Why does this contextmanager behave differently with dict understanding?

Question

I have a context decorator that has side effects. I noticed that side effects do not occur if I use a dict understanding.

from contextlib import contextmanager
import traceback
import sys

accumulated = []

@contextmanager
def accumulate(s):
    try:
        yield
    finally:
        print("Appending %r to accumulated" % s)
        accumulated.append(s)

def iterate_and_accumulate(iterable):
    for item in iterable:
        with accumulate(item):
            yield item

def boom_unless_zero(i):
    if i > 0:
        raise RuntimeError("Boom!")

try:
    {i: boom_unless_zero(i) for i in iterate_and_accumulate([0, 1])}
except:
    traceback.print_exc()

print(accumulated)

print('\n=====\n')

try:
    {i: boom_unless_zero(i) for i in iterate_and_accumulate([0, 1])}
except:
    traceback.print_exc()

print(accumulated)
print('Finished!')

Conclusion:

$ python2 boom3.py 
Appending 0 to accumulated
Traceback (most recent call last):
  File "boom3.py", line 25, in <module>
    {i: boom_unless_zero(i) for i in iterate_and_accumulate([0, 1])}
  File "boom3.py", line 25, in <dictcomp>
    {i: boom_unless_zero(i) for i in iterate_and_accumulate([0, 1])}
  File "boom3.py", line 22, in boom_unless_zero
    raise RuntimeError("Boom!")
RuntimeError: Boom!
[0]

=====

Appending 0 to accumulated
Appending 1 to accumulated
Traceback (most recent call last):
  File "boom3.py", line 34, in <module>
    {i: boom_unless_zero(i) for i in iterate_and_accumulate([0, 1])}
  File "boom3.py", line 34, in <dictcomp>
    {i: boom_unless_zero(i) for i in iterate_and_accumulate([0, 1])}
  File "boom3.py", line 22, in boom_unless_zero
    raise RuntimeError("Boom!")
RuntimeError: Boom!
[0, 0, 1]
Finished!
Appending 1 to accumulated

It is bizarre that a side effect occurs after my script is "finished". This means that users cannot use my contextdecorator if they use dict comprehension.

I noticed that this behavior disappears in Python 3, and the behavior also does not occur if I write [boom_unless_zero(i) for i in iterate_and_accumulate([0, 1])]instead of understanding the dict.

Why is this happening?

Answer 1

https://docs.python.org/2/reference/simple_stmts.html#the-yield-statement:

2.5 Python, yield try try... finally construct. , ( ), close-type-iterators, .

, , - , , (refcount ). , Python 2 Python 3 , .

-:

from contextlib import closing

try:
    with closing(iter(iterate_and_accumulate(a))) as it:
        {i: boom_unless_zero(i) for i in it}
except:
    traceback.print_exc()
print(accumulated)

---

; , , - , sys.exc_clear():

import sys

try:
    {i: boom_unless_zero(i) for i in iterate_and_accumulate(a)}
except:
    traceback.print_exc()
    try:
        sys.exc_clear()
    except AttributeError:
        pass
print(accumulated)

Python 3 (http://bugs.python.org/issue3021) , , sys.exc_clear() ( ).