Multiple keys per value

Question

Is it possible to assign multiple keys per value in a Python dictionary. One possible solution is to assign value to each key:

dict = {'k1':'v1', 'k2':'v1', 'k3':'v1', 'k4':'v2'} 

but this is not memory efficient since my data file is > 2 GB. Otherwise you could make a dictionary of dictionary keys:

key_dic = {'k1':'k1', 'k2':'k1', 'k3':'k1', 'k4':'k4'} dict = {'k1':'v1', 'k4':'v2'} main_key = key_dict['k2'] value = dict[main_key] 

This is also very time and effort consuming because I have to go through whole dictionary/file twice. Is there any other easy and inbuilt Python solution?

Note: my dictionary values are not simple string (as in the question 'v1', 'v2') rather complex objects (contains different other dictionary/list etc. and not possible to pickle them)

Note: the question seems similar as How can I use both a key and an index for the same dictionary value? But I am not looking for ordered/indexed dictionary and I am looking for other efficient solutions (if any) other then the two mentioned in this question.

Answer 1

What type are the values?

dict = {'k1':MyClass(1), 'k2':MyClass(1)} 

will give duplicate value objects, but

v1 = MyClass(1) dict = {'k1':v1, 'k2':v1} 

results in both keys referring to the same actual object.

In the original question, your values are strings: even though you're declaring the same string twice, I think they'll be interned to the same object in that case

---

NB. if you're not sure whether you've ended up with duplicates, you can find out like so:

if dict['k1'] is dict['k2']:     print("good: k1 and k2 refer to the same instance") else:     print("bad: k1 and k2 refer to different instances") 

(is check thanks to J.F.Sebastian, replacing id())

Answer 2

Check out this - it's an implementation of exactly what you're asking: multi_key_dict(ionary)

https://pypi.python.org/pypi/multi_key_dict (sources at https://github.com/formiaczek/python_data_structures/tree/master/multi_key_dict)

(on Unix platforms it possibly comes as a package and you can try to install it with something like:

sudo apt-get install python-multi-key-dict 

for Debian, or an equivalent for your distribution)

You can use different types for keys but also keys of the same type. Also you can iterate over items using key types of your choice, e.g.:

m = multi_key_dict() m['aa', 12] = 12 m['bb', 1] = 'cc and 1' m['cc', 13] = 'something else'  print m['aa']   # will print '12' print m[12]     # will also print '12'  # but also: for key, value in m.iteritems(int):     print key, ':', value # will print:1 # 1 : cc and 1 # 12 : 12 # 13 : something else  # and iterating by string keys: for key, value in m.iteritems(str):     print key, ':', value # will print: # aa : 12 # cc : something else # bb : cc and 1  m[12] = 20 # now update the value print m[12]   # will print '20' (updated value) print m['aa']   # will also print '20' (it maps to the same element) 

There is no limit to number of keys, so code like:

m['a', 3, 5, 'bb', 33] = 'something'  

is valid, and either of keys can be used to refer to so-created value (either to read / write or delete it).

Edit: From version 2.0 it should also work with python3.