Question

I have a dictionary which contains dictionaries, which may also contain dictionaries, e.g.

dictionary = {'ID': 0001, 'Name': 'made up name', 'Transactions':
               {'Transaction Ref': 'a1', 'Transaction Details':
                  {'Bill To': 'abc', 'Ship To': 'def', 'Product': 'Widget A'
                      ...} ...} ... }

Currently I’m unpacking to get the ‘Bill To’ for ID 001, ‘Transaction Ref’ a1 as follows:

if dictionary['ID'] == 001:
    transactions = dictionary['Transactions']
        if transactions['Transaction Ref'] == 'a1':
            transaction_details = transactions['Transaction Details']
            bill_to = transaction_details['Bill To']

I can’t help but think this is is a little clunky, especially the last two lines – I feel like something along the lines of the following should work:

bill_to = transactions['Transaction Details']['Bill To']

Is there a simpler approach for drilling down into nested dictionaries without having to unpack into interim variables?

Answer 1

bill_to = transactions['Transaction Details']['Bill To']

actually works. transactions['Transaction Details'] is an expression denoting a dict, so you can do lookup in it. For practical programs, I would prefer an OO approach to nested dicts, though. collections.namedtuple is particularly useful for quickly setting up a bunch of classes that only contain data (and no behavior of their own).

There’s one caveat: in some settings, you might want to catch KeyError when doing lookups, and in this setting, that works too, it’s hard to tell which dictionary lookup failed:

try:
    bill_to = transactions['Transaction Details']['Bill To']
except KeyError:
    # which of the two lookups failed?
    # we don't know unless we inspect the exception;
    # but it's easier to do the lookup and error handling in two steps