Python：为嵌套字典中的一个键附加多个值

Question

I have the below list of tuples:

p = [("01","Master"),("02","Node"),("03","Node"),("04","Server")]

I want my output to look like:

y = {
     "Master":{"number":["01"]},
     "Node":{"number":["02", "03"]},
     "Server":{"number":["04"]}
     }

I have tried the below code:

y = {}
for line in p:
     if line[1] in y:
          y[line[1]] = {}
          y[line[1]]["number"].append(line[0])
     else:
          y[line[1]] = {}
          y[line[1]]["number"] = [line[0]]

And I get the below error:

 Traceback (most recent call last):
         File "<stdin>", line 4, in <module>
 KeyError: 'number'

How do I solve this?

Answer 1

from collections import defaultdict

d = defaultdict(lambda: defaultdict(list))
for v, k in p:
    d[k]["number"].append(v)


print(d)

  defaultdict(<function <lambda> at 0x7f8005097578>, {'Node': defaultdict(<type 'list'>, {'number': ['02', '03']}), 'Master': defaultdict(<type 'list'>, {'number': ['01']}), 'Server': defaultdict(<type 'list'>, {'number': ['04']})})

without defaultdict:

d = {}
from pprint import pprint as pp
for v, k in p:
    d.setdefault(k,{"number":[]})
    d[k]["number"].append(v)


pp(d)

{'Master': {'number': ['01']},
 'Node': {'number': ['02', '03']},
 'Server': {'number': ['04']}}

Answer 2

Do not assign {} to key when key is already present in y.

y = {}
for line in p:
     try:
          y[line[1]]["number"].append(line[0])
     except:
          y[line[1]] = {}
          y[line[1]]["number"] = [line[0]]

OR Use defaultdict use:-

>>> from collections import defaultdict
>>> p = [("01","Master"),("02","Node"),("03","Node"),("04","Server")]
>>> d = defaultdict(list)
>>> for k, v in p:
...    d[v].append(k)
... 
>>> d
defaultdict(<type 'list'>, {'Node': ['02', '03'], 'Master': ['01'], 'Server': ['04']})

Answer 3

It's because you don't initialize your dictionary when needed, and you reset it when not needed.

Try this:

p = [("01","Master"),("02","Node"),("03","Node"),("04","Server")]

y = {}
for (number, category) in p:
    if not y.get(category, False):
        # initializes your sub-dictionary
        y[category] = {"number": []}

    # adds the correct number to the sub-dictionary
    y[category]["number"].append(number)

Note that using a tuple unpacking for (number, category) in p allows your code to be more readable inside your loop.

Answer 4

You are resetting the dictionary!

for line in p:
     if line[1] in y:
          #y[line[1]] = {}   -- RESET! ["number"] will now disappear.
                               #.. which leads to error in the next line.
          y[line[1]]["number"].append(line[0])
     else:
          y[line[1]] = {}
          y[line[1]]["number"] = [line[0]]

A more pythonic way of achieving the same thing would be by using a defaultdict as demonstrated in other answers.