Populating a Python dictionary

Question

Hi I'm building a dictionary where each key is a customer name and each value is a list of tuples which are purchased by each customer, like so: (product, quantity). For example:

{'customer1': (('milk', 3), ('bread', 5), ('eggs', 2)),
 'customer2': (('cheese', 2), ('cereal', 7))}

I am populating the dictionary based on the results of a SQL query. Being a Java programmer new to Python, can someone suggest the "pythonic" way to do this? Each row from the query contains customer name, product, quantity.

Answer 1

First of all, I'd use lists rather than tuples as dictionary entries. The principal difference is that lists are mutable, whereas tuples are not.

I think defaultdict is a good for for this problem:

from collections import defaultdict

customers = defaultdict(list)

You can add entries like so (of course in your case you'd do this in a loop):

customers['customer1'].append(('milk', 3))
customers['customer1'].append(('bread', 5))
customers['customer2'].append(('cereal', 7))

The result is:

>>> print dict(customers)
{'customer1': [('milk', 3), ('bread', 5)], 'customer2': [('cereal', 7)]}

Answer 2

Your inner structure should be a list, not a tuple, since the structure is homogenous.

{'customer1': [('milk', 3), ('bread', 5), ('eggs', 2)],
 'customer2': [('cheese', 2), ('cereal', 7)]}

This will also allow you to use .append() on them, and you can use collections.defaultdict to start off each value with an empty list for further simplification.

Answer 3

Im hoping that you have a list of lists from your database, so e.g.

rows = [('customer1', ('milk', 2)), ('customer12', ('bread', 4))] # etc etc

Then you can simply do.

for row in rows:
    cust_dict[row[0]] = row[1:]