Hmm, when exception occurs in some frame, its reference will be
retained even after exiting function? Indeed, extra
exception fixed problem.

import os

def foo():
    io = open("a.txt", "w")
    raise RuntimeError()

try:
    foo()
except:
    pass

try:
    raise RuntimeError()
except:
    pass

os.remove("a.txt") # fine

But still I'm little confused why this code prints "del".

class A:
    def __del__(self):
        print "del"

def foo():
    a = A()
    raise RuntimeError()

try:
    foo()
except:
    pass

I found this behavior when investigating issue3210. When
Lib/subprocess.py (814)'s CreateProcess() raises exception,
p2cread will be freed by refcount GC, it never happen before
os.remove(). And this error is also fixed by same hack.

import subprocess, os

file = open("filename", "w")
try:
    proc = subprocess.Popen("nosuchprogram", stdout=file)
except OSError:
    pass

try:
    raise RuntimeError() # hack
except:
    pass

file.close()
os.remove("filename")