT_PAGINATION_CLASS": "rest_framework.pagination.LimitOffsetPagination"

D. "DEFAULT_PAGINATION_CLASS": "rest_framework.pagination.CursorPagination"

Solution

1. Step 1: Identify correct class path

   LimitOffsetPagination is located at rest_framework.pagination.LimitOffsetPagination.

2. Step 2: Verify syntax for settings

   The setting key is DEFAULT_PAGINATION_CLASS and the value is the full class path as a string.

3. Final Answer:

   "DEFAULT_PAGINATION_CLASS": "rest_framework.pagination.LimitOffsetPagination" -> Option C

4. Quick Check:

   Correct class path and setting key = "DEFAULT_PAGINATION_CLASS": "rest_framework.pagination.LimitOffsetPagination" [OK]

Hint: Use full class path string for pagination in settings [OK]

Common Mistakes:

• Using OffsetPagination which does not exist
• Missing quotes around class path string
• Mixing pagination class names

3. Given this code snippet using CursorPagination, what will be the value of the next cursor if the current cursor is 'abc123' and page size is 2?

class MyCursorPagination(CursorPagination):
    page_size = 2

paginator = MyCursorPagination()
next_cursor = paginator.get_next_link()

medium

A. A tuple with offset and limit values

B. An integer representing the next page number

C. None, because get_next_link() returns nothing

D. A URL containing a cursor parameter with a new encoded cursor

Solution

1. Step 1: Understand CursorPagination behavior

   CursorPagination returns URLs with encoded cursor tokens for next pages, not page numbers or tuples.

2. Step 2: Analyze get_next_link() output

   get_next_link() returns a URL string containing the next cursor parameter for pagination.

3. Final Answer:

   A URL containing a cursor parameter with a new encoded cursor -> Option D

4. Quick Check:

   CursorPagination next link = URL with cursor [OK]

Hint: CursorPagination returns URLs with cursor tokens, not numbers [OK]

Common Mistakes:

• Expecting page numbers from CursorPagination
• Thinking get_next_link() returns None
• Confusing limit/offset with cursor

4. You have this Django REST Framework view using LimitOffsetPagination but it raises an error:

class MyLimitOffsetPagination(LimitOffsetPagination):
    default_limit = '10'

class MyView(ListAPIView):
    pagination_class = MyLimitOffsetPagination
    queryset = MyModel.objects.all()
    serializer_class = MySerializer

What is the likely cause of the error?

medium

A. pagination_class should be a string path, not a class

B. default_limit should be an integer, not a string

C. ListAPIView does not support pagination

D. queryset must be a list, not a QuerySet

Solution

1. Step 1: Check default_limit type

   default_limit must be an integer, but it is set as a string '10', causing a type error.

2. Step 2: Verify other parts

   pagination_class can be a class, ListAPIView supports pagination, queryset can be a QuerySet.

3. Final Answer:

   default_limit should be an integer, not a string -> Option B

4. Quick Check:

   default_limit type error = default_limit should be an integer, not a string [OK]

Hint: default_limit must be int, not quoted string [OK]

Common Mistakes:

• Setting default_limit as string instead of int
• Thinking pagination_class must be string path
• Assuming ListAPIView disables pagination

5. You want to implement pagination for a large dataset where new items are frequently added. Which pagination style is best to avoid duplicate or missing items when users navigate pages?

hard

A. CursorPagination

B. PageNumberPagination

C. LimitOffsetPagination

D. No pagination

Solution

1. Step 1: Understand pagination challenges with dynamic data

   PageNumber and LimitOffset can cause duplicates or missing items if data changes between requests.

2. Step 2: Identify pagination style that handles dynamic data well

   CursorPagination uses a stable cursor based on item order, preventing duplicates or skips when data changes.

3. Final Answer:

   CursorPagination -> Option A

4. Quick Check:

   Dynamic data needs CursorPagination [OK]

Hint: Use CursorPagination for changing data to avoid duplicates [OK]

Common Mistakes:

• Choosing PageNumberPagination for dynamic data
• Thinking LimitOffsetPagination handles data changes well
• Ignoring pagination for large datasets