在 Python 中将嵌套值插入到字典中

Question

say I have a simple dictionary d={'a':1}

I wish to run a line d['b']['c'] = 2 but I can't, I get: KeyError: 'b'

I don't want to insert b first with an empty dictionary because most of the time, this dictionary will contain b with more values except for c .

Is there an elegant way to do it so my final dictionary is:

d = {'a':1,
     'b':{'c':2}}

Answer 1

Well if you don't want to first assign an empty dict in order to erase nothing, you can first check if the dict is here or not, it's not one line only but quite clear I think:

d ={'a':1}
b = d.get('b', {})
b['c'] = 2
d['b'] = b

Answer 2

Is defaultdict sufficient for you?

from collections import defaultdict

d = defaultdict()

d['a'] = 1
print(d) # this gives back: defaultdict(None, {'a': 1})

d['b'] = {'c':2}
print(d) # this gives back: defaultdict(None, {'a': 1, 'b': {'c': 2}})

For a better example of defaultdict:

s = 'mississippi'
d = defaultdict(int)
for k in s:
    d[k] += 1

d.items() # this gives back: [('i', 4), ('p', 2), ('s', 4), ('m', 1)]

When a letter is first encountered, it is missing from the mapping, so the default_factory function calls int() to supply a default count of zero. The increment operation then builds up the count for each letter.