When an exception is raised in a __del__ method or a finalizer (i.e. a
weakref callback), only the exception name is printed out.
Unfortunately, arbitrarily complex code can be involved which makes
debugging quite tedious. It would be nice to display the full traceback
so that these errors are easier to diagnose (see issue7060 as an example).

Seconded. 
But such messages are also printed when the interpreter exits, when
poorly written __del__ methods access already disposed modules. Raising
a full traceback could cause users to think that the error is more
severe than it is.

Is there an already existing flag that could control this behaviour?
Otherwise, well, just ignore my remark.

Amaury asks: "Is there an already existing flag that could control this behaviour?"

I wonder if the warnings module could be used.

Submitting a patch proposing this format:

-----BEGIN UNRAISABLE EXCEPTION-----
Class: AttributeError
Instance: "'NoneType' object has no attribute 'someattr'"
Function: <bound method A.__del__ of <__main__.A object at 0x1007671d0>>
Traceback (most recent call last):
  File "/tmp/bla.py", line 4, in __del__
    None.someattr
-----END UNRAISABLE EXCEPTION-----

I've wrapped the exception information with header/footer differentiating it from a user's error handling code that also prints tracebacks (is it too much?).

I've considered using the warnings module, But I dislike the suppression of already warned messages. (2 instances will raise exception in __del__ but only one message will be printed) 

This is my first patch submission so feel free giving me a hard time.

The formatting isn't very conventional for Python.

Why not use the normal format? i.e.
 
Traceback (most recent call last):
  File "/tmp/bla.py", line 4, in __del__
    None.someattr
AttributeError: 'NoneType' object has no attribute 'someattr'

Why is this more likely to get confused with user input than other unhandled exceptions?

If you ARE going to deviate, it may be helpful to have it explain why this exception wasn't caught through the normal channels. Maybe the last line could be: "Failed to raise this exception in __del__/finalizer method." or similar.

Here's the next attempt (took your advice about the convention):

Exception ignored in: <bound method A.__del__ of <__main__.A object at 0x1007671d0>>
Traceback (most recent call last):
  File "/tmp/bla.py", line 4, in __del__
    None.someattr
AttributeError: 'NoneType' object has no attribute 'someattr'

reminder of the current format for comparison:

Exception AttributeError: "'NoneType' object has no attribute 'someattr'" in <bound method A.__del__ of <__main__.A object at 0x1007671d0>> ignored

I thought about a more elaborate explanation than "Exception ignored" but grepping this function through the code shows it can be called from various places making it too generic.

The reason I wanted to add a header/footer is for stating the message and the traceback go together (I print tracebacks to screen all the time), but it might be TMI..

I wish there was a less instrusive of saying "+1. I'm happy. Thanks." than posting a whole comment, but until then:

+1. I'm happy. Thanks.

(I haven't inspected the code - just based on the description.)