Return a counter on the channel ch.

        ch: string or integer.
            The channel index number or channel name.

        part: int or None
            The 0-based enumeration of a True part to return. This
            has an effect whether or not the mask or filter is turned
            on. Raise IndexError if the part does not exist.

        See `Counter
        <https://docs.python.org/2.7/library/collections.html#counter-objects>`_
        for the counter object returned.

def counter(self, ch, part=None):
        """Return a counter on the channel ch.

        ch: string or integer.
            The channel index number or channel name.

        part: int or None
            The 0-based enumeration of a True part to return. This
            has an effect whether or not the mask or filter is turned
            on. Raise IndexError if the part does not exist.

        See `Counter
        <https://docs.python.org/2.7/library/collections.html#counter-objects>`_
        for the counter object returned.

        """
        return Counter(self(self._key(ch), part=part))