nested_dict 1.0.4

Defaultdict extension for dictionaries with multiple levels of nesting

Overview

collections.defaultdict allows dictionaries to be created with default values.

For example, a dictionary of sets:

from collections import defaultdict

dd = defaultdict(set)

dd["1st group"].add(3)
dd["2nd group"].add(5)
dd["2nd group"].add(8)
dd["1st group"].add(4)
dd["2nd group"].add(5)

print dict(dd)

gives:

{'1st group': set([3, 4]), '2nd group': set([8, 5])})

This module extends this to dictionaries with more than one level of nesting:

from nested_dict import *

nd = nested_dict()

nd["a"]["b"]["c"] = 311
nd["d"]["e"] = 311

Each nested level is create magically when accessed, a process known as “auto-vivification” in perl.

How to use nested_dict:

“Auto-vivifying” nested dict

nd= nested_dict()
nd["mouse"]["chr1"]["+"] = 311
nd["mouse"]["chromosomes"]="completed"
nd["mouse"]["chr2"] = "2nd longest"
nd["mouse"]["chr3"] = "3rd longest"

for k, v in nd.iteritems_flat():
     print "%-30s=-%20s" % (k,v)

Gives:

('mouse', 'chr3')             =-         3rd longest
('mouse', 'chromosomes')      =-           completed
('mouse', 'chr2')             =-         2nd longest
('mouse', 'chr1', '+')        =-                 311

Specifying the autovivified object

If you wish the nested dictionary to hold a collection rather than a scalar, you have to write:

nd["mouse"]["chr2"] = list()
nd["mouse"]["chr2"].append(12)

or:

nd["mouse"]["chr2"] = set()
nd["mouse"]["chr2"].add(84)

Which doesn’t seem very “auto” at all!

Instead, specify the collection in the constructor of nested_dict:

# two levels of nesting
nd2 = nested_dict(2, list)
nd2["mouse"]["chr2"].append(12)

# three levels of nesting
nd3 = nested_dict(3, set)
nd3["mouse"]["chr2"]["categorised"].add(3)

# counts
nd4 = nested_dict(2, int)
nd4["mouse"]["chr2"]+=4
nd4["human"]["chr1"]+=3
nd4["human"]["chr3"]+=4