PEP 572: Tweaks suggested by Glenn Linderman and Tim Peters (#718)

* PEP 572: Tweaks suggested on python-dev by Glenn Linderman.
* Use Tim's rewording of the exceptional cases.
* Clarify that TargetScopeError is a *new* subclass of SyntaxError.

pep-0572.rst

@@ -148,7 +148,7 @@ in order to avoid ambiguities or user confusion:
     (y := f(x))  # Valid, though not recommended
 
   This rule is included to simplify the choice for the user between an
-  assignment statements and an assignment expression -- there is no
+  assignment statement and an assignment expression -- there is no
   syntactic position where both are valid.
 
 - Unparenthesized assignment expressions are prohibited at the top
@@ -235,13 +235,21 @@ comprehension, for example::
     partial_sums = [total := total + v for v in values]
     print("Total:", total)
 
-An exception to this special case applies when the target name is the
+However, an assignment expression target name cannot be the same as a
-same as a loop control variable for a comprehension containing it.
+``for``-target name appearing in any comprehension containing the
-This is invalid.  This exception exists to rule out edge cases of the
+assignment expression.  The latter names are local to the
-above scope rules as illustrated by ``[i := i+1 for i in range(5)]``
+comprehension in which they appear, so it would be contradictory for a
-or ``[[(j := j) for i in range(5)] for j in range(5)]``.  Note that
+contained use of the same name to refer to the scope containing the
-this exception also applies to ``[i := 0 for i, j in stuff]``, as well
+outermost comprehension instead.
-as to cases like ``[i+1 for i in i := stuff]``.
+
+For example, ``[i := i+1 for i in range(5)]`` is invalid: the ``for
+i`` part establishes that ``i`` is local to the comprehension, but the
+``i :=`` part insists that ``i`` is not local to the comprehension.
+The same reason makes these examples invalid too::
+
+    [[(j := j) for i in range(5)] for j in range(5)]
+    [i := 0 for i, j in stuff]
+    [i+1 for i in i := stuff]
 
 A further exception applies when an assignment expression occurs in a
 comprehension whose containing scope is a class scope.  If the rules
@@ -260,13 +268,13 @@ See Appendix B for some examples of how the rules for targets in
 comprehensions translate to equivalent code.
 
 The two invalid cases listed above raise ``TargetScopeError``, a
-subclass of ``SyntaxError`` (with the same signature).
+new subclass of ``SyntaxError`` (with the same signature).
 
 Relative precedence of ``:=``
 -----------------------------
 
 The ``:=`` operator groups more tightly than a comma in all syntactic
-positions where it is legal, but less tightly than all operators,
+positions where it is legal, but less tightly than all other operators,
 including ``or``, ``and`` and ``not``.  As follows from section
 "Exceptional cases" above, it is never allowed at the same level as
 ``=``.  In case a different grouping is desired, parentheses should be