Dictionaries in Python

Question

I am trying to get to speed on the use of dictionaries. I spent three hours last night searching the web for examples similar to some of the things I am trying to do. For example, suppose I have two dictionaries (actually I have two lists of dictionaries).

d1={key1:1, key2:2}
d2={key1:1, key2:'A', key4:4}

I want to update d1 so it looks like the following:

d1={key1:1, key2:[2,'A'], key3:3, key4:4}

Ii can't seem to find adequate examples to get me started. I have a fair number of books and I also reviewed them but they all seem to have the same type of examples that I am finding on the web.

Does anyone know of a place or a book that has explicit examples and descriptions of how to use dictionaries?

I think one of the problems I am having is that I am not understanding how references are maintained as I access the dictionary.

I can check to see if the two dictionaries have a common key:

for k in d1.keys():
    for k2 in d2.keys():
        if k==k2:
            print 'true'

but if they do I can't seem to combine the values into a list.

More than a direct answer to this particular example I would appreciate any suggestions about places where there are good examples of using dictionaries.

Answer 1

Try this:

import collections
merged = collections.defaultdict(list)
for k in d1:
   merged[k].append( d1[k] )
for k in d2:
   merged[k].append( d2[k] )

This may be what you're looking for.

Or possibly this.

import collections
merged = collections.defaultdict(set)
for k in d1:
   merged[k].add( d1[k] )
for k in d2:
   merged[k].add( d2[k] )

Answer 2

One good place to start is by getting iPython (easy_install ipython) then poking around with tab completion, ? and dir:

In [2]: dir {}
------> dir({})

Out[2]: 
['__class__',
 ...
 'keys',
 'pop',
 'popitem',
 'setdefault',
 'update',
 'values']

In [3]: {}.update?
Type:       dict
Base Class: <type 'dict'>
String Form:    {}
Namespace:  Interactive
Length:     0
Docstring:
    dict() -> new empty dictionary.
    dict(mapping) -> new dictionary initialized from a mapping object's
        (key, value) pairs.
    dict(seq) -> new dictionary initialized as if via:
        d = {}
        for k, v in seq:
            d[k] = v
    dict(**kwargs) -> new dictionary initialized with the name=value pairs
        in the keyword argument list.  For example:  dict(one=1, two=2)

(just for example)

Anyway, your problem with checking keys common between the two dictionaries: there are a few things to consider (maybe look at the set class?), but here's how I'd do it:

common_keys = [k for k in dict1 if k in dict2]

(ie, "each key k in dict1 if that key is also in dict2") (note, too, that testing for dictionary membership is an O(1) operation, so this will run in O(|dict1|))

edit: alright, so this doesn't solve the problem of merging the two dicts into one with lists... But Lott's answer is good for that, or you could use the setdefault method:

new = {}
for (k, v) in dict1.items():
    new.setdefault(k, []).append(v)
for (k, v) in dict2.items():
    new.setdefault(k, []).append(v)

Answer 3

I believe here is what you want:

>>> d1={'key1':1, 'key2':2}
>>> d2={'key1':1, 'key2':'A', 'key4':4}
>>> d = {}
>>> d.update(d1)
>>> for i in d2:
        if i in d and d2[i] != d[i]:
            d[i] = [d[i], d2[i]]
        else:
            d[i] = d2[i]            
>>> d
{'key2': [2, 'A'], 'key1': 1, 'key4': 4}

Answer 4

Here are some good links:

http://www.diveintopython.org/getting_to_know_python/dictionaries.html
http://docs.python.org/tutorial/datastructures.html#dictionaries