PEP 572: Updates in response to posting #3 :)

Author: Rosuav

pep-0572.rst

@@ -12,18 +12,23 @@ Post-History: 28-Feb-2018, 02-Mar-2018, 23-Mar-2018
 Abstract
 ========
 
-Programming is all about reusing code rather than duplicating it.  When
-an expression needs to be used twice in quick succession but never again,
-it is convenient to assign it to a temporary name with small scope.
-By permitting name bindings to exist within a single statement only, we
-make this both convenient and safe against name collisions.
+This is a proposal for permitting temporary name bindings
+which are limited to a single statement.
 
 
 Rationale
 =========
 
-When a subexpression is used multiple times in a list comprehension, there
-are currently several ways to spell this, none of which is universally
+Programmers generally prefer reusing code rather than duplicating it.  When
+an expression needs to be used twice in quick succession but never again,
+it is convenient to assign it to a temporary name with small scope.
+By permitting name bindings to exist within a single statement only, we
+make this both convenient and safe against name collisions.
+
+This is particularly notable in list/dict/set comprehensions and generator
+expressions, where refactoring a subexpression into an assignment statement
+is not possible. There are currently several ways to create a temporary name
+binding inside a list comprehension, none of which is universally
 accepted as ideal. A statement-local name allows any subexpression to be
 temporarily captured and then used multiple times.
 
@@ -43,7 +48,13 @@ and ``NAME`` is a simple name.
 
 The value of such a named expression is the same as the incorporated
 expression, with the additional side-effect that NAME is bound to that
-value for the remainder of the current statement.
+value for the remainder of the current statement. For example::
+
+    # Similar to the boolean 'or' but checking for None specifically
+    x = "default" if (spam().ham as eggs) is None else eggs
+
+    # Even complex expressions can be built up piece by piece
+    y = ((spam() as eggs), (eggs.method() as cheese), cheese[eggs])
 
 Just as function-local names shadow global names for the scope of the
 function, statement-local names shadow other names for that statement.
@@ -252,14 +263,14 @@ Both of these are forbidden; creating SLNBs in the headers of these statements
 will result in a SyntaxError.
 
 
-Alternative proposals
-=====================
+Alternative proposals and variants
+==================================
 
 Proposals broadly similar to this one have come up frequently on python-ideas.
 Below are a number of alternative syntaxes, some of them specific to
 comprehensions, which have been rejected in favour of the one given above.
 
-1. ``where``, ``let``, ``given``::
+1. ``where``, ``let``, or ``given``, in comprehensions only::
 
        stuff = [(y, x/y) where y = f(x) for x in range(5)]
        stuff = [(y, x/y) let y = f(x) for x in range(5)]
@@ -337,6 +348,15 @@ comprehensions, which have been rejected in favour of the one given above.
    ``f(x) < 0`` and you want to capture the value of ``f(x)``).  It also has
    no benefit to list comprehensions.
 
+8. Adding a ``where:`` to any statement to create local name bindings::
+
+       value = x**2 + 2*x where:
+           x = spam(1, 4, 7, q)
+
+   Execution order is inverted (the indented body is performed first, followed
+   by the "header").  This requires a new keyword, unless an existing keyword
+   is repurposed (most likely ``with:``).
+
 
 Discrepancies in the current implementation
 ===========================================