如果字典的键在frozensets中，则检索值

Question

I'm using frozensets to keep the keys of my dictionary to take an advantage of union, difference and intersection operations. But when I'm trying to retrieve values by keys from the dictionary through dict.get() it yields a None value.

newDict = {'a': 1, 'b': 2, 'c': 3, 'd': True}
stKeys = set(newDict)
stA = frozenset('a')
stB = frozenset('b')
stC = frozenset('c')
stD = frozenset('d')

print(stKeys)
print(newDict.get(stA & stKeys))
print(newDict.get(stB & stKeys))
print(newDict.get(stC & stKeys))
print(newDict.get(stD & stKeys))

Produce:

>>>None
>>>None
>>>None
>>>None

And even:

print(newDict.get(stA))
print(newDict.get(stB))
print(newDict.get(stC))
print(newDict.get(stD))

Produce:

>>>None
>>>None
>>>None
>>>None

How to retrieve values from the dictionary if your keys are in frozensets?

Thanks to Martijn Pieters ! The answer is DVO (Dictionary view objects) and the generator expression if you want to add the result to a list()

Answer 1

You can use dictionary view objects if you want to look for set intersections :

for key in newDict.viewkeys() & stA:
    # all keys that are in the intersection of stA and the dictionary

In Python 3, returning dictionary views is the default; you can use newDict.keys() here:

for key in newDict.keys() & stA:
    # all keys that are in the intersection of stA and the dictionary

Demo on Python 3:

>>> newDict = {'a': 1, 'b': 2, 'c': 3, 'd': True}
>>> stA = frozenset('a')
>>> stB = frozenset('b')
>>> stC = frozenset('c')
>>> stD = frozenset('d')
>>> newDict.keys() & stA
{'a'}
>>> for key in newDict.keys() & stA:
...     print(newDict[key])
... 
1

Answer 2

To create frozen set keys you need to actually create frozen sets and use them as keys:

newDict = {
    frozenset('a'): 1,
    frozenset('b'): 2,
    frozenset('c'): 3,
    frozenset('d'): True
}

Test:

>>> {frozenset('a'):1}[frozenset('a')]
1

Answer 3

You actually can do what you are trying to do, at least in 3.6.1 and I suspect 2.7.x as well:

newDict = {frozenset('a'): 1, frozenset('b'): 2, frozenset('c'): 3, 'd': True}
stKeys = set(newDict)
stA = frozenset('a')
print(stA)
stB = frozenset('b')
stC = frozenset('c')
stD = 'd'

print(newDict[stA])
print(newDict[stB])
print(newDict[stC])
print(newDict[stD])

Output:

frozenset({'a'})
1
2
3
True

The problem seems to be that the key was being assigned as a string object and not a frozenset, yet the search was set to look for a frozenset.