[development] Overiding #theme in hook_form_alter()

[Barry Jaspan]

I've written the module that wants to override the #theme of another 
module's form.  I discovered that in hook_form_alter I cannot set the 
#theme field because drupal_get_form overrides #theme after calling 
hook_form_alter it if the function theme_<formname> exists or the 
$callback argument is provided.

This seems like an unintentional restriction to me.  Is 
it?  Hook_form_alter can change everything else about a form, so why 
should it not be able to change the #theme field?  I know the theme 
function can be overridden by the site theme but that forces the 
module not to be self-contained.

The workaround I found is to add a callback to the form's #pre_render 
array that sets the form's #theme field.  This works fine (because 
#pre_render is invoked after drupal_get_form overrides #theme) but is 
certainly not intuitive; if this really is the best way to do it, 
I'll author a new page for the Forms API Tips and Tricks book section.

Thanks,

Barry

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