On VBScript - Hackdot

Re:On VBScript by duckerude on Thursday December 20, 2018 @03:19AM (#18723286)

RHS seems right to me because it means the actions on the LHS happen consecutively.

To expand your example a bit:

  class ShoutyContainer:
      def __setitem__(self, index, value):
          print("Assignment")
  
  def lhs(): print("LHS")
  def rhs(): print("RHS")
  a = ShoutyContainer()
  a[lhs()] = rhs()

Yields:

  RHS
  LHS
  Assignment

If LHS were evaluated first, then it would be separated from the final assignment step by the RHS evaluation. That would be odd.

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Re:On VBScript by alangpierce on Thursday December 20, 2018 @10:21AM (#18725602)

I think it depends on how you think about it. As mentioned elsewhere, "=" is a special statement syntax rather than an operator in Python, but if you were to think of it like an operator, it would be evaluated in three phases: (1) evaluate one side, (2) evaluate the other side, (3) perform operation. The __setitem__ method defines how the left side participates in step 3, but IMO there's a separate LHS evaluation step that's independent of the assignment itself.

You might call this code snippet analogous:

    class ShoutyNumber:
        def __init__(self, n):
            self.n = n
            print(f'Created {self.n}')
    
        def __add__(self, other):
            print(f'Added with {self.n}')
            return self.n + other.n
    
    ShoutyNumber(1) + ShoutyNumber(2)

Yields:

    Created 1
    Created 2
    Added with 1

But I understand that in some mental models and some implementations, it's cleaner to have the entire RHS value ready before starting any evaluation of the LHS. I think that avoids the need to have "assignable reference" as a special type of expression result.

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