Why a tuple with mutable items can't be used as an element in a set or dictionary?

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since tuple is immutable, it's hash value will never change, even though it's items are allowed to change (mutate). so why sets and dictionaries can't use a tuple with mutable objects.

example code: tuple = (1, "mayank", [1, "mayank"]) set = {1, "mayank", tuple}
error: TypeError: unhashable type: 'list'

extra explanation: tuple contains the 'id' (reference) of objects which will not change throughout the lifetime of respective objects. so the hash of a tuple containing mutable objects should also not change even though the elements are allowed to change (mutate). why then dictionaries and sets can't contain tuple with mutable elements.

What I have tried:

searching the internet.
reading more about the concepts.
testing on the command line.

Posted 25-Nov-21 0:34am

mayank mahajan 2021

Updated 25-Nov-21 0:48am

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Richard MacCutchan · Answer 1 · 2021-11-25T00:48:00

The issue is that the set type does not allow a list as an element, due to its rule of no duplicates. As demonstrated by:

Python

foo = {1, 'foo', [1,2,3]}


Traceback (most recent call last):
  File "C:\Users\rjmac\Documents\VSCode\Python\atest.py", line 21, in <module>
    result = pytest()
  File "C:\Users\rjmac\Documents\VSCode\Python\atest.py", line 12, in pytest
    foo = {1, 'foo', [1,2,3]}
TypeError: unhashable type: 'list'