There is a class matplotlib.axes.AxesSubplot, but the module matplotlib.axes has no attribute AxesSubplot

Heh. That’s because there is no AxesSubplot class.. until one is needed, when one is built from SubplotBase. This is done by some magic in axes.py:

def subplot_class_factory(axes_class=None):
    # This makes a new class that inherits from SubplotBase and the
    # given axes_class (which is assumed to be a subclass of Axes).
    # This is perhaps a little bit roundabout to make a new class on
    # the fly like this, but it means that a new Subplot class does
    # not have to be created for every type of Axes.
    if axes_class is None:
        axes_class = Axes

    new_class = _subplot_classes.get(axes_class)
    if new_class is None:
        new_class = new.classobj("%sSubplot" % (axes_class.__name__),
                                 (SubplotBase, axes_class),
                                 {'_axes_class': axes_class})
        _subplot_classes[axes_class] = new_class

    return new_class

So it’s made on the fly, but it’s a subclass of SubplotBase:

>>> import matplotlib.pyplot as plt
>>> fig = plt.figure()
>>> ax = fig.add_subplot(111)
>>> print type(ax)
<class 'matplotlib.axes.AxesSubplot'>
>>> b = type(ax)
>>> import matplotlib.axes
>>> issubclass(b, matplotlib.axes.SubplotBase)
True