Tool: Pyflakes
Processed Files:
    reviewboard/admin/siteconfig.py
    reviewboard/admin/forms.py

Ignored Files:
    reviewboard/templates/admin/general_settings.html



Tool: PEP8 Style Checker
Processed Files:
    reviewboard/admin/siteconfig.py
    reviewboard/admin/forms.py

Ignored Files:
    reviewboard/templates/admin/general_settings.html

 'elasticsearch' imported but unused

Can we make a constant on the class and then use that both here and in the SEARCH_ENGINE_CHOICES tuple?

Since there's only one variable being formatted in, we can just use %s and positional-based formatting, which should shorten this code a bit.

How about "reviewboard"?

You can use the new constant(s) here, too.

You can use the new constant(s) here, too.

Add a blank line between these?

Hmm, this is a little weird. I think I'd prefer just attaching the change handler and then calling updateCacheFormDisplay() at the end of the function.

This should fit on one line.

Same deal here.

Tool: Pyflakes
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    reviewboard/admin/siteconfig.py
    reviewboard/admin/forms.py

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Tool: PEP8 Style Checker
Processed Files:
    reviewboard/admin/siteconfig.py
    reviewboard/admin/forms.py

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Tool: Pyflakes
Processed Files:
    reviewboard/admin/siteconfig.py
    reviewboard/admin/forms.py
    reviewboard/search/engines.py

Ignored Files:
    reviewboard/templates/admin/general_settings.html



Tool: PEP8 Style Checker
Processed Files:
    reviewboard/admin/siteconfig.py
    reviewboard/admin/forms.py
    reviewboard/search/engines.py

Ignored Files:
    reviewboard/templates/admin/general_settings.html

 'elasticsearch' imported but unused

 'elasticsearch' imported but unused

Since these are POD constants, the variable names should be all caps. As it is where you used it I mistook it for a class name.
Probably also don't need the blank line between them.

You could just make the jquery inline here rather than defining a variable and using add():

$('#row-search_results_per_page, #row-search_engine')
    .toggle(searchEnabled);

You could just make the jquery inline here rather than defining a variable.

You could just make the jquery inline here rather than defining a variable and using add():

$('#row-elasticsearch_index_name, #row-elasticsearch_url')
    .toggle(searchEnabled && searchEngine === 'elasticsearch');

There's an extra line here.

Something terrible has happened here with your editor. Please reindent.
You can also combine the selectors into a single string with a comma.

Tool: Pyflakes
Processed Files:
    reviewboard/admin/siteconfig.py
    reviewboard/admin/forms.py
    setup.py
    reviewboard/search/engines.py

Ignored Files:
    reviewboard/templates/admin/general_settings.html



Tool: PEP8 Style Checker
Processed Files:
    reviewboard/admin/siteconfig.py
    reviewboard/admin/forms.py
    setup.py
    reviewboard/search/engines.py

Ignored Files:
    reviewboard/templates/admin/general_settings.html

 'elasticsearch' imported but unused

I think something went horribly wrong with search/replace. We're now shouting WOOSH and ELASTICSEARCH in a lot of places that we shouldn't be. Only the variable names should be all-caps.

Can be:

$('#id_search_engine, #id_search_enable')
    .change(updateSearchFormDisplay);

I think rather than hard-coding everything for Whoosh and Elasticsearch, we should do what we do for auth, hosting services, etc. and have a concept of backends that are registered as entrypoints, with config forms. It's a little more work up-front, but it makes it a lot easier down the road for us or third-parties to support additional search engines (say, AWS's search services).
We'd still go through Haystack for all this, so any extension providing such things would need to deal with that, but we'd have our side covered when it comes to settings and configuration.

Let's swap these fields. The index depends on the URL.
We should also increase the width of both fields. The URL especially.

Let's rename during import to search_engines, so it's clear and won't conflict in the future.

Missing docstring.

Swap these.

Let's rename here as well.

Missing a file docstring.

Can we document both of these?

Why the "WHOOSH"?

? This isn't how it's listed in pypi.

Tool: Pyflakes
Processed Files:
    reviewboard/admin/siteconfig.py
    reviewboard/admin/forms.py
    reviewboard/search/search_engines.py

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    reviewboard/templates/admin/general_settings.html



Tool: PEP8 Style Checker
Processed Files:
    reviewboard/admin/siteconfig.py
    reviewboard/admin/forms.py
    reviewboard/search/search_engines.py

Ignored Files:
    reviewboard/templates/admin/general_settings.html

 'elasticsearch' imported but unused

This looks good to me as-is, but I agree with Christian's comment about making this more pluggable. Perhaps in a follow-up change?