Using collections.defaultdict for ease:
from collections import defaultdict
v = defaultdict[list]
for key, value in sorted[d.items[]]:
v[value].append[key]
but you can do it with a bog-standard dict too, using dict.setdefault[]:
v = {}
for key, value in sorted[d.items[]]:
v.setdefault[value, []].append[key]
The above sorts keys first; sorting the values of the output dictionary later is much more cumbersome and inefficient.
If anyone would not need the output to be sorted, you can drop the sorted[] call, and use sets [the keys in the input dictionary are guaranteed to be unique, so no information is lost]:
v = {}
for key, value in d.items[]:
v.setdefault[value, set[]].add[key]
to produce:
{6: {1}, 1: {2, 3, 6}, 9: {4, 5}}
[that the output of the set values is sorted is a coincidence, a side-effect of how hash values for integers are implemented; sets are unordered structures].
Group List of Dictionary Data by Particular Key in Python can be done using itertools.groupby[] method.
Itertools.groupby[]
This method calculates the keys for each element present in iterable. It returns key and iterable of grouped items.
Syntax: itertools.groupby[iterable, key_func]
Parameters:
- iterable: Iterable can be of any kind [list, tuple, dictionary].
- key_func: A function that calculates keys for each element present in iterable.
Return type: It returns consecutive keys and groups from the iterable. If the key function is not specified or is None, key defaults to an identity function and returns the element unchanged.
Let’s see the examples: Example 1: Suppose we have list of dictionary of employee and company.
INFO = [
{'employee': 'XYZ_1', 'company': 'ABC_1'},
{'employee': 'XYZ_2', 'company': 'ABC_2'},
{'employee': 'XYZ_3', 'company': 'ABC_3'},
{'employee': 'XYZ_4', 'company': 'ABC_3'},
{'employee': 'XYZ_5', 'company': 'ABC_2'},
{'employee': 'XYZ_6', 'company': 'ABC_3'},
{'employee': 'XYZ_7', 'company': 'ABC_1'},
{'employee': 'XYZ_8', 'company': 'ABC_2'},
{'employee': 'XYZ_9', 'company': 'ABC_1'}
]Now we need to display all the data group by the ‘company’ key name.
Code:
Python3
from itertools import groupby
INFO = [
{'employee': 'XYZ_1', 'company': 'ABC_1'},
{'employee': 'XYZ_2', 'company': 'ABC_2'},
{'employee': 'XYZ_3', 'company': 'ABC_3'},
{'employee': 'XYZ_4', 'company': 'ABC_3'},
{'employee': 'XYZ_5', 'company': 'ABC_2'},
{'employee': 'XYZ_6', 'company': 'ABC_3'},
{'employee': 'XYZ_7', 'company': 'ABC_1'},
{'employee': 'XYZ_8', 'company': 'ABC_2'},
{'employee': 'XYZ_9', 'company': 'ABC_1'}
]
def key_func[k]:
return k['company']
INFO = sorted[INFO, key=key_func]
for key, value in groupby[INFO, key_func]:
print[key]
print[list[value]]
Output:
ABC_1 [{’employee’: ‘XYZ_1’, ‘company’: ‘ABC_1′}, {’employee’: ‘XYZ_7’, ‘company’: ‘ABC_1′}, {’employee’: ‘XYZ_9’, ‘company’: ‘ABC_1′}] ABC_2 [{’employee’: ‘XYZ_2’, ‘company’: ‘ABC_2′}, {’employee’: ‘XYZ_5’, ‘company’: ‘ABC_2′}, {’employee’: ‘XYZ_8’, ‘company’: ‘ABC_2′}] ABC_3 [{’employee’: ‘XYZ_3’, ‘company’: ‘ABC_3′}, {’employee’: ‘XYZ_4’, ‘company’: ‘ABC_3′}, {’employee’: ‘XYZ_6’, ‘company’: ‘ABC_3’}]
Example 2: Suppose we have list of dictionary of student grades and marks.
students = [
{'mark': '65','grade': 'C'},
{'mark': '86','grade': 'A'},
{'mark': '73','grade': 'B'},
{'mark': '49','grade': 'D'},
{'mark': '91','grade': 'A'},
{'mark': '79','grade': 'B'}
]Now we need to display all the data group by the ‘grade’ key.
Code:
Python3
from itertools import groupby
from operator import itemgetter
students = [
{'mark': '65', 'grade': 'C'},
{'mark': '86', 'grade': 'A'},
{'mark': '73', 'grade': 'B'},
{'mark': '49', 'grade': 'D'},
{'mark': '91', 'grade': 'A'},
{'mark': '79', 'grade': 'B'}
]
students = sorted[students,
key = itemgetter['grade']]
for key, value in groupby[students,
key = itemgetter['grade']]:
print[key]
for k in value:
print[k]
Output:
A
{'mark': '86', 'grade': 'A'}
{'mark': '91', 'grade': 'A'}
B
{'mark': '73', 'grade': 'B'}
{'mark': '79', 'grade': 'B'}
C
{'mark': '65', 'grade': 'C'}
D
{'mark': '49', 'grade': 'D'}