def scan(root_dir: str) -> list:
"""
Finds all "collections" under a given directory.
Note:
A collection is defined as a flat folder (IE, no sub-folders) with image contents.
Any non-image files will be ignored. So long as a folder contains one or more
images and has no sub-directories, it will be treated as a collection.
:param root_dir: path to directory to recursively scan through.
:return: list of Collection objects.
"""
collection_list = []
for dir_tuple in (x for x in walk(root_dir)):
if is_collection(dir_tuple):
collection_list.append(Collection.from_dir_tuple(dir_tuple))
return collection_list
