How to use a dictionary as key for a dictionary in Python

Question

All -

I have a very basic question today ... but it has been preventing me from moving on productively in my programming, so I am posting it here.

I want to create a dictionary that takes a dictionary as a key. Presumably I can only pass a reference to the dictionary as key ... only I don't know how to do that in Python. Here is a toy reduction of what I am trying to do:

def test( dict ):
    a={}
    b={1:1}
    a[ dict ] = b
    return a

a = {0:0}
print test( a ) 

I would like b to be a dictionary of the form { {0:0} : {1:1} }.

Any help with this is much appreciated.

Kind regards -

Pat

Answer 1

Keys for dictionaries must be hashable items; unfortunately, dictionaries themselves are not hashable (they are mutable, which disqualifies them from being hashable).

Use their item list, converted to a sorted tuple, instead:

a[tuple(sorted(dct.items()))] = b

Mutable objects are not hashable because they can be changed in-place, making later key-lookups fail. What would the expected outcome be if you added or removed items from the dictionary you used as the key, for example?

Answer 2

Dictionaries can only use hashable objects as keys, which means they must be immutable . The default dictionary isn't either, so that won't work. You can try a frozendict though.

Answer 3

Python wiki :

To be used as a dictionary key, an object must support the hash function (eg through __hash__), equality comparison (eg through __eq__ or __cmp__), and must satisfy the correctness condition above.

So trying this out:

a = {0: 0}
hash(a)

yields the error:

Traceback (most recent call last):
  File "<pyshell#1>", line 1, in <module>
    hash(a)
TypeError: unhashable type: 'dict'

basically just confirming what mgilson already said. You are out of luck as far as using a dictionary (or a list, etc) as a dictionary key.

Answer 4

a dict can't be used as a key as it is unhashable.

hash(a)

so i would rethink your problem or if you really need to hash a dict you could represent is aa string , or tuples and hash by that:

hash(tuple(dict.items()))
hash(''.join(["%s%s" %(k, v) for k,v in dict.items()]))

Answer 5

Thanks to all who replied. I know this was a fairly basic question and all the infos on hashability and mutability is much appreciated.

I will indeed pursue the approach of mapping an input vector (a dictionary itself) to the sorted tuple of its items. This characterises a given dictionary unambiguously and is a hashable data structure such that it can be used as a dictionary key.

def get_key( dict1, dict2 ):

    list = dict1.items() + dict2.items()
    list.sort()
    return tuple( list )

a,b = {'feature1':0}, {'feature1':1}
print get_key( a, b )