Pseudo-dicts as properties

Question

I have a Python class C which should have two pseudo-dicts a and b. The term pseudo-dicts means that the dictionaries don't actually exist and that they are “recomputed” each time a key is accessed.

In pseudocode this would look like this:

class C:
    def a.__getitem__(self, key):
        return 'a'
    def b.__getitem__(self, key):
        return 'b'

>>> c = C()
>>> c.a['foo']
'a'
>>> c.b['bar']
'b'

I could implement a class for a and b, but since both have just a few short methods, I wonder whether there is a more elegant and compact way to do this.

Answer 1

Why not just define your own class?

class PseudoDict(object):
    def __init__(self, c):
        self.c = c

    def __getitem__(self, key):
        return self.c.somethingmagical()

class C(object):
    def __init__(self):
        self.a = PseudoDict(self)
        self.b = PseudoDict(self)

c = C()
print c.a['foo']
print c.b['bar']

I'm not sure where the values for these 'pseudo-dicts' are coming from, so you'll have to update the __getitem__ method.

Answer 2

Like this?

from collections import defaultdict
class C:
    a = defaultdict(lambda:'a')
    b = defaultdict(lambda:'b')

c=C()
print c.a['foo']
print c.b['bar']

Or maybe like this for real calculation functions?

from collections import defaultdict

class C:
    def __init__(self):
        self.a = defaultdict(self.geta)
        self.b = defaultdict(self.getb)
    def geta(self):
        return 'a'
    def getb(self):
        return 'b'

c=C()
print c.a['foo']
print c.b['bar']