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found a pretty annoying bug caused by with blocks. A with block
terminates the debugging session and the program keeps running. It's not
possible to go to the next line with 'n'. 's' steps into the open() call.
# pdbtest.py
import pdb
pdb.set_trace()
print("before with")
with open("/etc/passwd") as fd:
 data = fd.read()
print("after with")
print("end of program")
$ ./python pdbtest.py
> /home/heimes/dev/python/py3k/pdbtest.py(3)<module>()
-> print("before with")
(Pdb) n
before with
> /home/heimes/dev/python/py3k/pdbtest.py(4)<module>()
-> with open("/etc/passwd") as fd:
(Pdb) n
after with
end of program

The bug makes debugging a crux :(

Corrected by revision 58958.
"with" is innocent. The problem is this function in abc.py which uses a
generator expression:
 def __instancecheck__(cls, instance):
 """Override for isinstance(instance, cls)."""
 return any(cls.__subclasscheck__(c)
 for c in {instance.__class__, type(instance)})
It is called both by pdb (deep inside 'print') and the debugged code (in
PyObject_MethodCall), and this reveals the bug.

__instancecheck__ should be simpler: all classes are new-style, how is
it possible to have different results for
 instance.__class__
and
 type(instance)
?

Good work Amaury! :)
I also wonder how type(cls) != cls.__class__ is possible with new style
classes. So far I found only one way and it ain't beautiful:
>>> class Meta(type):
... def __getattribute__(self, key):
... if key == "__class__": return object
... return type.__getattribute__(self, key)
...
>>> class Example(metaclass=Meta): pass
...
>>> Example.__class__
<type 'object'>
>>> type(Example)
<class '__main__.Meta'>

Good work indeed!
Here's another way:
>>> class C:
... @property
... def __class__(self): return D
... 
>>> class D: pass
... 
>>> C().__class__
<class '__main__.D'>
>>>

BTW: if you can show (e.g. with your unittest) that this indeed breaks
in 2.6 and 2.5, please do backport it.
BTW**2: I've noticed that abc.py's __instancecheck__ gets called a lot
at times when I don't expect it. Can you research this a bit?

PPSS. When backporting, a note in Misc/NEWS is appreciated.

> BTW**2: I've noticed that abc.py's __instancecheck__ gets called a lot
> at times when I don't expect it. Can you research this a bit?
In classobject.c, method_call() calls PyObject_IsInstance() on the first
arg when the method is unbound.
This happens a lot in io.py, each time the code calls a base class method.