e.g:
>>> op.normpath(directory).startswith(base_path)
False
>>> op.normpath(directory)
u'/home/projects/mywebsite/static/utils'
>>> base_path
u'/home/projects/mywebsite/admin/../static'
>>> base_path = op.normpath(base_path)
>>> op.normpath(directory).startswith(base_path)
True