PEP 572: Update based on implementation experimentation

pep-0572.rst

@@ -35,9 +35,9 @@ Syntax and semantics
 ====================
 
 In any context where arbitrary Python expressions can be used, a **named
-expression** can appear. This can be parenthesized for clarity, and is of
+expression** can appear. This is of the form ``target := expr`` where
-the form ``(target := expr)`` where ``expr`` is any valid Python expression,
+``expr`` is any valid Python expression, and ``target`` is any valid
-and ``target`` is any valid assignment target.
+assignment target. (NOTE: See 'Open questions' below for precedence.)
 
 The value of such a named expression is the same as the incorporated
 expression, with the additional side-effect that the target is assigned
@@ -58,6 +58,9 @@ that value::
 Differences from regular assignment statements
 ----------------------------------------------
 
+Most importantly, since ``:=`` is an expression, it can be used in contexts
+where statements are illegal, including lambda functions and comprehensions.
+
 An assignment statement can assign to multiple targets::
 
     x = y = z = 0
@@ -232,7 +235,7 @@ Capturing condition values
 Assignment expressions can be used to good effect in the header of
 an ``if`` or ``while`` statement::
 
-    # Proposed alternative to the above
+    # Proposed syntax
     while (command := input("> ")) != "quit":
         print("You entered:", command)
 
@@ -523,6 +526,26 @@ Translated into longhand, this would become::
 ie utilizing the same early-binding technique that is used at class scope.
 
 
+Operator precedence
+-------------------
+
+There are two logical precedences for the ``:=`` operator. Either it should
+bind as loosely as possible, as does statement-assignment; or it should bind
+more tightly than comparison operators. Placing its precedence between the
+comparison and arithmetic operators (to be precise: just lower than bitwise
+OR) allows most uses inside ``while`` and ``if`` conditions to be spelled
+without parentheses, as it is most likely that you wish to capture the value
+of something, then perform a comparison on it::
+
+    pos = -1
+    while pos := buffer.find(search_term, pos + 1) >= 0:
+        ...
+
+Once find() returns -1, the loop terminates. If ``:=`` binds as loosely as
+``=`` does, this would capture the result of the comparison (generally either
+``True`` or ``False``), which is less useful.
+
+
 Frequently Raised Objections
 ============================