# Buggy code
def main():
    # construct a dict where 'values' are all the processes
    # having 'key' as their parent
    tree = collections.defaultdict(list)
    for p in psutil.process_iter():
        try:
            tree[p.ppid()].append(p.pid)
        except psutil.NoSuchProcess:
            pass
    # on systems supporting PID 0, PID 0's parent is usually 0
    if 0 in tree and 0 in tree[0]:
        tree[0].remove(0)
    print_tree(min(tree), tree)


# Traceback (psutil._error.NoSuchProcess)
How to reproduce:

1. start Photoshop CS6 on a Mountain Lion OSX
2. import psutil; [x.as_dict() for x in psutil.process_iter()] # (in .py file, ipython)



What is the expected output?
A long list of processes and related information



What do you see instead?


$ python test.py
Traceback (most recent call last):
File "test.py", line 3, in <module>
[x.as_dict() for x in psutil.process_iter() if x.is_running()]
File
"/Users/rico/.virtualenvs/temp/lib/python2.7/site-packages/psutil/__init__.py",
line 225, in as_dict
ret = attr()
File
"/Users/rico/.virtualenvs/temp/lib/python2.7/site-packages/psutil/__init__.py",
line 414, in get_nice
return self._platform_impl.get_process_nice()
File
"/Users/rico/.virtualenvs/temp/lib/python2.7/site-packages/psutil/_psosx.py",
line 151, in wrapper
raise NoSuchProcess(self.pid, self._process_name)
psutil._error.NoSuchProcess: process no longer exists (pid=46244)

or within iPython notebook:
[x.as_dict() for x in psutil.process_iter() if x.is_running()]
---------------------------------------------------------------------------
NoSuchProcess                             Traceback (most recent call last)
<ipython-input-108-a71c6dffe397> in <module>()
----> 1 [x.as_dict() for x in psutil.process_iter() if x.is_running()]

/Users/rico/.virtualenvs/temp/lib/python2.7/site-packages/psutil/__init__.pyc
in as_dict(self, attrs, ad_value)
223                         ret = attr(interval=0)
224                     else:
--> 225                         ret = attr()
226                 else:
227                     ret = attr

/Users/rico/.virtualenvs/temp/lib/python2.7/site-packages/psutil/__init__.pyc
in get_nice(self)
412     def get_nice(self):
413         """Get process niceness (priority)."""
--> 414         return self._platform_impl.get_process_nice()
415
416     @_assert_pid_not_reused

/Users/rico/.virtualenvs/temp/lib/python2.7/site-packages/psutil/_psosx.pyc in
wrapper(self, *args, **kwargs)
149             err = sys.exc_info()[1]
150             if err.errno == errno.ESRCH:
--> 151                 raise NoSuchProcess(self.pid, self._process_name)
152             if err.errno in (errno.EPERM, errno.EACCES):
153                 raise AccessDenied(self.pid, self._process_name)

NoSuchProcess: process no longer exists (pid=46243, name='adobe_licutil')

When I close Photoshop, the error will not show up. When starting it again the
error reappears.
An additional is_running() check within the list comprehension does not change
a thing and running the code several times will not change the reported pid.

# Fixed code
def main():
    # construct a dict where 'values' are all the processes
    # having 'key' as their parent
    tree = collections.defaultdict(list)
    for p in psutil.process_iter():
        try:
            tree[p.ppid()].append(p.pid)
        except (psutil.NoSuchProcess, psutil.ZombieProcess):
            pass
    # on systems supporting PID 0, PID 0's parent is usually 0
    if 0 in tree and 0 in tree[0]:
        tree[0].remove(0)
    print_tree(min(tree), tree)