Hacker News Reader

Comment by tobinfricke | Hacker News Reader

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.

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...

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.

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

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.

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).

Reply on news.ycombinator.com