Add __hash__ method for permissions.OperandHolder class

[vanya909]

OperandHolder is not hashable, so need to add __hash__ method

Description

OperandHolder is not hashable after #8710, need to add __hash__ method to it to provide convenient syntax to filter unique permissions using set or dict.fromkeys

Ways to represent

1. from rest_framework import permissions
2. set([permissions.IsAuthenticated & permissions.IsAdminUser])

Fixes

• Remove __eq__ method from OperandHolder
  or
• Provide __hash__ method to OperandHolder

Second way is provided in this PR

[peterthomassen]

lgtm, sorry being slow.

[auvipy]

thanks anyway. merged as it was small, after cross checking

[adamchainz]

We should really have tests for this. @vanya909 can you make a follow-up PR with a test that hashes an instance?

[tomchristie]

Can you explain why using set(...) on these classes is useful?

[vanya909]

@tomchristie

Can you explain why using set(...) on these classes is useful?

It's useful in cases when we want to filter out unique permissions in order to avoid calculating of some complex permissions two or more times

Of course it can be done by just iterating over permissions list and appending in a result list those permissions which haven't been added yet, but using set() is more simple

In case when we want to keep original order of permissions dict.fromkeys() can be used, I would say dict.fromkeys() is preferable, but it also requires all permissions to be hashable

[peterthomassen]

Also, this fixes the regression introduced by #8710, so don't think it requires justification beyond that