How to make a dictionary, in which multiple values access one value(Python)

Question

I want to make a Python dictionary. I want values like 0.25, 0.30, 0.35 to be keys in this dictionary. The problems is that I have values like 0.264, 0.313, 0.367. I want this values to access the keys e.g. I want every value from 0.25(inclusive) to 0.30(exclusive) to access the value under the key 0.25. Any ideas how to do this? I think I've done that before somehow, but I have no ideas right now. Thanks in advance.

Answer 1

Create a subclass of dict with adjusted __getitem__, __setitem__, __delitem__, __contains__, get(), pop() and update() methods that round the key:

class RoundingDict(dict):
    def _round(self, key):
        return int(key * 20) / 20.0

    def __contains__(self, key):
        return super(RoundingDict, self).__contains__(self._round(key))

    def __getitem__(self, key):
        return super(RoundingDict, self).__getitem__(self._round(key))

    def __setitem__(self, key, value):
        super(RoundingDict, self).__setitem__(self._round(key), value)

    def __delitem__(self, key):
        super(RoundingDict, self).__delitem__(self._round(key))

    def get(self, key, default=None):
        return super(RoundingDict, self).get(self._round(key), default)

    def pop(self, key, default=None):
        return super(RoundingDict, self).pop(self._round(key), default)

    def update(self, iterable):
        try:
            super(RoundingDict, self).update({
                self._round(k): v for k, v in iterable.iteritems()})
        except AttributeError:
            super(RoundingDict, self).update((
                (self._round(k), v) for k, v in iterable))

This floors any key down to the nearest 0.05 multiple when getting, setting or deleting individual keys, as well as when updating the dictionary with multiple keys:

>>> d = RoundingDict()
>>> d[0.346] = 'Foo'
>>> d[0.34]
'Foo'
>>> d[0.30]
'Foo'
>>> d[0.35]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<string>", line 6, in __getitem__
KeyError: 0.35
>>> d.get(0.34)
'Foo'
>>> d.get(0.35, 'default')
'default'

Answer 2

What you need is to make a custom dictionary class where the __getitem__ method rounds down the value before calling the standard __getitem__ method.