Multilevel Dictionaries in Python

content.md

As an experienced Perl programmer, I have been dealing with nested hashrefs of hashrefs. (Those not familiar with Perl: hashref is short for hash reference. Hash is also known as associative array.)

In Perl, we can just arbitrarily create hashrefs of hashrefs on the fly, e.g. hashref-hashref.pl

In Python, not so easy. Something like multilevel-dict-broken.py will give you a KeyError exception.

My friend Kamal Advani (thanks, man!) suggested something like multilevel-dict-solution.py to me.

Use at your own risk. I do not provide any support for this. You have my permission to copy it.

hashref-hashref.pl

#!/usr/bin/perl

use strict;
use Data::Dumper;

my $x;
$x->{a}->{b}->{c}->{d} = 1;
print Dumper($x)

=begin output
$VAR1 = {
          'a' => {
                   'b' => {
                            'c' => {
                                     'd' => 1
                                   }
                          }
                 }
        };
=end output

multilevel-dict-broken.py

#!/usr/bin/env python

x = {}
x['a']['b']['c']['d'] = 1

multilevel-dict-solution.py

#!/usr/bin/env python

import collections
import pprint

dict_factory = lambda: collections.defaultdict(dict_factory)
x = collections.defaultdict(dict_factory)
x['a']['b']['c']['d'] = 1
print x['a']['b']['c']['d']
print pprint.pformat(x)  # looks a bit messy, but it gives you the idea

"""
Sample output (the addresses [in hexadecimal] will likely be different):
1
defaultdict(<function <lambda> at 0x7f733afa6230>, {'a': defaultdict(<function <lambda> at 0x7f733afa6230>, {'b': defaultdict(<function <lambda> at 0x7f733afa6230>, {'c': defaultdict(<function <lambda> at 0x7f733afa6230>, {'d': 1})})})})
"""