Python 2.5.1 (r251:54863, Apr 18 2007, 08:51:08) [MSC v.1310 32 bit
(Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import pstats
>>> ps = pstats.Stats("profile.log")
>>> ps.add("profile.log")
<pstats.Stats instance at 0x01358BC0>
>>> ps.print_callers()
   Random listing order was used

{method 'append' of 'list' objects}                                    
           <-
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Python25\lib\pstats.py", line 388, in print_callers
    self.print_call_line(width, func, callers, "<-")
  File "C:\Python25\lib\pstats.py", line 417, in print_call_line
    nc, cc, tt, ct = value
ValueError: too many values to unpack

I hit the same issue, and I think the problem is that at line 515 in
pstats.py add_callers() the two stats instead of adding them
member-wise. As a result, each time add() is called, the number of stats
associated with each func grows by 4. Then, when print_call_line() is
called by print_callers() or print_callees(), there are "too many values
to unpack" at line 417.

This change to pstats.py modifies add_callers() to add the stats
together instead of concatenating the tuples.

515c515
<             new_callers[func] = caller + new_callers[func]
---
>             new_callers[func] = map(lambda x,y: x+y, caller +
new_callers[func])

1315 is a duplicate of this, and I end up with a very similar solution.

Here's my patch against trunk, with one test. Please review!

Committed as r60149. Thanks for the patch!