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devel / comp.lang.python / Re: Why no list as dict key?

Re: Why no list as dict key?

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From: ros...@gmail.com (Chris Angelico)
Newsgroups: comp.lang.python
Subject: Re: Why no list as dict key?
Date: Thu, 21 Apr 2022 04:28:41 +1000
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by: Chris Angelico - Wed, 20 Apr 2022 18:28 UTC

On Thu, 21 Apr 2022 at 04:23, Abdur-Rahmaan Janhangeer
<arj.python@gmail.com> wrote:
>
> Greetings list,

Greetings tuple,

> Using Python3.9, i cannot assign a list [1, 2] as key
> to a dictionary. Why is that so? Thanks in advanced!
>

Because a list can be changed, which would change what it's equal to:

>>> spam = [1, 2]
>>> ham = [1, 2, 3]
>>> spam == ham
False
>>> spam.append(3)
>>> spam == ham
True

If you use spam as a dict key, then mutate it in any way, it would
break dict invariants of all kinds (eg you could also have used ham as
a key, and then you'd have duplicate keys).

Instead, use a tuple, which can't be mutated, is always equal to the
same things, and is hashable, which means it can be used as a key:

>>> spam = (1, 2)
>>> ham = (1, 2, 3)
>>> {spam: "spam", ham: "ham"}
{(1, 2): 'spam', (1, 2, 3): 'ham'}

ChrisA

Subject	Author
Re: Why no list as dict key?	Chris Angelico