Double-indexed dictionary

Question

I want to be able to store and look up values in a dictionary based on two integer values.

So when I look up a value I want to use the keys read_length and min_size to access the element, like so:

number_of_read_lengths[read_length][min_size]

I know I can create nested dictionaries, but that is a slight hassle.

Is there a simple way of doing what I want to do?

Answer 1

You can use any immutable and hashable object as key, including tuples

number_of_read_lengths = {}  number_of_read_lengths[14,3] = "Your value" 

Answer 2

Using tuples could be quite annoying -- you got to remember to place the tuple during indexing.

I would recommend a nested dict, but a defaultdict, like so:

from collections import defaultdict  number_of_read_lengths = defaultdict(dict)  number_of_read_lengths[1][2] = 3  print(number_of_read_lengths) 

This code would give:

defaultdict(<type 'dict'>, {1: {2: 3}}) 

This way, any non-existing element in the number_of_read_lengths dict will be created as a dict when accessing or setting it. Simple and effective.

More info on defaultdict: http://docs.python.org/library/collections.html#collections.defaultdict There are also examples: http://docs.python.org/library/collections.html#defaultdict-examples

Answer 3

You could try to use tuples as keys:

number_of_read_lengths[(read_length, min_size)]

Answer 4

Just to expand a bit more from the comment I made:

A dict key must be hashable, which a simple tuple is. However, a tuple that contains unhashable values such as lists, is not hashable (even though it is immutable!) and therefore cannot be used as dict key:

>>> bad = ([12],[32])
# still immutable
>>> bad[1] = [21]
Traceback (most recent call last):
File "<input>", line 1, in <module>
TypeError: 'tuple' object does not support item assignment

# but not hashable!
>>> d = {}
>>> d[bad] = 2
Traceback (most recent call last):
  File "<input>", line 1, in <module>
TypeError: unhashable type: 'list'

You can even have mutable and hashable objects as dict keys, but it's not really useful and should be avoided.