[Python-checkins] r42621 - peps/trunk/pep-0343.txt

[guido.van.rossum]

Author: guido.van.rossum
Date: Mon Feb 27 22:08:43 2006
New Revision: 42621

Modified:
   peps/trunk/pep-0343.txt
Log:
Change the translation slightly.
ctx.__exit__ is now bound before ctx.__enter__;
ctx.__enter__ is still called before the try-block
is entered, but VAR is assigned *inside* the try-block.
Clarify what happens to break, continue, return.


Modified: peps/trunk/pep-0343.txt
==============================================================================
--- peps/trunk/pep-0343.txt	(original)
+++ peps/trunk/pep-0343.txt	Mon Feb 27 22:08:43 2006
@@ -225,25 +225,34 @@
 
     The translation of the above statement is:
 
-        abc = (EXPR).__context__()
+        ctx = (EXPR).__context__()
+        exit = exc.__exit__  # Not calling it yet
+        res = ctx.__enter__()
         exc = (None, None, None)
-        VAR = abc.__enter__()
         try:
             try:
+                VAR = res    # Only if "as VAR" is present
                 BLOCK
             except:
                 exc = sys.exc_info()
                 raise
         finally:
-            abc.__exit__(*exc)
+            exit(*exc)
 
-    Here, the variables 'abc' and 'exc' are internal variables and not
+    Here, the lowercase variables are internal variables and not
     accessible to the user; they will most likely be implemented as
     special registers or stack positions.
 
-    The above translation is fairly literal - if any of the relevant
-    methods are not found as expected, the interpreter will raise
-    AttributeError.
+    The details of the above translation are intended to prescribe the
+    exact semantics.  If any of the relevant methods are not found as
+    expected, the interpreter will raise AttributeError, in the order
+    that they are tried (__context__, __exit__, __enter__).
+    Similarly, if any of the calls raises an exception, the effect is
+    exactly as it would be in the above code.  Finally, if BLOCK
+    contains a break, continue or return statement, the __exit__()
+    method is called with three None arguments just as if BLOCK
+    completed normally.  (I.e. these "pseudo-exceptions" are not seen
+    as exceptions by __exit__().)
 
     The call to the __context__() method serves a similar purpose to
     that of the __iter__() method of iterator and iterables. An