The SharedDataMiddleware (and serving.run_simple(static_files=...) respectively) takes a dict, mapping file and directory names in the URL to files, directories or modules on disk. However, there are no dict-lookups involved in that logic, but instead dict item's are iterated, until a first match is found. But since dict items are retrieved in arbitrary order, the behavior is unpredictable, in scenarios like this:

serving.run_simple(static_files={
  '/static/': static_files_dir,
  '/': os.path.join(static_files_dir, 'index.html')
})

If the key / is yielded first, all requests resolve to index.html. Even using an OrderedDict doesn't help, since the SharedDataMiddleware internally creates another dict.

It seems to make most sense to just use lists, both to be passed in when using the API, and for the internal data structure. However, for backwards compatibility I keep supporting dict-like objects as well, in this PR.