Question

I can not really find a satisfying answer to the following question:

What would be the best practice for a module to add a new product attribute to all attribute_sets (and possibly at a certain location).

All approaches I can find seem to refer to adding a new attribute to just one attribute set.

Answer 1

There is a magento way for it
Example:

$installer->addAttribute('catalog_product', 'test_me', array(
'label'         => 'test Me',
'input'         => 'textarea',
'type'          => 'text',
'class'         => '',
'global'        => true,
'visible'       => true,
'required'      => false,
'user_defined'  => true,
'default'       => '',
'apply_to'      => 'simple,configurable',
'visible_on_front' => true,
'is_configurable' => false,
'wysiwyg_enabled' => true,
'used_in_product_listing' => true,
'is_html_allowed_on_front' => true,
'group'         => 'Config',
'sort_order'    => 25

));

To add attribute to every attribute_set you need to set 'user_defined' = true and set 'group' = 'your_group_name'. If needed magento will add your group to every attribute set, and attach attribute to it.