Python, TypeError: unhashable type: 'list'

Question

I'm receiving the following error in my program. The traceback:

Traceback (most recent call last):
File "C:\Python33\Archive\PythonGrafos\Alpha.py", line 126, in <module>
menugrafos()
File "C:\Python33\Archive\PythonGrafos\Alpha.py", line 97, in menugrafos
zetta = Beta.caminhografo(grafo,va,vb)
File "C:\Python33\Archive\PythonGrafos\Beta.py", line 129, in caminhografo
if ([vo, a]) in vat == ([vo,vq]) in vat:
TypeError: unhashable type: 'list'

The program is meant to make an adjacency list which works fine, and then proceed to search if there is a path between vertex va and vb. I used a dictionary of lists in collection/defaultdict to adequately append adjacent vertex.

The problem is in the if clauses after the list are created at the end of the program. I can't find a way to properly use the if-clauses with the dict to see if there is a valid path between vertex. Also, grafo is a graph class.

Here is the code:

class graph:
    v = 0
    a = 0
    node = []

class vertex:
    ta = []
    adj = {}
    
def caminhografo(grafo, va, vb):
    vat = defaultdict(list)
    i = 0
    a = 0
    z = 0
    vo = int(va)
    vq = int(vb)
    vz = int(va)
    vw = int(vb)
    x = len(grafo.node)
    if vz < vw:
        for vz in range (vw+1):
            a = 0
            x = len(grafo.node)
            for a in range (x):
                if [int(vz),int(a)] in grafo.node:
                    vat[vz].append(a)                   
    if vz > vw:
        while vz > vw:
            a = 0
            x = len(grafo.node)
            for a in range (x):
                if[int(va),int(a)] in grafo.node:
                    vat[vz].append(a)
            vz = vz - 1
    a = 0
    x = len(grafo.node)
    print(vat)
    for a in range (x):
       if ([vo, a]) in vat == ([vo,vq]) in vat:
           print("""
    ==============================================
               Existe Caminho
    ==============================================
    """)
           break
       elif ([vo,a]) in vat:
           vo = a
       else:           
           print("""
    ==============================================
             Não Existe Caminho
    ==============================================
        """)
           break

Answer 1

The problem is that you can't use a list as the key in a dict, since dict keys need to be immutable. Use a tuple instead.

This is a list:

[x, y]

This is a tuple:

(x, y)

Note that in most cases, the ( and ) are optional, since , is what actually defines a tuple (as long as it's not surrounded by [] or {}, or used as a function argument).

You might find the section on tuples in the Python tutorial useful:

Though tuples may seem similar to lists, they are often used in different situations and for different purposes. Tuples are immutable, and usually contain an heterogeneous sequence of elements that are accessed via unpacking (see later in this section) or indexing (or even by attribute in the case of namedtuples). Lists are mutable, and their elements are usually homogeneous and are accessed by iterating over the list.

And in the section on dictionaries:

Unlike sequences, which are indexed by a range of numbers, dictionaries are indexed by keys, which can be any immutable type; strings and numbers can always be keys. Tuples can be used as keys if they contain only strings, numbers, or tuples; if a tuple contains any mutable object either directly or indirectly, it cannot be used as a key. You can’t use lists as keys, since lists can be modified in place using index assignments, slice assignments, or methods like append() and extend().

---

In case you're wondering what the error message means, it's complaining because there's no built-in hash function for lists (by design), and dictionaries are implemented as hash tables.

Answer 2

If you arrived at this post because you got the error in the title, apart from OP's problem (where a list was being used a key to a dict), there are a couple more cases where this may occur.

1. A list is being passed into a set

Just like why lists cannot be dictionary keys, lists cannot be a set element. If a tuple is to be added to it, is shouldn't contain a list either.

s = {(1, 2), [3, 4]}    # <---- TypeError: unhashable type: 'list'
s = {(1, 2), (3, 4)}    # <---- OK

s.add((5, [6]))         # <---- TypeError because the element to be added contains a list
s.add((5, 6))           # <---- OK because (5, 6) is a tuple

2. Pandas groupby on lists

Another common way this error occurs is if a pandas dataframe column stores a list and is used as a grouper in a groupby operation. A solution is similar as above, convert the lists into tuples and groupby using the column of tuples.

import pandas as pd
df = pd.DataFrame({'group': [[1, 2], [3, 4], [5, 6]], 'value': [0, 1, 2]})

# group  value
# [1, 2]     0
# [3, 4]     1
# [5, 6]     2

df.groupby('group')['value'].mean()                  # <---- TypeError
df.groupby(df['group'].agg(tuple))['value'].mean()   # <---- OK
#          ^^^^^^^^^^^^^^^^^^^^^^  <--- convert each list into a tuple

3. Pandas index/column labels contain a list

Pandas column label cannot be a list (because it is analogous to a dictionary key), so if you attempt to rename() it by a list, it will show this error. A solution is to convert the list into a tuple (or even into a MultiIndex).

df = pd.DataFrame({'group': range(3)})
df.rename(columns={'group': ['col', 'one']})               # TypeError
df.rename(columns={'group': ('col', 'one')})               # OK
df.columns = pd.MultiIndex.from_tuples([('col', 'one')])   # OK

Pandas index can contain a list as a value but if you try to index that row, it will throw this error. A solution is to convert the list into a tuple or simply "clean" the data (probably the index shouldn't contain a list/tuple to begin with) such as converting it into a MultiIndex.

df = pd.DataFrame({'group': range(3)}, index=[['a'], 'b', 'c'])
df.loc['b']           # TypeError

4. collections.Counter is called on an object containing a list

Because Counter creates a dict-like object, each value should be immutable for the very same reason, so if an object contains a list, this error will be shown. A solution is probably to convert the list into a tuple

from collections import Counter
lst = ['a', 'b', ['c']]
Counter(lst)                  # TypeError

Counter(['a', 'b', ('c',)])   # OK