From dad6766f2b3665f75e78287a983542fe444edf4c Mon Sep 17 00:00:00 2001
From: Guido van Rossum <guido@python.org>
Date: Mon, 19 Oct 2020 15:30:33 -0700
Subject: [PATCH] PEP 634: change walrus pattern to AS pattern (#1661)

Co-authored-by: Brandt Bucher <brandtbucher@gmail.com>
---
 pep-0634.rst | 52 +++++++++++++++++-----------------------------------
 1 file changed, 17 insertions(+), 35 deletions(-)

diff --git a/pep-0634.rst b/pep-0634.rst
index 0d588713f..0eab4daeb 100644
--- a/pep-0634.rst
+++ b/pep-0634.rst
@@ -158,7 +158,7 @@ Irrefutable case blocks
 
 A pattern is considered irrefutable if we can prove from its syntax
 alone that it will always succeed.  In particular, capture patterns
-and wildcard patterns are irrefutable, as are walrus patterns whose
+and wildcard patterns are irrefutable, and so are AS patterns whose
 right-hand side is irrefutable, OR patterns containing at least
 one irrefutable pattern, and parenthesized irrefutable patterns.
 
@@ -177,8 +177,8 @@ Patterns
 The top-level syntax for patterns is as follows::
 
   patterns: open_sequence_pattern | pattern
-  pattern: walrus_pattern | or_pattern
-  walrus_pattern: capture_pattern ':=' or_pattern
+  pattern: as_pattern | or_pattern
+  as_pattern: or_pattern 'as' capture_pattern
   or_pattern: '|'.closed_pattern+
   closed_pattern:
       | literal_pattern
@@ -191,34 +191,19 @@ The top-level syntax for patterns is as follows::
       | class_pattern
 
 
-Walrus Patterns
-^^^^^^^^^^^^^^^
+AS Patterns
+^^^^^^^^^^^
 
 Syntax::
 
-  walrus_pattern: capture_pattern ':=' or_pattern
+  as_pattern: or_pattern 'as' capture_pattern
 
-(Note: the name on the left may not be ``_``.)
+(Note: the name on the right may not be ``_``.)
 
-A walrus pattern matches the OR pattern on the right of the ``:=``
-operator against the subject.  If this fails, the walrus pattern fails.
-Otherwise, the walrus pattern binds the subject to the name on the left
-of the ``:=`` operator and succeeds.
-
-Open Issue
-~~~~~~~~~~
-
-An alternate syntax for this construct has been put forward, whose
-syntax would be::
-
-  walrus_pattern: or_pattern 'as' capture_pattern
-
-The semantics would be the same: it matches the OR pattern against the
-subject and on success binds the subject to the name in the capture
-pattern.
-
-We leave it to the Steering Council to decide which form to prefer (we
-would rename "walrus pattern" to "AS pattern").
+An AS pattern matches the OR pattern on the left of the ``as``
+keyword against the subject.  If this fails, the AS pattern fails.
+Otherwise, the AS pattern binds the subject to the name on the right
+of the ``as`` keyword and succeeds.
 
 
 OR Patterns
@@ -415,7 +400,7 @@ Syntax::
   mapping_pattern: '{' [items_pattern] '}'
   items_pattern: ','.key_value_pattern+ ','?
   key_value_pattern:
-      | (literal_pattern | value_pattern) ':' or_pattern
+      | (literal_pattern | value_pattern) ':' pattern
       | double_star_pattern
   double_star_pattern: '**' capture_pattern
 
@@ -463,10 +448,7 @@ Syntax::
       | keyword_patterns
   positional_patterns: ','.pattern+
   keyword_patterns: ','.keyword_pattern+
-  keyword_pattern: NAME '=' or_pattern
-
-(Note that positional patterns may be unparenthesized walrus patterns,
-but keyword patterns may not.)
+  keyword_pattern: NAME '=' pattern
 
 A class pattern may not repeat the same keyword multiple times.
 
@@ -596,8 +578,8 @@ Other notation used beyond standard EBNF:
   guard: 'if' named_expression
 
   patterns: open_sequence_pattern | pattern
-  pattern: walrus_pattern | or_pattern
-  walrus_pattern: capture_pattern ':=' or_pattern
+  pattern: as_pattern | or_pattern
+  as_pattern: or_pattern 'as' capture_pattern
   or_pattern: '|'.closed_pattern+
   closed_pattern:
       | literal_pattern
@@ -640,7 +622,7 @@ Other notation used beyond standard EBNF:
   mapping_pattern: '{' [items_pattern] '}'
   items_pattern: ','.key_value_pattern+ ','?
   key_value_pattern:
-      | (literal_pattern | value_pattern) ':' or_pattern
+      | (literal_pattern | value_pattern) ':' pattern
       | double_star_pattern
   double_star_pattern: '**' capture_pattern
 
@@ -651,7 +633,7 @@ Other notation used beyond standard EBNF:
       | keyword_patterns
   positional_patterns: ','.pattern+
   keyword_patterns: ','.keyword_pattern+
-  keyword_pattern: NAME '=' or_pattern
+  keyword_pattern: NAME '=' pattern
 
 
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