python: unhashable type error

Question

Traceback (most recent call last):
  File "<pyshell#80>", line 1, in <module>
    do_work()
  File "C:\pythonwork\readthefile080410.py", line 14, in do_work
    populate_frequency5(e,data)
  File "C:\pythonwork\readthefile080410.py", line 157, in populate_frequency5
    data=medications_minimum3(data,[drug.upper()],1)
  File "C:\pythonwork\readthefile080410.py", line 120, in medications_minimum3
    counter[row[11]]+=1
TypeError: unhashable type: 'list'

I am getting the above error on this line:

data=medications_minimum3(data,[drug.upper()],1)

(I have also tried drug.upper() without brackets)

Here is a preview of this function:

def medications_minimum3(c,drug_input,sample_cutoff): #return sample cut off for # medications/physician
  d=[]
  counter=collections.defaultdict(int)
  for row in c:
    counter[row[11]]+=1
  for row in c:
    if counter[row[11]]>=sample_cutoff:
      d.append(row) 
  write_file(d,'/pythonwork/medications_minimum3.csv')
  return d

Does anyone know what I am doing wrong here?

I know that what must be wrong is the way I am calling this function, because I call this function from a different location and it works fine:

d=medications_minimum3(c,drug_input,50)

Thank you very much for your help!

Answer 1

counter[row[11]]+=1

You don't show what data is, but apparently when you loop through its rows, row[11] is turning out to be a list. Lists are mutable objects which means they cannot be used as dictionary keys. Trying to use row[11] as a key causes the defaultdict to complain that it is a mutable, i.e. unhashable, object.

The easiest fix is to change row[11] from a list to a tuple. Either by doing

counter[tuple(row[11])] += 1

or by fixing it in the caller before data is passed to medications_minimum3. A tuple simply an immutable list, so it behaves exactly like a list does except you cannot change it once it is created.