CODEWITHSUNDEEP
pythondictionarykey
fsociety
fsociety

Reputation: 1027

python list as dict key

I have the following: I see the following object has __hash__ attribute

a=[1,2,3]

>>> dir(a)
['__add__', '__class__', '__contains__', '__delattr__', '__delitem__', '__delsli
ce__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__getit
em__', '__getslice__', '__gt__', '__hash__', '__iadd__', '__imul__', '__init__',
 '__iter__', '__le__', '__len__', '__lt__', '__mul__', '__ne__', '__new__', '__r
educe__', '__reduce_ex__', '__repr__', '__reversed__', '__rmul__', '__setattr__'
, '__setitem__', '__setslice__', '__sizeof__', '__str__', '__subclasshook__', 'a
ppend', 'count', 'extend', 'index', 'insert', 'pop', 'remove', 'reverse', 'sort'
]

>>> dict1={a:'55'}
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'list'

Q:even though it has __hash__ attribute, still it cannot be used as key.Why?

Upvotes: 0

Views: 209

Answers (1)

BrenBarn
BrenBarn

Reputation: 251363

Did you try looking at the value of the attribute?

>>> print(a.__hash__)
None

Just because something has a __hash__ attribute doesn't mean it's hashable; the __hash__ attribute has to be a callable that actually hashes it (instead of, e.g., raising an exception).

Upvotes: 3

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