How could I create a dictionary in Python with a list as a key?

Question

I want to create a dictionary in Python where the keys are lists.

The issue I want to face is that I have a list of multiple integer values (from 1 to 300), and depending different value-ranges, I want to convert those into characters. For example:

Values from 1 to 100 into character 'A'.
Values from 101 to 200 into character 'B'.
Values from 201 to 300 into character 'C'.

I tried this way but it didn't work:

dictionary = {[1, 100]:'A', [101, 200]:'B', [201, 300]:'C'}

But I get the error:

TypeError: unhashable type: 'list'

How could I do it?

Convert the lists to tuples. Tuples are hashable. Lists are not. — khelwood, May 25 '17 at 13:20
@khelwood, why not post that as an answer, unless this is a duplicate? — Krupip, May 25 '17 at 13:24
if the list does not contain repeated values than you can also use frozenset instead of list — Marco, May 25 '17 at 13:24
Dictionary keys have to be immutable, meaning that once they're created, they cannot be changed. Lists are mutable. Strings, tuples, and numbers are immutable and thus can be keys as khelwood points out. — sizzzzlerz, May 25 '17 at 13:25
Maybe [Why can't I use a list as a dict key in python?](https://stackoverflow.com/questions/7257588/why-cant-i-use-a-list-as-a-dict-key-in-python)? — Ender Look, May 25 '17 at 13:25

score 3 · Accepted Answer · answered May 25 '17 at 13:25

3

Convert the lists to tuples. Tuples are hashable. Lists are not.

dictionary = {(1, 100):'A', (101, 200):'B', (201, 300):'C'}

answered May 25 '17 at 13:25

khelwood

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Thanks, it helped me a lot! But how can I check in which group is a value? I mean, if I have a value X, how could I search into the dictionary to check if it must be converted into 'A', 'B', or 'C'? – jartymcfly May 25 '17 at 14:00
@jartymcfly Sounds like you have a new question to ask. – khelwood May 25 '17 at 14:01

score 0 · Answer 2 · answered May 25 '17 at 13:33

You can use a tuple, a JSON representation of the list, or a repr of the list:

>>> d1={repr([1, 100]):'A', repr([101, 200]):'B', repr([201, 300]):'C'}
>>> import json
>>> d2={json.dumps([1, 100]):'A', json.dumps([101, 200]):'B', json.dumps([201, 300]):'C'}
>>> d2
{'[101, 200]': 'B', '[1, 100]': 'A', '[201, 300]': 'C'}
>>> d1
{'[101, 200]': 'B', '[1, 100]': 'A', '[201, 300]': 'C'}

score 0 · Answer 3 · answered May 25 '17 at 13:40

I don't know if I understood you correctly but you try to convert a list of ints in range [1, 300] to a list of characters with the characters A,B,C. Depending on the integer value. I don't see why you need a dict with keys of lists...

You can do it with a simple for loop and a few if statements.

import random

ints = [random.randint(1,300) for i in xrange(10)]

chars = []

for el in ints:
    if el > 0 and el <= 300:
        if el <= 100:
            chars.append("A")
        elif el <= 200:
            chars.append("B")
        else:
            chars.append("C")
    else:
        raise RuntimeError("element {} not in valid range.".format(el))

print(ints)
print(chars)

which returns

[23, 260, 276, 31, 170, 42, 16, 125, 102, 39]
['A', 'C', 'C', 'A', 'B', 'A', 'A', 'B', 'B', 'A']

How could I create a dictionary in Python with a list as a key?

3 Answers3