dumpdata error message should note pyyaml requirement

[abeld@…]

When trying to dumping some data to yaml, if pyyaml is not installed, one gets the following error message:

$ ./manage-dev dumpdata --indent=2 --format=yaml auth
Error: Unknown serialization format: yaml

It would be very helpful, if the error message would mention the lack of pyyaml, for example by saying:
"Error: Unknown serialization format: yaml (you need to install PyYAML perhaps?)"

Also, the PyYAML requirement should be mentioned on the ​http://docs.djangoproject.com/en/dev/howto/initial-data/ page, as well.

The point is that although the need for pyyaml is mentioned on the "Serializing Django objects" page, when one uses the documentation as a reference, one might miss that page resulting in some head-scratching as to how come yaml doesn't work even though the documentation claims it does.

[Adam Vandenberg]

The "serialization formats" page does note the PyYAML requirement: ​http://docs.djangoproject.com/en/dev/topics/serialization/#serialization-formats

[Aymeric Augustin]

Change UI/UX from NULL to False.

[Aymeric Augustin]

Change Easy pickings from NULL to False.

[Tim Graham]

I don't think there's a need to special case the error message for yaml to note the needed dependency.

[Chris Curvey]

not that my opinon counts, but I agree with the original request. How many folks will google for 'django dumpdata unknown serialization format' only to end up on this page.

[Russell Keith-Magee]

Yeah - I agree with the original request as well. We're swallowing an ImportError, and reporting it as a "missing backend" error. The two errors are distinct, and should be reported as such. It's not about special casing YAML - it's about modifying our error reporting in the general case so that if a backend has an import dependency, we accurately report the underlying problem.

[berto]

I've created a pull request here: ​https://github.com/django/django/pull/1564

Thanks!

[berto]

New PR: ​https://github.com/django/django/pull/1582

[Russell Keith-Magee <russell@…>]

In 4f5faa1916e7c8cb72cc9ebf1a1fd964ba6e707b:

Merge pull request #1582 from rca/12756-missing-yaml-module-serializer-error-message

Fixed #12756: Improved error message when yaml module is missing.

[Russell Keith-Magee <russell@…>]

In 3df9647ad96eb0f1919be921bb96e949f1a518a0:

[1.6.x] Merge pull request #1582 from rca/12756-missing-yaml-module-serializer-error-message

Fixed #12756: Improved error message when yaml module is missing.

Backport of 4f5faa1916e7c8cb72cc9ebf1a1fd964ba6e707b from master.