Provide Sound.set_volume(pct) (#259)

Closes #258

Author: ddemidov

ev3dev/core.py

@@ -50,7 +50,7 @@
 import time
 from os.path import abspath
 from struct import pack, unpack
-from subprocess import Popen
+from subprocess import Popen, check_output
 
 try:
     # This is a linux-specific module.
@@ -3048,3 +3048,29 @@ def speak(text, espeak_opts='-a 200 -s 130'):
         with open(os.devnull, 'w') as n:
             cmd_line = '/usr/bin/espeak --stdout {0} "{1}" | /usr/bin/aplay -q'.format(espeak_opts, text)
             return Popen(cmd_line, stdout=n, shell=True)
+
+    @staticmethod
+    def set_volume(pct, channel=None):
+        """
+        Sets the sound volume to the given percentage [0-100] by calling
+        ``amixer -q set <channel> <pct>%``.
+        If the channel is not specified, it tries to determine the default one
+        by running ``amixer scontrols``. If that fails as well, it uses the
+        ``Playback`` channel, as that is the only channel on the EV3.
+        """
+        if channel is None:
+            # Get default channel as the first one that pops up in
+            # 'amixer scontrols' output, which contains strings in the
+            # following format:
+            #
+            #     Simple mixer control 'Master',0
+            #     Simple mixer control 'Capture',0
+            out = check_output(['amixer', 'scontrols']).decode()
+            m = re.search("'(?P<channel>[^']+)'", out)
+            if m:
+                channel = m.group('channel')
+            else:
+                channel = 'Playback'
+
+        cmd_line = '/usr/bin/amixer -q set {0} {1:d}%'.format(channel, pct)
+        Popen(cmd_line, shell=True).wait()