[Python-checkins] r42942 - peps/trunk/pep-0343.txt

[guido.van.rossum]

Author: guido.van.rossum
Date: Thu Mar  9 18:39:53 2006
New Revision: 42942

Modified:
   peps/trunk/pep-0343.txt
Log:
Update this PEP to reflect the new design: exceptions are swallowed only
when __exit__ returns a true value.


Modified: peps/trunk/pep-0343.txt
==============================================================================
--- peps/trunk/pep-0343.txt	(original)
+++ peps/trunk/pep-0343.txt	Thu Mar  9 18:39:53 2006
@@ -236,7 +236,9 @@
             except:
                 # The exceptional case is handled here
                 exc = False
-                exit(*sys.exc_info())
+                if not exit(*sys.exc_info()):
+                    raise
+                # The exception is swallowed if exit() returns true
         finally:
             # The normal and non-local-goto cases are handled here
             if exc:
@@ -277,17 +279,21 @@
     BLOCK, ctx.__exit__() is called with three arguments representing
     the exception type, value, and traceback.
 
-    IMPORTANT: if ctx.__exit__() is passed exception information, it
-    *must* re-raise the exception, unless it wants the exception to be
-    ignored.  That is, if ctx.__exit__() returns normally, execution
+    IMPORTANT: if ctx.__exit__() returns a "true" value, the exception
+    is "swallowed".  That is, if it returns "true", execution
     continues at the next statement after the with-statement, even if
     an exception happened inside the with-statement.  However, if the
     with-statement was left via a non-local goto (break, continue or
     return), this non-local return is resumed when ctx.__exit__()
-    returns normally.  The motivation for this detail is to make a
-    try/except block in a context manager created from a decorated
-    generator behave exactly as if the body of the generator were
-    expanded in-line at the place of the with-statement.
+    returns regardless of the return value.  The motivation for this
+    detail is to make it possible for ctx.__exit__() to swallow
+    exceptions, without making it too easy (since the default return
+    value, None, is false and this causes the exception to be
+    re-raised).  The main use case for swallowing exceptions is to
+    make it possible to write the @contextmanager decorator so thatn
+    that a try/except block in a decorated generator behaves exactly
+    as if the body of the generator were expanded in-line at the place
+    of the with-statement.
 
     The motivation for passing the exception details to __exit__(), as
     opposed to the argument-less __exit__() from PEP 310, was given by
@@ -351,10 +357,9 @@
                else:
                    try:
                        self.gen.throw(type, value, traceback)
-                   except (type, StopIteration):
-                       return
-                   else:
-                       raise RuntimeError("generator caught exception")
+                       return True
+                   except StopIteration:
+                       return True
 
         def contextmanager(func):
            def helper(*args, **kwds):