How to sum all the values that belong to the same key?

Question

I'm pulling data from the database and assuming i have something like this:

    Product Name    Quantity
    a               3
    a               5
    b               2
    c               7

I want to sum the Quantity based on Product name, so this is what i want:

    product = {'a':8, 'b':2, 'c':7 }

Here's what I'm trying to do after fetching the data from the database:

    for row in result:
       product[row['product_name']] += row['quantity']

but this will give me: 'a'=5 only, not 8.

Answer 1

If you insist on using loops, you can do this:

# fake data to make the script runnable
result = [
  {'product_name': 'a', 'quantity': 3},
  {'product_name': 'a', 'quantity': 5},
  {'product_name': 'b', 'quantity': 2},
  {'product_name': 'c', 'quantity': 7}
]

# solution with defaultdict and loops
from collections import defaultdict

d = defaultdict(int)
for row in result:
  d[row['product_name']] += row['quantity']

print(dict(d))

The output:

{'a': 8, 'b': 2, 'c': 7}