Group by multiple keys and summarize/average values of a list of dictionaries

Question

What is the most pythonic way to group by multiple keys and summarize/average values of a list of dictionaries in Python please? Say I have a list of dictionaries as below:<

Answer 1

You can put the quantities and the number of their occurrences in one big default dict:

from collections import defaultdict

counts = defaultdict(lambda: [0, 0])
for line in input_data:
    entry = counts[(line['dept'], line['sku'])]
    entry[0] += line['qty']
    entry[1] += 1

Now it is only the question to get the numbers into a list of dicts:

sums_dict = [{'dept': k[0], 'sku': k[1], 'qty': v[0]} 
              for k, v in counts.items()]
avg_dict = [{'dept': k[0], 'sku': k[1], 'avg': float(v[0]) / v[1]} for 
             k, v in counts.items()]

The results for the sums:

sums_dict

[{'dept': '002', 'qty': 600, 'sku': 'qux'},
 {'dept': '001', 'qty': 400, 'sku': 'foo'},
 {'dept': '003', 'qty': 700, 'sku': 'foo'},
 {'dept': '002', 'qty': 900, 'sku': 'baz'},
 {'dept': '001', 'qty': 200, 'sku': 'bar'}]

and for the averages:

avg_dict

[{'avg': 600.0, 'dept': '002', 'sku': 'qux'},
 {'avg': 200.0, 'dept': '001', 'sku': 'foo'},
 {'avg': 700.0, 'dept': '003', 'sku': 'foo'},
 {'avg': 450.0, 'dept': '002', 'sku': 'baz'},
 {'avg': 200.0, 'dept': '001', 'sku': 'bar'}]

An alternative version without the default dict:

counts = {}
for line in input_data:
    entry = counts.setdefault((line['dept'], line['sku']), [0, 0])
    entry[0] += line['qty']
    entry[1] += 1

The rest is the same:

sums_dict = [{'dept': k[0], 'sku': k[1], 'qty': v[0]} 
              for k, v in counts.items()]
avg_dict = [{'dept': k[0], 'sku': k[1], 'avg': float(v[0]) / v[1]} for 
             k, v in counts.items()]