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?
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.