仅听python中对cloud firestore集合的添加

只有将新文档添加到firebase中的特定集合时，才会触发代码

我已经在javascript中找到了解决此问题的方法，但我无法找到python的解决方案。

中包含了一个如何解决此问题的示例（请查看“python”选项卡）：

def on_snapshot(col_snapshot, changes, read_time):
    print(u'Callback received query snapshot.')
    print(u'Current cities in California: ')
    for change in changes:
        if change.type.name == 'ADDED':
            print(u'New city: {}'.format(change.document.id))
        # Removed other cases

col_query = db.collection(u'cities').where(u'state', u'==', u'CA')

# Watch the collection query
query_watch = col_query.on_snapshot(on_snapshot)