如何在Python中动态构建字典

Question

i want to dynamically build a dictionary in python. based upon a set of shows that has one dimension.

this is what i have currently do when looping through the entries

class Show(object):
    def __init__(self, json_data):
        self.name = json_data.get("abstract_name")
        self.id = json_data.get("abstract_key")
        self.station = json_data.get("station")
        self.first_letter = json_data.get("first_letter")


    def to_dict(self):
        return self.__dict__


 shows = {} 
 for entry in json.get("material"):
        show = Show(entry)
        first_letter = show.first_letter

        if shows.get(first_letter, None) is None:
            shows.update(first_letter, {})

        if shows.get(first_letter).get(show.id, None) is None:
            shows.get(first_letter).update(show.id, {})

        shows.get(first_letter).get(show.id).update(show.to_dict())

What i want to do is create the indexes where it is empty. But i can't seem to find a way to do this

Answer 1

First, you're not using dict.update() the right way. dict.update() expects either:

1/ a dict (or dict-like) object

d.update({"foo":"bar"})

2/ a sequence (list or any iterable) of (key, value) pairs

d.update([("foo", "bar")])

3/ keyword args

d.update(foo="bar")

Note that all those three examples could be written much more simply as

d["foo"] = "bar"

dict.update() is mostly useful if you want to set many keys at once and/or already have another dict or a sequence of key,value pairs.

Also, you don't have to .get() a key to test if it's already set, you can test for containment with key in yourdict (negating it with key not in yourdict ) ie:

if first_letter not in shows:
    shows[first_letter] = {}

But in your case, the simplest solution is to use a defaultdict :

from collections import defaultdict

# we want a defaultdict of defauldicts of dicts,
# so we need a custom factory function
def defaultdict_of_dicts():
    return defaultdict(dict)

shows = defaultdict(defaultdict_of_dicts)

for entry in json.get("material"):
    show = Show(entry)
    shows[show.first_letter][show.id].update(show.to_dict())             

As a last note: your show.to_dict() method may have unwanted side effects:

>>> s = Show({"abstract_name":"name", "abstract_key": "key", "station":"station", "first_letter":"first_letter"}) 
>>> s.first_letter
'first_letter'
>>> d = s.to_dict()
>>> d
{'first_letter': 'first_letter', 'station': 'station', 'name': 'name', 'id': 'key'}
>>> d["first_letter"] = "gotcha"
>>> s.first_letter
'gotcha'
>>> 

It would be safer to implement to_dict() as return self.__dict__.copy() (well, might not be that important in this exact example, but better safe than sorry...)