PEP 671: Add section on evaluation order (#2652)

* PEP 671: Add section on evaluation order

This has been a point of some confusion, so I'm just writing in some defaults explicitly.

* PEP 671: Fix backticks

pep-0671.rst

@@ -72,36 +72,41 @@ allows the expression to refer to other arguments.
 
 Multiple late-bound arguments are evaluated from left to right, and can refer
 to previously-defined values. Order is defined by the function, regardless of
-the order in which keyword arguments may be passed. Using names of later
-arguments should not be relied upon, and while this MAY work in some Python
-implementations, it should be considered dubious::
+the order in which keyword arguments may be passed.
 
     def prevref(word="foo", a=>len(word), b=>a//2): # Valid
-    def selfref(spam=>spam): # Highly likely to give an error
-    def spaminate(sausage=>eggs + 1, eggs=>sausage - 1): # Confusing, may fail
-    def frob(n=>len(items), items=[]): # May fail, may succeed
+    def selfref(spam=>spam): # UnboundLocalError
+    def spaminate(sausage=>eggs + 1, eggs=>sausage - 1): # Confusing, don't do this
+    def frob(n=>len(items), items=[]): # See below
 
-Moreover, even if syntactically and semantically legal, this kind of construct
-is highly confusing to other programmers, and should be avoided.
+Evaluation order is left-to-right; however, implementations MAY choose to do so
+in two separate passes, first for all passed arguments and early-bound defaults,
+and then a second pass for late-bound defaults. Otherwise, all arguments will be
+assigned strictly left-to-right.
 
-
-Choice of spelling
-------------------
+Rejected choices of spelling
+----------------------------
 
 While this document specifies a single syntax ``name=>expression``, alternate
-spellings are similarly plausible. Open for consideration are the following::
+spellings are similarly plausible. The following spellings were considered::
 
     def bisect(a, hi=>len(a)):
     def bisect(a, hi:=len(a)):
     def bisect(a, hi?=len(a)):
-
-An alternative reference implementation is under consideration, which would
-use this syntax::
-
     def bisect(a, @hi=len(a)):
 
 Since default arguments behave largely the same whether they're early or late
-bound, the syntax is deliberately similar to the existing early-bind syntax.
+bound, the chosen syntax ``hi=>len(a)`` is deliberately similar to the existing
+early-bind syntax.
+
+One reason for rejection of the ``:=`` syntax is its behaviour with annotations.
+Annotations go before the default, so in all syntax options, it must be
+unambiguous (both to the human and the parser) whether this is an annotation,
+a default, or both. The alternate syntax ``target:=expr`` runs the risk of
+being misinterpreted as ``target:int=expr`` with the annotation omitted in
+error, and may thus mask bugs. The chosen syntax ``target=>expr`` does not
+have this problem.
+
 
 How to Teach This
 =================
@@ -137,17 +142,6 @@ be defined by the function. Additionally, dedicated sentinel objects can be
 used as dictionary lookup keys, where :pep:`671` does not apply.
 
 
-Interaction with annotations
-============================
-
-Annotations go before the default, so in all syntax options, it must be
-unambiguous (both to the human and the parser) whether this is an annotation,
-a default, or both. The alternate syntax ``target:=expr`` runs the risk of
-being misinterpreted as ``target:int=expr`` with the annotation omitted in
-error, and may thus mask bugs. The preferred syntax ``target=>expr`` does not
-have this problem.
-
-
 Implementation details
 ======================
 
@@ -210,14 +204,3 @@ Copyright
 
 This document is placed in the public domain or under the
 CC0-1.0-Universal license, whichever is more permissive.
-
-
-
-..
-   Local Variables:
-   mode: indented-text
-   indent-tabs-mode: nil
-   sentence-end-double-space: t
-   fill-column: 70
-   coding: utf-8
-   End: