Paginator.Page() should throw an exception for invalid [next|previous]_page_number()

[mehta.apurva@…]

The next_page_number() and previous_page_number() currently return an incremented or decremented number without verifying if the page is in the range or not. They should instead throw an exception.

Repro Steps
=======

>>> from django.core.paginator import Paginator
>>> objects = ['john', 'paul', 'george', 'ringo', 'bill', 'gates', 'steve', 'jobs']
>>> p = Paginator(objects, 2)
>>> p.count
8
>>> p.num_pages
4
>>> p.page_range
[1, 2, 3, 4]
>>> p.page(1).has_previous()
False
>>> p.page(4).has_next()
False
>>> p.page(1).previous_page_number()  ### Should throw Exception
0   
>>> p.page(4).next_page_number() ### Should throw Exception
5

[Julien Phalip]

That makes sense. It would be a matter of calling Page.paginator.validate_number().

[Apurva Mehta]

Patch for the issue

[Apurva Mehta]

Updated the functions to call Page.paginator.validate_number()

[Aymeric Augustin]

Please don't mark tickets as fixed until the fix is actually committed to the code base.

[Julien Phalip]

Thank you. This would now need some tests and documentation (​https://docs.djangoproject.com/en/dev/topics/pagination/).

[neaf]

Previous patch with tests and documentation update.

[neaf]

Attached another patch with tests and documentation.

I'm not sure if it isn't compatibility breaking change. I'm pretty sure no-one wants to display 0 and out-of-range pages but I don't know if we should throw exceptions where we haven't in the ast.

[Julien Phalip]

Quick note: This is slightly backward-compatible and some release notes should be added.

[Claude Paroz <claude@…>]

In [fc40a6504b78e604a6ba90a3b85b1ed54ee238a2]:

Fixed #17159 -- Validated returned number of next|previous_page_number

Thanks mehta.apurva at gmail.com for the report and the initial patch
and neaf for the complete patch.