The old pattern¶

Iterating over processes and collecting attributes requires more boilerplate than it should. A process returned by psutil.process_iter() may disappear before you access it, or require elevated privileges, so every lookup has to be guarded with a try / except:

>>> import psutil
>>> for proc in psutil.process_iter():
...     try:
...         pinfo = proc.as_dict(attrs=['pid', 'name'])
...     except (psutil.NoSuchProcess, psutil.AccessDenied):
...         pass
...     else:
...         print(pinfo)
...
{'pid': 1, 'name': 'systemd'}
{'pid': 2, 'name': 'kthreadd'}
{'pid': 3, 'name': 'ksoftirqd/0'}

This is not decorative. It's necessary to avoid the race condition.

The new pattern¶

5.3.0 adds attrs and ad_value parameters to psutil.process_iter(). With these, the loop body becomes:

>>> import psutil
>>> for proc in psutil.process_iter(attrs=['pid', 'name']):
...     print(proc.info)
...
{'pid': 1, 'name': 'systemd'}
{'pid': 2, 'name': 'kthreadd'}
{'pid': 3, 'name': 'ksoftirqd/0'}

Internally, process_iter() attaches an info dict to the Process instance. The attributes are pre-fetched in one shot. Processes that disappear during iteration are silently skipped, and attributes that would raise AccessDenied get assigned ad_value, which defaults to None:

for p in psutil.process_iter(['name', 'username'], ad_value="N/A"):
    print(p.name(), p.username())

Performance¶

Beyond the syntactic win, the new syntax is also faster than calling individual methods in a loop. process_iter(attrs=[...]) is equivalent to using Process.oneshot() on each process (see Making psutil twice as fast for how that works): attributes that share a syscall or a /proc file are fetched together instead of re-read on every method call, which is a lot faster.

Comprehensions¶

With the exception boilerplate out of the way, comprehensions finally work cleanly. E.g. getting processes owned by the current user can be written as:

>>> import getpass
>>> from pprint import pprint as pp
>>> pp([(p.pid, p.info['name']) for p in psutil.process_iter(attrs=['name', 'username']) if p.info['username'] == getpass.getuser()])
[(16832, 'bash'),
 (19772, 'ssh'),
 (20492, 'python')]