Nested, infinite dictionary in python

Some time ago I have rewritten my own project from PHP to Python. I needed something similar to arrays in PHP - “automatic” generation of the values (and the keys) which don’t exist.

PHP example.

<?php
  $test = array();
  print_r($test);
  $test['a']['b'] = 1;
  print_r($test);
  $test['a']['b'] += 1;
  print_r($test);
  $test['b']['a'] += 1;
  print_r($test);
?>

Test.

eshlox@eshlox ~/Temp/php $ php test.php
Array
(
)
Array
(
    [a] => Array
        (
            [b] => 1
        )

)
Array
(
    [a] => Array
        (
            [b] => 2
        )

)
Array
(
    [a] => Array
        (
            [b] => 2
        )

    [b] => Array
        (
            [a] => 1
        )

)

Solution.

class NestedDict(dict):
    def __getitem__(self, key):
        if key in self:
            return self.get(key)
        return self.setdefault(key, NestedDict())

    def __add__(self, other):
        return other

    def __sub__(self, other):
        return other

>>> test = NestedDict()
>>> test
{}
>>> test['a']['b'] = 1
>>> test
{'a': {'b': 1}}
>>> test['a']['b'] += 1
>>> test
{'a': {'b': 2}}
>>> test['b']['a'] += 1
>>> test
{'a': {'b': 2}, 'b': {'a': 1}}

Thread at Stackoverflow.