PEP 637 allow for unpacking of starargs (#1622)

pep-0637.rst

@@ -184,7 +184,7 @@ have anonymous argument semantic, exactly like the indexing operation. However,
             print(index)
 
 The index operation basically forwards the content of the square brackets "as is"
-in the ``index`` argument:: 
+in the ``index`` argument::
 
     >>> x=X()
     >>> x[0]
@@ -270,7 +270,7 @@ The successful implementation of this PEP will result in the following behavior:
    
    This remains the case even if the index is followed by keywords; see point 5 below.
 
-3. Comma-seperated arguments are still parsed as a tuple and passed as
+3. Comma-separated arguments are still parsed as a tuple and passed as
    a single positional argument::
 
        obj[spam, eggs]
@@ -347,17 +347,26 @@ The successful implementation of this PEP will result in the following behavior:
    - but if no ``**kwargs`` parameter is defined, it is an error.
 
 
-7. Sequence unpacking remains a syntax error inside subscripts::
+7. Sequence unpacking is allowed inside subscripts::
 
        obj[*items]
 
-   Reason: unpacking items would result it being immediately repacked into
-   a tuple. Anyone using sequence unpacking in the subscript is probably
-   confused as to what is happening, and it is best if they receive an
-   immediate syntax error with an informative error message.
+   This allows notations such as ``[:, *args, :]``, which could be treated 
+   as ``[(slice(None), *args, slice(None))]``.
 
-   This restriction has however been considered arbitrary by some, and it might
-   be lifted at a later stage for symmetry with kwargs unpacking, see next.
+   The following notation equivalence should be honored::
+
+       obj[*()]        
+       # Error. Equivalent to obj[]
+       
+       obj[*(), foo=3] 
+       # Equivalent to obj[foo=3]
+
+       obj[*(x,)]      
+       # Equivalent to obj[x] 
+       
+       obj[*(x,),]     
+       # Equivalent to obj[x,]
 
 8. Dict unpacking is permitted::
 
@@ -365,6 +374,14 @@ The successful implementation of this PEP will result in the following behavior:
        obj[index, **items]
        # equivalent to obj[index, spam=1, eggs=2]
 
+   The following notation equivalent should be honored::
+
+       obj[**{}]    
+       # Error. Equivalent to obj[]
+       
+       obj[3, **{}] 
+       # Equivalent to obj[3]
+
 9. Keyword-only subscripts are permitted. The positional index will be the empty tuple::
 
        obj[spam=1, eggs=2]
@@ -779,7 +796,7 @@ more than this::
     obj[1, k=3]      # __getitem__(1, k=3).      Integer
     obj[1, 2, k=3]   # __getitem__((1, 2), k=3). Tuple
 
-With the first more in line with a \*args semantics for calling a routine with
+With the first more in line with a ``*args`` semantics for calling a routine with
 no positional arguments::
 
     >>> def foo(*args, **kwargs):