From 9b82c2499689cc46f6fcfc2792060d3bcee8b28a Mon Sep 17 00:00:00 2001
From: Guido van Rossum <guido@python.org>
Date: Tue, 3 May 2005 22:23:32 +0000
Subject: [PATCH] Some clarifications after Raymond's email.

---
 pep-0340.txt | 37 +++++++++++++++++++++++++++++++++++--
 1 file changed, 35 insertions(+), 2 deletions(-)

diff --git a/pep-0340.txt b/pep-0340.txt
index 93050b21c..37533da3d 100644
--- a/pep-0340.txt
+++ b/pep-0340.txt
@@ -122,7 +122,7 @@ Specification: a Change to the 'for' Loop
     will be translated as follows:
 
         itr = iter(EXPR1)
-        arg = None
+        arg = None    # Set by "continue EXPR2", see below
         brk = False
         while True:
             try:
@@ -496,7 +496,7 @@ Examples
         block locking(myLock):
             # Code here executes with myLock held.  The lock is
             # guaranteed to be released when the block is left (even
-            # if by an uncaught exception).
+            # if via return or by an uncaught exception).
 
     2. A template for opening a file that ensures the file is closed
        when the block is left:
@@ -557,6 +557,11 @@ Examples
             for line in f:
                 print line.rstrip()
 
+       (If this example confuses you, consider that it is equivalent
+       to using a for-loop with a yield in its body in a regular
+       generator which is invoking another iterator or generator
+       recursively; see for example the source code for os.walk().)
+
     6. It is possible to write a regular iterator with the
        semantics of example 1:
 
@@ -581,6 +586,10 @@ Examples
                    self.lock.release()
                raise type, value, traceback
 
+       (This example is easily modified to implement the other
+       examples; it shows how much simpler generators are for the same
+       purpose.)
+
     7. Redirect stdout temporarily:
 
         def redirecting_stdout(new_stdout):
@@ -597,6 +606,30 @@ Examples
             block redirecting_stdout(f):
                 print "Hello world"
 
+    8. A variant on opening() that also returns an error condition:
+
+        def opening_w_error(filename, mode="r"):
+	    try:
+	        f = open(filename, mode)
+	    except IOError, err:
+	        yield None, err
+	    else:
+                try:
+		    yield f, None
+		finally:
+		    f.close()
+
+       Used as follows:
+
+        block opening_w_error("/etc/passwd", "a") as f, err:
+            if err:
+	        print "IOError:", err
+	    else:
+	        f.write("guido::0:0::/:/bin/sh\n")
+
+    9. More examples are needed: showing "continue EXPR", and the use
+       of continue, break and return in a block-statement.
+
 Acknowledgements
 
     In no useful order: Alex Martelli, Barry Warsaw, Bob Ippolito,