Merge #493: Use shutil.which to discover udevadm, groupadd and usermo…

…d binaries

09a747a Use shutil.which to discover udevadm, groupadd and usermod binaries (Pavol Rusnak)

Pull request description:

  Fixes #491

  [`shutil.which`](https://docs.python.org/3/library/shutil.html#shutil.which) is available since python 3.3

ACKs for top commit:
  achow101:
    ACK 09a747a

Tree-SHA512: 93bcaa94f3f2b8d760ca8111f3ddb78f31bee0aec52af926a51df2caf85ebe472f5729d82cd17f1ccf937497c5e678e2515ca1a59c01c021d433a7f0fd506910

hwilib/udevinstaller.py

@@ -8,7 +8,7 @@
 from .errors import NeedsRootError
 
 from subprocess import check_call, CalledProcessError, DEVNULL
-from shutil import copy
+from shutil import copy, which
 from os import path, listdir, getlogin, geteuid
 
 class UDevInstaller(object):
@@ -39,9 +39,9 @@ def install(source: str, location: str) -> bool:
         return True
 
     def __init__(self) -> None:
-        self._udevadm = '/sbin/udevadm'
-        self._groupadd = '/usr/sbin/groupadd'
-        self._usermod = '/usr/sbin/usermod'
+        self._udevadm = which('udevadm')
+        self._groupadd = which('groupadd')
+        self._usermod = which('usermod')
 
     def _execute(self, cmd: str, *args: str) -> None:
         command = [cmd] + list(args)
@@ -51,12 +51,14 @@ def trigger(self) -> None:
         """
         Run ``udevadm trigger``
         """
+        assert self._udevadm
         self._execute(self._udevadm, 'trigger')
 
     def reload_rules(self) -> None:
         """
         Run ``udevadm control --reload-rules``
         """
+        assert self._udevadm
         self._execute(self._udevadm, 'control', '--reload-rules')
 
     def add_user_plugdev_group(self) -> None:
@@ -67,13 +69,15 @@ def add_user_plugdev_group(self) -> None:
         self._add_user_to_group(getlogin(), 'plugdev')
 
     def _create_group(self, name: str) -> None:
+        assert self._groupadd
         try:
             self._execute(self._groupadd, name)
         except CalledProcessError as e:
             if e.returncode != 9: # group already exists
                 raise
 
     def _add_user_to_group(self, user: str, group: str) -> None:
+        assert self._usermod
         self._execute(self._usermod, '-aG', group, user)
 
     def copy_udev_rule_files(self, source: str, location: str) -> None: