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I have the following expression:

>>> a = 3
>>> b = 2
>>> a == (a := b)
False

Now, a == 2 after the operation, as expected. And the result is what I would want, i.e., comparison of a to RHS of assignment before assignment.

Reversing the order of the equality operator reverses the result:

>>> a = 3
>>> b = 2
>>> (a := b) == a
True

There does not appear to be anything directly relevant to this corner case in PEP-572, relative precedence section. The next section, change to evaluation order mentions that the evaluation order is left-to-right. Is that what is going on here (stash the value of a, update it, and compare vs update a, then compare against its new value)?

Where is this behavior defined, and how reliable is it?

Mad Physicist
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    Hmm to me this just seems to be a consequence that ` comp_op ` gets evaluated from left to right. – juanpa.arrivillaga Sep 13 '21 at 18:58
  • @juanpa.arrivillaga. Having put more thought into what "evaluation" means, I'm beginning to see that. I was originally confused because the syntax. I'll keep the question around because it may still be useful. – Mad Physicist Sep 13 '21 at 19:00
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    Not just the `:=` operator; you get the same behavior for `a == f(b)` and `f(b) == a` where `f` is a function changing the value of the (global) variable `a` (or for `a == ++a` and `++a == a` in languages that have `++/--`). – tobias_k Sep 13 '21 at 19:06
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    @tobias_k: Actually, in languages like C/C++, stuff like `a == ++a` and `++a == a` are often not sequenced; the language can choose to evaluate them in either order (I *think* the latest C++ standard did stuff to make stuff like that sequenced, but it's a very recent change); `a == ++a` could evaluate the left or right side first and the whole expression would evaluate to either true or false depending on which the compiler decided to do first. For example see [Is `C == C++` undefined behaviour?](https://stackoverflow.com/q/25965372/364696). Python is stricter than most on left-to-right rules. – ShadowRanger Sep 14 '21 at 02:50
  • @ShadowRanger Thanks for pointing that out, I was lazy and only tried in Java... – tobias_k Sep 14 '21 at 07:20

1 Answers1

7

Neither of those PEP sections have to do with this. You just have a == comparison, and the general Evaluation order applies: "Python evaluates expressions from left to right."

So your (a := b) == a simply first evaluates the left side (a := b), assigning something to a and evaluating to the same value. And then evaluate the right side a, which is of course still the same (just-assigned) value, so you get True.

About those PEP sections:

What that first PEP section says is that := groups less tightly than ==, so it would apply if you didn't have parentheses:

  • a == a := b would mean (a == a) := b (you'd get a syntax error for trying to assign to a comparison).
  • a := b == a would mean a := (b == a), where with your values the b == a evaluates to False and that gets assigned to a and becomes the result of the entire expression. (Note that at statement-level, you'd have to write (a := b == a).)

What that second PEP section does is just to point out something bad that had already existed but which the := made "more visible", so they suggested to finally fix it. The issue was that a dict comprehension like {X: Y for ...} evaluated Y before X, against the general left-to-right rule and against dict displays like {X: Y} which already did evaluate X before Y as expected. Consider this:

>>> a, b = 3, 2
>>> {a: (a := b) for _ in '_'}
{3: 2}

With that old behavior, it would've resulted in {2: 2}. And given that people might write something like that when := became available, it became more of an issue.

no comment
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