Why there is an error

Question

bad_dict = {
[1, 2, 3]: "one two three",
}

Answer 1

because dictionaries only can have immutable keys, int, strings and tuples by example.

in your example, the list [1,2,3] is a invalid key because the list is a mutable data type.

Answer 2

The key of dictionary should be hashable, but list is unhashable so that code gives you error.

Answer 3

This appears (to me) to mix the syntax for dictionaries:

{index:value, index:value, ...}

lists...in this case I'd expect it on the right hand side of equals (=):
... = [value, value, value]

and assignment (left-hand side of equals):

a = value
a,b = value1,value2
[a,b] = [value1, value2]

*** Note...this does NOT fix it:
{
[1,2,3]:["a","b","c"]
}
TypeError: unhashable type: 'list'

Which brings me back to mixing syntax.