PEP 671: Change recommendation from permissive to strict

pep-0671.rst

@@ -68,13 +68,14 @@ Notably, the expression is evaluated in the function's run-time scope, NOT the
 scope in which the function was defined (as are early-bound defaults). This
 allows the expression to refer to other arguments.
 
-Self-referential expressions will result in UnboundLocalError::
-
-    def spam(eggs=>eggs): # Nope
-
 Multiple late-bound arguments are evaluated from left to right, and can refer
-to previously-calculated values. Order is defined by the function, regardless
-of the order in which keyword arguments may be passed.
+to previously-defined values. Order is defined by the function, regardless of
+the order in which keyword arguments may be passed. Using names of other
+arguments is a SyntaxError at function definition time::
+
+    def spaminate(sausage=>eggs + 1, eggs=>sausage - 1): # SyntaxError
+    def selfref(spam=>spam): # SyntaxError
+    def frob(n=>len(items), items=[]): # SyntaxError
 
 
 Choice of spelling
@@ -131,6 +132,13 @@ Open Issues
 - annotations? They go before the default, so is there any way an anno could
   want to end with ``=>``?
 
+- Rather than banning future refs, these could be permitted, at the price of
+  harder-to-explain semantics. Arguments would be resolved first with those
+  passed and those with early-bound defaults, and then late-bound ones would
+  be evaluated, left-to-right; the consequences for getting it wrong would
+  then be UnboundLocalError at call time, rather than SyntaxError from the
+  function definition.
+
 
 References
 ==========