Accessing values nested within dictionaries

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Question :

Accessing values nested within dictionaries

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:

    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
Answered By: Fred Foo

Answer #2:

You can use something like this:

>>> def lookup(dic, key, *keys):
...     if keys:
...         return lookup(dic.get(key, {}), *keys)
...     return dic.get(key)
>>> d = {'a':{'b':{'c':5}}}
>>> print lookup(d, 'a', 'b', 'c')
>>> print lookup(d, 'a', 'c')

Additionally, if you don’t want to define your search keys as individual parameters, you can just pass them in as a list like this:

>>> print lookup(d, *['a', 'b', 'c'])
>>> print lookup(d, *['a', 'c'])

Answer #3:

Following is another way of accessing nested dictionaries

>>> dbo={'m':{'d':{'v':{'version':1}}}}
>>> name='m__d__v__version' # it'll refer to 'dbo['m']['d']['v']['version']', '__' is the separator
>>> version = reduce(dict.get, name.split('__'), dbo)
>>> print version

Here, variable ‘name’ refers to ‘dbo[‘m’][‘d’][‘v’][‘version’]’, which seems much shorter and neat.

This method will not throw KeyError. If a key is not found then you’ll get ‘None’.


Answered By: blackfyre

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