In a Python dict of dicts, how do you emulate Perl's auto-vivification behavior?

Question

Both Google and the online docs are not delivering much insight on my query, so I thought I would ask the community here.

In Perl, you can easily setup a hash-of-a-hash-of-a-hash and test the final key like so:

my $hash = {};
$hash{"element1"}{"sub1"}{"subsub1"} = "value1";
if (exists($hash{"element1"}{"sub1"}{"subsub1"})) {
   print "found value\n";
}

What's the 'best-practice' equivalent in Python?

Answer 1

The closest equivalent is probably something like the following:

import collections

def hasher():
  return collections.defaultdict(hasher)

hash = hasher()
hash['element1']['sub1']['subsub1'] = 'value1'
if 'subsub1' in hash['element1']['sub1']:
  print 'found value'

Answer 2

adict={'element1':{'sub1':{'subsub1':'value1'}}}

If you want to test that all the keys exist, you could use try...except KeyError:

try:
    adict['element1']['sub1']['subsub1']
except KeyError:
    print('no dice')
else:
    print('found value')
# found value

try:
    adict['phony']['baloney']
except KeyError:
    print('no dice')
else:
    print('found value')
# no dice

If you only want to test the existence of the final key:

if 'subsub1' in adict['element1']['sub1']:
    print('found value')

Answer 3

As to whether this is a best practice in Python is up to debate:

hash = {}
hash['element1', 'sub1', 'subsub1'] = 'value'
if ('element1', 'sub1', 'subsub1') in hash:
    print "found value"

But, it certainly works and is very elegant, if it works for you.

The major drawback is that you don't have intermediate access. You can't do:

if ('element1', 'sub1') in hash:
   print "found value"

Answer 4

I don't know if I'll get any agreement, but this is how I typically declare dictionaries of dictionaries:

someObj = {
  'element1': {
    'sub1': {
      'subsub1': 'value1'
    }
  }
}

As for checking for the presence of an element, I agree with this approach:

try:
  someObj['element1']['sub1']['subsub1']
except KeyError:
  print('no value found')
else:
  print('found value')