[Apply SF patch #504943]

This patch makes it possible to pass Warning instances as the first
argument to warnings.warn. In this case the category argument
will be ignored. The message text used will be str(warninginstance).

Doc/lib/libwarnings.tex

@@ -145,7 +145,10 @@ message to \code{sys.stderr}).
 \begin{funcdesc}{warn}{message\optional{, category\optional{, stacklevel}}}
 Issue a warning, or maybe ignore it or raise an exception.  The
 \var{category} argument, if given, must be a warning category class
-(see above); it defaults to \exception{UserWarning}.  This function
+(see above); it defaults to \exception{UserWarning}.  Alternatively
+\var{message} can be a \exception{Warning} instance, in which case
+\var{category} will be ignore and \code{message.__class__} will be used.
+In this case the message text will be \code{str(message)}. This function
 raises an exception if the particular warning issued is changed
 into an error by the warnings filter see above.  The \var{stacklevel}
 argument can be used by wrapper functions written in Python, like
@@ -169,6 +172,9 @@ filename and line number, and optionally the module name and the
 registry (which should be the \code{__warningregistry__} dictionary of
 the module).  The module name defaults to the filename with \code{.py}
 stripped; if no registry is passed, the warning is never suppressed.
+\var{message} must be a string and \var{category} a subclass of
+\exception{Warning} or \var{message} may be a \exception{Warning} instance,
+in which case \var{category} will be ignored.
 \end{funcdesc}
 
 \begin{funcdesc}{showwarning}{message, category, filename,

Lib/warnings.py

@@ -11,6 +11,9 @@ onceregistry = {}
 
 def warn(message, category=None, stacklevel=1):
     """Issue a warning, or maybe ignore it or raise an exception."""
+    # Check if message is already a Warning object
+    if isinstance(message, Warning):
+       category = message.__class__
     # Check category argument
     if category is None:
         category = UserWarning
@@ -49,14 +52,20 @@ def warn_explicit(message, category, filename, lineno,
             module = module[:-3] # XXX What about leading pathname?
     if registry is None:
         registry = {}
-    key = (message, category, lineno)
+    if isinstance(message, Warning):
+       text = str(message)
+       category = message.__class__
+    else:
+       text = message
+       message = category(message)
+    key = (text, category, lineno)
     # Quick test for common case
     if registry.get(key):
         return
     # Search the filters
     for item in filters:
         action, msg, cat, mod, ln = item
-        if (msg.match(message) and
+        if (msg.match(text) and
             issubclass(category, cat) and
             mod.match(module) and
             (ln == 0 or lineno == ln)):
@@ -68,11 +77,11 @@ def warn_explicit(message, category, filename, lineno,
         registry[key] = 1
         return
     if action == "error":
-        raise category(message)
+        raise message
     # Other actions
     if action == "once":
         registry[key] = 1
-        oncekey = (message, category)
+        oncekey = (text, category)
         if onceregistry.get(oncekey):
             return
         onceregistry[oncekey] = 1
@@ -80,7 +89,7 @@ def warn_explicit(message, category, filename, lineno,
         pass
     elif action == "module":
         registry[key] = 1
-        altkey = (message, category, 0)
+        altkey = (text, category, 0)
         if registry.get(altkey):
             return
         registry[altkey] = 1

Misc/NEWS

@@ -70,6 +70,8 @@ Library
 
 - distutils bdist commands now offer a --skip-build option.
 
+- warnings.warn now accepts a Warning instance as first argument.
+
 Tools/Demos
 
 Build