Limiting validation to the create page not the edit page

What is the best way to limit your custom validation check to the add page an开发者_C百科d not the edit page?

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You could use the arg() function to check that you're on the /node/add/... page. Try changing the first line inside your hook_form_alter implementation to read...

if($form_id === 'external_favourite_node_form' && arg(1) == 'add') {