Python dict like surjective multiple key → value container
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22-10-2019 - |
Question
I'm currently in the need for a Python container class with similar functionality like the builtin dict
type. Basically what I need is a dictionary, where an arbitrary number of keys beside a primary key, which map to the very same value. However when iterating over it, it should iterate only over the (primary_key, value)
pairs and only the primary key if the list of keys is requested.
If this has already been implemented I'd rather not reinvent the wheel. So is there already a module providing such a container? If not, I'm going to implement it myself.
Solution
Here is a quick implementation:
class MultipleKeyDict(dict):
__slots__ = ["_primary_keys"]
def __init__(self, arg=None, **kwargs):
self._primary_keys = {}
self.update(arg, **kwargs)
def __setitem__(self, key, value):
super(MultipleKeyDict, self).__setitem__(key, value)
self._primary_keys.setdefault(value, key)
def __delitem__(self, key):
value = self[key]
super(MultipleKeyDict, self).__delitem__(key)
if self._primary_keys[value] == key:
del self._primary_keys[value]
for k, v in super(MultipleKeyDict, self).iteritems():
if v == value:
self._primary_keys[value] = k
break
def __iter__(self):
return self.iterkeys()
def update(self, arg=None, **kwargs):
if arg is not None:
if isinstance(arg, collections.Mapping):
for k in arg:
self[k] = arg[k]
else:
for k, v in arg:
self[k] = v
for k in kwargs:
self[k] = kwargs[k]
def clear(self):
super(MultipleKeyDict, self).clear()
self._primary_keys.clear()
def iteritems(self):
for v, k in self._primary_keys.iteritems():
yield k, v
def items(self):
return list(self.iteritems())
def itervalues(self):
return self._primary_keys.iterkeys()
def values(self):
return self._primary_keys.keys()
def iterkeys(self):
return self._primary_keys.itervalues()
def keys(self):
return self._primary_keys.values()
The only messy bit is that it has to search the whole dict in case a primary key gets deleted.
I omitted copy()
, pop()
, popitem()
and setdefault()
. If you need them, you'll have to implement them yourself.
OTHER TIPS
The simplest and easiest solution would be to use two dictionaries, one of which maps secondary keys to a primary key. If for some reason you need a reverse mapping, that could be included in the primary dictionary.
sec = {'one': 'blue', 'two': 'red', 'three': 'blue', # seconary keys
'blue': 'blue', 'red': 'red'} # include identity mapping for primaries
dict = {'blue': ('doll', '$9.43', ('one', 'three')),
'red': ('truck', '$14.99', ('two',)) }
record = dict[sec['two']]
print('Toy=', record[0], 'Price=', record[1])
There is now a multiple key dictionary python package.
https://pypi.python.org/pypi/multi_key_dict/1.0.2
From the link:
from multi_key_dict import multi_key_dict k = multi_key_dict() k[1000, 'kilo', 'k'] = 'kilo (x1000)' print k[1000] # will print 'kilo (x1000)' print k['k'] # will also print 'kilo (x1000)' # the same way objects can be updated, deleted: # and if an object is updated using one key, the new value will # be accessible using any other key, e.g. for example above: k['kilo'] = 'kilo' print k[1000] # will now print 'kilo' as value was updated