PEP 653: Fix example. Use get(key, sentinel) instead of __getitem__(k…

…ey) for mappings. (#1902)

pep-0653.rst

@@ -53,7 +53,7 @@ By allowing classes to choose which kinds of pattern they match, other classes c
 For example, using ``sympy``, we might want to write::
 
     # a*a == a**2
-    case Mul(args=[a, b]) if a == b:
+    case Mul(args=[Symbol(a), Symbol(b)]) if a == b:
         return Pow(a, 2)
 
 Which requires the sympy class ``Symbol`` to "self" match.
@@ -273,14 +273,16 @@ A pattern not including a double-star pattern::
 
 translates to::
 
+    $sentinel = object()
     $kind = type($value).__match_container__
     if $kind & MATCH_MAPPING == 0:
         FAIL
-    if not $value.keys() >= $KEYWORD_PATTERNS.keys():
-        FAIL
     # $KEYWORD_PATTERNS is a meta-variable mapping names to variables.
     for $KEYWORD in $KEYWORD_PATTERNS:
-        $KEYWORD_PATTERNS[$KEYWORD] = $value[QUOTE($KEYWORD)]
+        $tmp = $value.get(QUOTE($KEYWORD), $sentinel)
+        if $tmp is $sentinel:
+            FAIL
+        $KEYWORD_PATTERNS[$KEYWORD] = $tmp
 
 Example: [4]_
 
@@ -293,10 +295,10 @@ translates to::
     $kind = type($value).__match_container__
     if $kind & MATCH_MAPPING == 0:
         FAIL
-    if not $value.keys() >= $KEYWORD_PATTERNS.keys():
-        FAIL:
     # $KEYWORD_PATTERNS is a meta-variable mapping names to variables.
     $tmp = dict($value)
+    if not $tmp.keys() >= $KEYWORD_PATTERNS.keys():
+        FAIL:
     for $KEYWORD in $KEYWORD_PATTERNS:
         $KEYWORD_PATTERNS[$KEYWORD] = $tmp.pop(QUOTE($KEYWORD))
     $DOUBLE_STARRED_PATTERN = $tmp
@@ -502,15 +504,14 @@ on the naive implementation.
 When performing matching, implementations are allowed
 to treat the following functions and methods as pure:
 
-For any class supporting ``MATCH_SEQUENCE`` or ``MATCH_MAPPING``::
+For any class supporting ``MATCH_SEQUENCE``::
 
 * ``cls.__len__()``
 * ``cls.__getitem__()``
 
 For any class supporting ``MATCH_MAPPING``::
 
-* ``cls.keys()``
-* ``cls.__contains__()``
+* ``cls.get()`` (Two argument form only)
 
 Implementations are allowed to make the following assumptions:
 
@@ -519,6 +520,8 @@ Implementations are allowed to make the following assumptions:
 * Reading any of ``__match_container__``, ``__match_class__`` or ``__match_args__`` is a pure operation, and may be cached.
 * Sequences, that is any class for which ``__match_container__&MATCH_SEQUENCE`` is not zero, are not modified by iteration, subscripting or calls to ``len()``.
   Consequently, those operations can be freely substituted for each other where they would be equivalent when applied to an immutable sequence.
+* Mappings, that is any class for which ``__match_container__&MATCH_MAPPING`` is not zero, will not capture the second argument of the ``get()`` method.
+  So, the ``$sentinel`` value may be freely re-used.
 
 In fact, implementations are encouraged to make these assumptions, as it is likely to result in signficantly better performance.
 
@@ -779,10 +782,14 @@ translates to::
     $kind = type($value).__match_container__
     if $kind & MATCH_MAPPING == 0:
         FAIL
-    if $value.keys() != {"x", "y"}:
+    $tmp = $value.get("x", $sentinel)
+    if $tmp is $sentinel:
         FAIL
-    x = $value["x"]
-    y = $value["y"]
+    x = $tmp
+    $tmp = $value.get("y", $sentinel)
+    if $tmp is $sentinel:
+        FAIL
+    y = $tmp
     if not x > 2:
         FAIL
 
@@ -797,9 +804,9 @@ translates to::
     $kind = type($value).__match_container__
     if $kind & MATCH_MAPPING == 0:
         FAIL
-    if not $value.keys() >= {"x", "y"}:
-        FAIL
     $tmp = dict($value)
+    if not $tmp.keys() >= {"x", "y"}:
+        FAIL
     x = $tmp.pop("x")
     y = $tmp.pop("y")
     z = $tmp