def appendlist(self, key, value):
        "Appends an item to the internal list associated with key"
        self.setlistdefault(key, [])
        dict.__setitem__(self, key, self.getlist(key) + [value])
Appends an item to the internal list associated with key
stackoverflow/venv/lib/python3.6/site-packages/scrapy/utils/datatypes.py