PEP 758: Force parentheses when using 'as' (#4073)

peps/pep-0758.rst

@@ -12,12 +12,12 @@ Abstract
 ========
 
 This PEP [1]_ proposes to allow unparenthesized ``except`` and ``except*``
-blocks in Python's exception handling syntax. Currently, when catching multiple
-exceptions, parentheses are required around the exception types. This was a
-Python 2 remnant. This PEP suggests allowing the omission of these parentheses,
-simplifying the syntax, making it more consistent with other parts of the syntax
-that make parentheses optional, and improving readability in certain cases.
-
+blocks in Python's exception handling syntax only when not using the ``as``
+clause. Currently, when catching multiple exceptions, parentheses are required
+around the exception types. This was a Python 2 remnant. This PEP suggests allowing
+the omission of these parentheses, simplifying the syntax, making it more consistent
+with other parts of the syntax that make parentheses optional, and improving readability
+in certain cases.
 
 Motivation
 ==========
@@ -57,14 +57,18 @@ The same change would apply to ``except*`` expressions. For example:
     except* ExceptionA, ExceptionB, ExceptionC:
         ...
 
-Both forms will also allow the use of the ``as`` clause to capture the exception
-instance as before:
+When using the ``as`` clause to capture the exception instance paretheses must
+be used as before. Some users have expressed that they would find it confusing not
+to require parentheses as it would be unclear what exactly is being assigned to
+the target since in other parts of the language multiple ``as`` clauses can be used
+in similar situations (like in imports and context managers). This means that if
+an ``as`` clause its being added to the previous example it must be done as:
 
 .. code-block:: python
 
     try:
         ...
-    except ExceptionA, ExceptionB, ExceptionC as e:
+    except (ExceptionA, ExceptionB, ExceptionC) as e:
         ...
 
 
@@ -93,14 +97,16 @@ list of exception types. The grammar will be updated as follows:
 .. code-block:: peg
 
     except_block:
-        | 'except' expressions ['as' NAME] ':' block
+        | 'except' expressions ':' block
+        | 'except' expression 'as' NAME ':' block
         | 'except' ':' block
 
     except_star_block
-        | 'except' '*' expressions ['as' NAME] ':' block 
+        | 'except' '*' expressions ':' block 
+        | 'except' '*' expression 'as' NAME ':' block 
 
 This allows both the current parenthesized syntax and the new unparenthesized
-syntax:
+syntax while requiring parentheses when the ``as`` keyword is used:
 
 .. code-block:: python
 
@@ -110,6 +116,8 @@ syntax:
         ...
     except ExceptionC, ExceptionD:    # New syntax
         ...
+    except (ExceptionE, ExceptionF) as e: # Parentheses still required
+        ...
 
 The semantics of exception handling remain unchanged. The interpreter will catch
 any of the listed exceptions, regardless of whether they are parenthesized or
@@ -191,6 +199,16 @@ Rejected Ideas
    This was rejected due to the potential for confusion and to maintain a clear
    distinction between the two styles.
 
+Deferred Ideas
+==============
+
+1. Allowing unparenthesized expressions when the ``as`` keyword is used. The reason
+   we have decided to defer this particular form given that there isn't clear
+   consensus either way and there are reasonable arguments for both positions, the safest
+   approach is keeping the parentheses requirement since it can be removed later if users
+   find the disconnect too acute, while taking it out and users deciding it was a bad idea
+   doesn’t make it easy to put back.
+
 Footnotes
 =========