Python Multiple Assignment Is a Puzzle

Re:Python Multiple Assignment Is a Puzzle by todd8 on Sunday July 27, 2014 @07:46AM (#8092512)

I think it's easier to see what is going on without the nested subscripting:

    >>> i = 0
    >>> a = [0, 0]
    >>> i, a[i] = 1, 10
    >>> a
    [0, 10]

versus

    >>> i = 0
    >>> a = [0, 0]
    >>> a[i], i = 10, 1
    >>> a
    [10, 0]

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Re:Python Multiple Assignment Is a Puzzle by mst on Sunday July 27, 2014 @11:54AM (#8093061)

It's notable that in the same situation perl will do basically the same thing as python ... unless it detects a dependency (i.e. same variable referenced on both sides), in which case it constructs a temporary list, fills that from the RHS, then evaluates the LHS to get a list of lvalues, then assigns (roughly, I've not been in that part of the VM code for a bit).

i.e. from perl's POV, python's behaviour is an optimisation, which if over-applied introduces a bug ... but, y'know, DWIM vs. regularity etc., I'm just surprised that python's behaviour is sloppier than perl's here :)

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Re:Python Multiple Assignment Is a Puzzle by tobinfricke on Sunday July 27, 2014 @08:02AM (#8092537)

Does the language specify that things will work this way, or is it undefined / implementation-dependent behavior?

To me, it smells like the result of a sloppy specification.

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Re:Python Multiple Assignment Is a Puzzle by todd8 on Sunday July 27, 2014 @08:29AM (#8092567)

Yes, this behavior is spelled out in detail in the Python Language Reference [1], in particular section 6.14 Evaluation Order. It explicitly states that the order of evaluation is left to right with the right hand side of assignments being executed before the left hand side. It further gives this example where expressions are evaluated in the order of their suffixes:

    expr3, expr4 = expr1, expr2

Although evaluation order is handled differently by different programming languages, it seems that Python is behaving logically here.

[1] https://docs.python.org/3.4/reference/expressions.html#evalu...

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Re:Python Multiple Assignment Is a Puzzle by Someone on Sunday July 27, 2014 @01:59PM (#8093423)

I think that specification still can be improved. It is clear about order of evaluation, but not on the order of assignment.

Does it do tmp3 = expr1 tmp4 = expr2

   expr3 = tmp3
   expr4 = tmp4

   expr3 = expr1
   expr4 = expr2

? I guess it is the latter, but the text does not make that clear.

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Re:Python Multiple Assignment Is a Puzzle by zwegner on Sunday July 27, 2014 @02:11PM (#8093449)

The first--the latter isn't in order (expr3 is "evaluated" before expr2, which in this case means assignment).

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Re:Python Multiple Assignment Is a Puzzle by Someone on Sunday July 27, 2014 @03:30PM (#8093703)

That's precisely the place where I think the description can be improved. With complex assignments, there are two parts: compute the place to store a value, and store the value there. In my mental model, the latter is better called assignment than evaluation.

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Re:Python Multiple Assignment Is a Puzzle by zwegner on Sunday July 27, 2014 @04:42PM (#8093913)

With Python's evaluation/assignment order, both parts of a complex assignment happen at once--which is the source of the weird behavior in the article (A[0] is assigned to before the subscript of A[A[0] - 1] is evaluated).

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