1        class Radio():                               #1) remove parentheses
 2            def __init__(self):                      #2) constructor must be named __init__
 3                self._powerOn = False
 4                self._volume = 5
 5                self._station = 90.7
 6                self._presets = [ 90.7, 92.3, 94.7, 98.1, 105.7, 107.7 ]
 7                
 8            def togglePower(self):
 9                self._powerOn = not self._powerOn
10                
11            def setPreset(self, ind):
12                self._presets[ind] = self._station   #3) _station is an instance variable
13                
14            def gotoPreset(self, ind):
15                self._station = self._presets[ind]
16                
17            def increaseVolume(self):                #4) Indentation of this line must be consistent with rest of the Radio class definition block
18                self._volume = self._volume + 1
19                
20            def decreaseVolume(self):
21                self._volume = self._volume - 1
22                
23            def increaseStation(self):               #5) Must use colon when starting a new block
24                self.station = self.station + .2
25                
26            def decreaseStation(self):
27                self.station = self.station - .2

The output is

9
15
a is 3, b is 6

Those values remain part of the state of the instance at the onset of the call bar(3). The key difference in this method is that the first two lines assign to local variables a and b. Given the arithmetic, those values will be 6 and 9 respectively, leading to the returned sum of 15 that is displayed on the second line of output.

However, note well that the internal values self._a and self._b remain 3 and 6 respectively after the call to bar. So the final command of print it produces the output a is 3, b is 6 based on those values and the format resulting from the __str__ method.

    def scale(self, factor):
        for k in range(len(self._coords)):
            self._coords[k] *= factor

    def __add__(self, other):
        dim = len(self._coords)
        combined = [self._coords[k] + other._coords[k] for k in	range(dim)]
        return Point(dim, combined)

The simplest approach is to modify the togglePower method so as to read

    def togglePower(self):
      self._powerOn = not self._powerOn
      self._muted = False                 # will not remain muted after power is toggled