From 189ff13add50d1c4e7cf79d339465e6d498fddba 2012-06-07 21:23:02
From: Bradley M. Froehle <brad.froehle@gmail.com>
Date: 2012-06-07 21:23:02
Subject: [PATCH] Remove args/kwargs handling in TryNext.

This allows the error message in magic_paste to propagate correctly,
closing gh-1736.

---

diff --git a/IPython/core/error.py b/IPython/core/error.py
index cf7d546..25ff887 100644
--- a/IPython/core/error.py
+++ b/IPython/core/error.py
@@ -35,22 +35,9 @@ class TryNext(IPythonCoreError):
     """Try next hook exception.
 
     Raise this in your hook function to indicate that the next hook handler
-    should be used to handle the operation.  If you pass arguments to the
-    constructor those arguments will be used by the next hook instead of the
-    original ones.
-    
-    A _msg argument will not be passed on, so it can be used as a displayable
-    error message.
+    should be used to handle the operation.
     """
 
-    def __init__(self, _msg="", *args, **kwargs):
-        self.args = args
-        self.kwargs = kwargs
-        self.msg = _msg
-    
-    def __str__(self):
-        return str(self.msg)
-
 class UsageError(IPythonCoreError):
     """Error in magic function arguments, etc.
 
diff --git a/IPython/core/hooks.py b/IPython/core/hooks.py
index 81340d2..742393f 100644
--- a/IPython/core/hooks.py
+++ b/IPython/core/hooks.py
@@ -131,17 +131,15 @@ class CommandChainDispatcher:
         This will call all funcs in chain with the same args as were given to
         this function, and return the result of first func that didn't raise
         TryNext"""
-
+        last_exc = TryNext()
         for prio,cmd in self.chain:
             #print "prio",prio,"cmd",cmd #dbg
             try:
                 return cmd(*args, **kw)
-            except TryNext, exc:
-                if exc.args or exc.kwargs:
-                    args = exc.args
-                    kw = exc.kwargs
+            except TryNext as exc:
+                last_exc = exc
         # if no function will accept it, raise TryNext up to the caller
-        raise TryNext(*args, **kw)
+        raise last_exc
 
     def __str__(self):
         return str(self.chain)
diff --git a/IPython/lib/clipboard.py b/IPython/lib/clipboard.py
index 30974b3..deef0da 100644
--- a/IPython/lib/clipboard.py
+++ b/IPython/lib/clipboard.py
@@ -15,9 +15,8 @@ def win32_clipboard_get():
     try:
         import win32clipboard
     except ImportError:
-        message = ("Getting text from the clipboard requires the pywin32 "
-            "extensions: http://sourceforge.net/projects/pywin32/")
-        raise TryNext(_msg=message)
+        raise TryNext("Getting text from the clipboard requires the pywin32 "
+                      "extensions: http://sourceforge.net/projects/pywin32/")
     win32clipboard.OpenClipboard()
     text = win32clipboard.GetClipboardData(win32clipboard.CF_TEXT)
     # FIXME: convert \r\n to \n?
@@ -44,9 +43,8 @@ def tkinter_clipboard_get():
     try:
         import Tkinter
     except ImportError:
-        message = ("Getting text from the clipboard on this platform "
-            "requires Tkinter.")
-        raise TryNext(_msg=message)
+        raise TryNext("Getting text from the clipboard on this platform "
+                      "requires Tkinter.")
     root = Tkinter.Tk()
     root.withdraw()
     text = root.clipboard_get()
diff --git a/docs/source/whatsnew/development.txt b/docs/source/whatsnew/development.txt
index b8f65c6..dc1d31b 100644
--- a/docs/source/whatsnew/development.txt
+++ b/docs/source/whatsnew/development.txt
@@ -25,3 +25,11 @@ Other new features
   :meth:`~IPython.core.interactiveshell.InteractiveShell.run_cell` to
   :meth:`~IPython.core.interactiveshell.InteractiveShell.run_ast_nodes`
   is now configurable.
+
+Backwards incompatible changes
+------------------------------
+
+* The exception :exc:`IPython.core.error.TryNext` previously accepted
+  arguments and keyword arguments to be passed to the next implementation
+  of the hook. This feature was removed as it made error message propagation
+  difficult and violated the principle of loose coupling.