--- a/mercurial/cmdutil.py	Thu Nov 06 10:39:52 2008 +0100
+++ b/mercurial/cmdutil.py	Thu Nov 06 11:17:38 2008 +0100
@@ -214,9 +214,12 @@
 
 def make_file(repo, pat, node=None,
               total=None, seqno=None, revwidth=None, mode='wb', pathname=None):
+
+    writable = 'w' in mode or 'a' in mode
+
     if not pat or pat == '-':
-        return 'w' in mode and sys.stdout or sys.stdin
-    if hasattr(pat, 'write') and 'w' in mode:
+        return writable and sys.stdout or sys.stdin
+    if hasattr(pat, 'write') and writable:
         return pat
     if hasattr(pat, 'read') and 'r' in mode:
         return pat

--- a/mercurial/patch.py	Thu Nov 06 10:39:52 2008 +0100
+++ b/mercurial/patch.py	Thu Nov 06 11:17:38 2008 +0100
@@ -1294,7 +1294,8 @@
 
         if not fp:
             fp = cmdutil.make_file(repo, template, node, total=total,
-                                   seqno=seqno, revwidth=revwidth)
+                                   seqno=seqno, revwidth=revwidth,
+                                   mode='ab')
         if fp != sys.stdout and hasattr(fp, 'name'):
             repo.ui.note("%s\n" % fp.name)
 

--- a/tests/test-export	Thu Nov 06 10:39:52 2008 +0100
+++ b/tests/test-export	Thu Nov 06 11:17:38 2008 +0100
@@ -13,3 +13,9 @@
     echo "# foo-$out.patch"
     hg export -v -o "foo-$out.patch" 2:tip
 done
+
+echo "# exporting 4 changesets to a file"
+hg export -o export_internal 1 2 3 4
+grep HG export_internal | wc -l
+echo "# exporting 4 changesets to a file"
+hg export 1 2 3 4 | grep HG | wc -l

--- a/tests/test-export.out	Thu Nov 06 10:39:52 2008 +0100
+++ b/tests/test-export.out	Thu Nov 06 11:17:38 2008 +0100
@@ -58,3 +58,7 @@
 foo-09.patch
 foo-10.patch
 foo-11.patch
+# exporting 4 changesets to a file
+4
+# exporting 4 changesets to a file
+4