Please describe the problem:
while gtk.<SomeWidget>.connect() accepts bound methods,
gtk.<SomeWidget>.handler_block_by_func() does not with a strangish error:
SystemError: Objects/funcobject.c:128: bad argument to internal function
and somewhat incorrect python backtrace

Steps to reproduce:
#!/usr/bin/python
import gtk
class A:
  def __init__(self):
    self.win = gtk.Window()
    self.con = gtk.VBox()
    self.win.add(self.con)
    for i in range(2):
      but = gtk.ToggleButton("Stubborn button %d" % i)
      but.connect("toggled", self.callback)
      self.con.add(but)
    self.win.show_all()

  def callback(self, w):
    for ww in w.get_parent().get_children():
      ww.handler_block_by_func(self.callback)
      ww.set_active(0)
      ww.handler_unblock_by_func(self.callback)
                           
a = A()
gtk.main()

Actual results:
Traceback (most recent call last):

+ Trace 86707

# button indeed untoggles itself and neighbours
# an error is printed
# python backtrace is incorrect (offending line is actually one below)

Expected results:
button untoggles itself and neighbours (hence stubborn)
no errors occur
if errors do occur backtrace should point to the offending line - ww.handler_block_by_func(self.callback)

Does this happen every time?
yes

Other information: