Can anyone explain to me what a two-level dictionary is in Python

Question

I am struggling to find any documentation anywhere on what this actually is. I understand just an ordinary dictionary. This consists of key and value pairs so if you search for a key its corresponding value is returned, For example:

myDict = {‘dog’ : ’fido’, ‘cat’ : ’tiddles’, ‘fish’ : ’bubbles’, ’rabbit’ : ’thumper’}

And then you can invoke certain methods on this like:

myDict[‘fish’]

returns

'bubbles'

or

myDict.has_key(‘tiddles’)

returns

True

How would a two-level dictionary compare to this?

It appears nested dictionaries was what I was looking for.

One more question, say I have a nested dictionary which links words to text files where the first integer is the number of the text file and the second is the number of occurrences:

myDict = {'through':{1:18,2:27,3:2,4:15,5:63}, 'one':{1:27,2:15,3:24,4:9,5:32}, 'clock':{1:2,2:5,3:9,4:6,5:15}

How would I use the file numbers to work out the total number of text files that were present? i.e is there a way of extracting the number of key / value pairs in the inner dictionary?

"Two-level dictionary" isn't really an official term, but it's a reasonably common way to describe a nested dictionary of exactly two levels—that is, a dictionary which has only dictionaries for values (but those sub-dictionaries don't have dictionaries for values). — abarnert, Nov 10 '14 at 21:18
tiddles is a value, not a key, it should return False if he doesn't mean a two-way dict. I give an answer below — Russia Must Remove Putin, Nov 10 '14 at 21:22
Yes I think a dictionary of dictionaries is what I am looking for. I have another question though? — KeyboardNinja, Nov 10 '14 at 22:28

score 4 · Accepted Answer · answered Nov 10 '14 at 21:14

4

I guess a two level dictionary could be a dictionary of dictionaries i.e

dict = {'a':{"cool":1,"dict":2}}

you could use it like

dict['a']['cool'] 
>> 1

so you can do

dict['a'].has_key('cool')
>> True

answered Nov 10 '14 at 21:14

user3684792

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score 0 · Answer 2 · answered Nov 10 '14 at 21:16

It is just a nested dictionary, meaning it contains other dictionaries, for example

d = {'mike':{'age':10, 'gender':'male'}, 'jen':{'age':12, 'gender':'female'}}

I can access inner values such as

>>> d['mike']['age']
10

Common examples of deeply nested dictionaries in Python are reading and writing of JSON.

score 0 · Answer 3 · edited May 23 '17 at 11:57

I believe you mean a two-way dictionary, here's a recipe (from Two way/reverse map):

class TwoWayDict(dict):
    def __setitem__(self, key, value):
        # Remove any previous connections with these values
        if key in self:
            del self[key]
        if value in self:
            del self[value]
        dict.__setitem__(self, key, value)
        dict.__setitem__(self, value, key)

    def __delitem__(self, key):
        dict.__delitem__(self, self[key])
        dict.__delitem__(self, key)

    def __len__(self):
        """Returns the number of connections"""
        return dict.__len__(self) // 2

usage:

myDict = {‘dog’ : ’fido’, ‘cat’ : ’tiddles’, ‘fish’ : ’bubbles’, ’rabbit’ : ’thumper’}

twowaydict = TwoWayDict() # can't instantiate with old dict, need to setitem
for key in myDict: 
    twowaydict[key] = myDict[key]

twowaydict.has_key('tiddles')

returns True

If you mean a dict of dicts, that's fairly common construct, where the values of the containing dict are also dicts.

dofd = {'key1': {'subkey1': 'value1,1', 'subkey2': 'value1,2'}
        'key2': {'subkey1': 'value2,1', 'subkey2': 'value2,2'}
       }

and you'd access the internal values like this:

dofd['key1']['subkey2']

should return value1,2

I might be wrong, but beware with `return dict.__len__(self) // 2`. What happens if `twowaydict['a'] = 'a'`? — spalac24, Nov 10 '14 at 21:39

Can anyone explain to me what a two-level dictionary is in Python

3 Answers3