Add traceback.format_exc().

Doc/lib/libtraceback.tex

@@ -48,6 +48,11 @@ information in a thread-safe way instead of using the deprecated
 variables.)
 \end{funcdesc}
 
+\begin{funcdesc}{format_exc}{\optional{limit\optional{, file}}}
+This is like \code{print_exc(\var{limit})} but returns a string
+instead of printing to a file.
+\end{funcdesc}
+
 \begin{funcdesc}{print_last}{\optional{limit\optional{, file}}}
 This is a shorthand for \code{print_exception(sys.last_type,
 sys.last_value, sys.last_traceback, \var{limit}, \var{file})}.

Lib/threading.py

@@ -8,9 +8,8 @@ except ImportError:
     del _sys.modules[__name__]
     raise
 
-from StringIO import StringIO as _StringIO
 from time import time as _time, sleep as _sleep
-from traceback import print_exc as _print_exc
+from traceback import format_exc as _format_exc
 
 # Rename some stuff so "from threading import *" is safe
 __all__ = ['activeCount', 'Condition', 'currentThread', 'enumerate', 'Event',
@@ -440,10 +439,8 @@ class Thread(_Verbose):
             except:
                 if __debug__:
                     self._note("%s.__bootstrap(): unhandled exception", self)
-                s = _StringIO()
-                _print_exc(file=s)
                 _sys.stderr.write("Exception in thread %s:\n%s\n" %
-                                 (self.getName(), s.getvalue()))
+                                  (self.getName(), _format_exc()))
             else:
                 if __debug__:
                     self._note("%s.__bootstrap(): normal return", self)

Lib/traceback.py

@@ -6,8 +6,8 @@ import types
 
 __all__ = ['extract_stack', 'extract_tb', 'format_exception',
            'format_exception_only', 'format_list', 'format_stack',
-           'format_tb', 'print_exc', 'print_exception', 'print_last',
-           'print_stack', 'print_tb', 'tb_lineno']
+           'format_tb', 'print_exc', 'format_exc', 'print_exception',
+           'print_last', 'print_stack', 'print_tb', 'tb_lineno']
 
 def _print(file, str='', terminator='\n'):
     file.write(str+terminator)
@@ -211,6 +211,16 @@ def print_exc(limit=None, file=None):
     finally:
         etype = value = tb = None
 
+
+def format_exc(limit=None):
+    """Like print_exc() but return a string."""
+    try:
+        etype, value, tb = sys.exc_info()
+        return ''.join(format_exception(etype, value, tb, limit))
+    finally:
+        etype = value = tb = None
+    
+
 def print_last(limit=None, file=None):
     """This is a shorthand for 'print_exception(sys.last_type,
     sys.last_value, sys.last_traceback, limit, file)'."""

Misc/NEWS

@@ -101,6 +101,9 @@ Extension modules
 Library
 -------
 
+- traceback.format_exc has been added (similar to print_exc but it returns
+  a string).
+
 - xmlrpclib.MultiCall has been added.
 
 - poplib.POP3_SSL has been added.