human
Solve the following python programming problem given the following code:
def categories(collection = None, collections = None, reverse = None, order_by = None, _pod = None, use_cache = False) : 
    if isinstance (collection, collection_lib.Collection) : 
        collection = collection 
else : 
        if isinstance (collection, basestring) : 
            collection = _pod.get_collection (collection) 
else : 
            text = '{} must be a Collection instance or a collection path, found: {}.' 
            raise ValueError (text.format (collection, type (collection))) 
category_list = collection.list_categories () 
    def order_func(doc) : 
        try : 
            return category_list.index (doc.category) 
except ValueError : 
            return 0 
docs = [order_func for doc in collection.list_docs (reverse = reverse)] 
    docs = sorted (docs, key = order_func) 
    items = itertools.groupby (docs, key = order_func) 
    return ((category_list [index], pages) for (index, pages) in items) 

    
    

    Fix the buggy line: docs = [order_func for doc in collection.list_docs (reverse = reverse)]
assistant
#Here is Python program to fix the buggy line: docs = [order_func for doc in collection.

docs = [doc for doc in collection.list_docs (reverse = reverse)]