Importing exceptions: documentation should be more explicit and clearer

[Daniele Procida]

At ​https://docs.djangoproject.com/en/dev/ref/exceptions, for some of the listed exceptions information is provided on how to import it.

For example: "The UnreadablePostError is raised when a user cancels an upload. It is available from django.http."

For others though (e.g. ​https://docs.djangoproject.com/en/dev/ref/exceptions/#noreversematch) there isn't clear explicit information about where to get them from.

Also, when the information is provided, it's not always provided consistently.

My suggestion is that each section (Database Exceptions, Http Exceptions and so on) there should begin with a paragraph on its own: Core exceptions are provided in django.core.exceptions, Database exceptions are provided in django.db and so on.

FInally, Django-specific Exceptions seems a bit of a misnomer, since others not in that section are also Django-specific. Calling them Core exceptions would seem to make more sense.

[Dominic Rodger]

This has bugged me in the past too - I'll take a look at fixing it.

[Dominic Rodger]

Pull request created at ​https://github.com/django/django/pull/1425.

There are a couple of extra commits in the pull request that tidy up two other things on this page (one typo, and the section on DoesNotExist). I'll happily separate those out into a separate pull request if that'd be easier.

Do let me know if there's anything else I can do to make getting this merged easier, this is my first (non-single-line!) submission to Django.

[Daniele Procida]

I'm taking a look at your pull request now. Eventually, your commits should be collapsed into one, which will require a git rebase. There's no reason to make multiple pull requests.

Anyway, it looks fundamentally good; I'll send you a pull request myself if I think any minor tweaks to your improvements are required.

[Tim Graham <timograham@…>]

In 920b242e307297778c6e5010f23502c4f41f299c:

Fixed #20786 -- Cleaned up docs/ref/exceptions.txt

Thanks Daniele Procida for the suggestion and edits.

[Tim Graham <timograham@…>]

In e710f6dc56731b52f84864e7cb199dc4a8f881e7:

[1.6.x] Fixed #20786 -- Cleaned up docs/ref/exceptions.txt

Thanks Daniele Procida for the suggestion and edits.

Backport of 920b242e30 from master