Is there a 'multimap' implementation in Python?

Question

I am new to Python, and I am familiar with implementations of Multimaps in other languages. Does Python have such a data structure built-in, or available in a commonly-used library?

To illustrate what I mean by "multimap":

a = multidict() a[1] = 'a' a[1] = 'b' a[2] = 'c'  print(a[1])  # prints: ['a', 'b'] print(a[2])  # prints: ['c'] 

Answer 1

Such a thing is not present in the standard library. You can use a defaultdict though:

>>> from collections import defaultdict >>> md = defaultdict(list) >>> md[1].append('a') >>> md[1].append('b') >>> md[2].append('c') >>> md[1] ['a', 'b'] >>> md[2] ['c'] 

(Instead of list you may want to use set, in which case you'd call .add instead of .append.)

---

As an aside: look at these two lines you wrote:

a[1] = 'a' a[1] = 'b' 

This seems to indicate that you want the expression a[1] to be equal to two distinct values. This is not possible with dictionaries because their keys are unique and each of them is associated with a single value. What you can do, however, is extract all values inside the list associated with a given key, one by one. You can use iter followed by successive calls to next for that. Or you can just use two loops:

>>> for k, v in md.items(): ...     for w in v: ...         print("md[%d] = '%s'" % (k, w)) ...  md[1] = 'a' md[1] = 'b' md[2] = 'c' 

Answer 2

Just for future visitors. Currently there is a python implementation of Multimap. It's available via pypi