如何在Python中为特定键创建列表作为字典条目？

Question

I am iterating through a file looking for certain attributes in each line, and if the line matches I want to insert it as an item in a list for a particular dictionary key.

For example:

list_of_names = ['aaron', 'boo', 'charlie']
for name in list_of_names
    if color contains 'a':
        #add here: add to list in dict['has_a']
print dict['has_a']

Should print ['aaron', 'charlie'].

The reason I'm asking this is because I'm not sure how else to create multiple entries for a key in a dictionary.

Answer 1

You can use python's defaultdict for this purpose. It will automatically generate a list as a default value for the dictionary.

from collections import defaultdict

mydict = defaultdict(list)
list_of_names = ['aaron', 'boo', 'charlie']
for name in list_of_names:
    if  'a' in name:
        mydict['has_a'].append(name)
print mydict['has_a']

Output:

['aaron', 'charlie']

---

The OP has indicated in a comment that he wants heterogenous values in his dictionary. In that case a defaultdict may not be appropriate and instead he should just special case those two cases.

# Initialize our dictionary with list values for the two special cases.
mydict = {'has_a' : [], 'has_b' : []}
list_of_names = ['aaron', 'boo', 'charlie']
for name in list_of_names:
    if  'a' in name:
        mydict['has_a'].append(name)
    # When not in a special case, just use the dictionary like normal to assign values.
print mydict['has_a']

Answer 2

I think it is a good use case for the setdefault method of the dict object:

d = dict()
for name in list_of_names:
  if 'a' in name:
    d.setdefault("has_a", []).append(name)

Answer 3

You can use key function to get list of keys and check if adding is needed. Then append as always.

list_of_names = ['aaron', 'boo', 'charlie'] has_dictionary = {} for name in list_of_names: if name.find('a') != -1: if 'has_a' not in has_dictionary.keys(): has_dictionary['has_a'] = [] has_dictionary['has_a'].append(name) print(has_dictionary['has_a'])