TypeError: list objects are unhashable

Question

totalCost = problem.getCostOfActions(self.actions)

Answer 1

Looks like you are trying to use a list as key in a dictionary or something like that. Lists are not hashable, and so they cannot be used as dictionary keys or in sets.

On another note, python gives you a stacktrace when such an error happens, and that includes file-names and line-numbers. You should be able to track down the offending code with that.

Edit About stacktraces:

cat > script.py
foo = [1,2,3]
bar = {}
bar[foo] = "Boom"
print "Never happens"

python script.py
Traceback (most recent call last):
  File "script.py", line 3, in <module> // this is the file and the line-number
   bar[foo] = "Boom"
TypeError: unhashable type: 'list'

Answer 2

You've probably attempted to use mutable objects such as lists, as the key for a dictionary, or as a member of a set. Mutable items cannot be tracked for such uses efficiently and predictably so they do not provide the hash special attribute.

Answer 3

The error is produced when an unhashable type is added to sets.

>>> s=set((1,2))
>>> a.add([3,4])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'list'

I think this may be also your case. Use tuple instead of list:

>> a.add((3,4))
>>> 

Answer 4

Maybe the line should be like this instead

totalCost = sum(map(problem.getCostOfActions,self.actions))

or if you prefer a generator expression

totalCost = sum(problem.getCostOfActions(action) for action in self.actions)

Since I can't see your code, I assumed that problem.getCostOfActions() returns the cost of a single action, as that might cause the error you are getting if self.actions is a list

Alternatively you could fix the function problem.getCostOfActions() so that it returns the total cost of a list of actions as the name suggests.

Consider adding the body of this function to your question if you want help fixing it