Python: the type of the key must be immutable in the dictionary, but why can I let an instance of the node class be the key?
Regardless of a dictionary in python or a hashmap in Java, the key can be an instance of the node class.
But when I read the python tutorial it says:
The keys are unique in the dictionary, and the values may not be available. Dictionary values can be of any type, but keys must be immutable data types such as strings, numbers, or tuples.
I feel embarrassed! # Sorry for my bad expression! :(
Sample code:
class RandomListNode:
def __init__(self, x):
self.label = x
self.next = None
self.random = None
dict = {}
node1 = RandomListNode(10)
node2 = RandomListNode(5)
dict[node1] = node2
print dict[node1].label #5
Summary: Hashable (hash value will not be changed), or immutable object can be a key value. Ref: https://docs.python.org/2/glossary.html#term-hashable
source to share
By default, instances of a class are unique and therefore can be used as keys.
The actual limitation is the presence of a method __hash__
in the class. If you add your own method __eq__
, you must also add your own method __hash__
, which is still considered "immutable", but make sure your hash value is not mutated or your entries will not be fetched from set
and dict
s.
source to share