Understanding Python's @Property decorator usage

photos.py

import os
import hashlib
import time
import urllib.request
import numpy as np


class Photo(object):
    sources = None

    def __init__(self, source_type):
        self.source_type = source_type
        self.uploads_folder = 'uploads'
        self.min_width = self.min_height = 200

    @property
    def filename(self):
        return self.__filename

    @filename.setter
    def filename(self, ext='jpg'):
        filehash = hashlib.sha1()
        filehash.update(str(time.time()).encode('utf-8'))
        filename = filehash.hexdigest()[:18] + '.' + ext

        if not os.path.exists(self.uploads_folder):
            os.makedirs(self.uploads_folder)

        self.__filename = os.path.join(self.uploads_folder,
        filename)

    @classmethod
    def set_source(self, source_type):
        if self.sources is None:
            self.sources = {}
            for source_class in self.__subclasses__():
                source = source_class()
                self.sources[source.source_type] = source
        return self.sources[source_type]


class FacebookPhoto(Photo):
    def __init__(self):
        super(FacebookPhoto, self).__init__('facebook')

    def save_file(self, url):
        with urllib.request.urlopen(url) as url:
            f = np.asarray(bytearray(url.read()), dtype='uint8')
        f = cv2.imdecode(f, cv2.IMREAD_COLOR)

        cv2.imwrite(self.filename, f)


class LocalPhoto(Photo):
    def __init__(self):
        super(LocalPhoto, self).__init__('local')

    def save_file(self, formdata):
        ext = formdata.mimetype.split('/')[1]
        formdata.save(self.filename(ext))