// Note that plt.gca().figure == figure
... = FigureCanvasTkAgg(figure, ...)
plt.sca(plt.gca())

class FigureCanvasBase:
    def __init__(self, figure):
        ...
        figure.set_canvas(self)
        self.figure = figure
        **# Here it is setting canvas.manager to None!!**
        self.manager = None
        ...

def sca(ax):
    """
    Set the current Axes to *ax* and the current Figure to the parent of *ax*.
    """
    if not hasattr(ax.figure.canvas, "manager"):
        raise ValueError("Axes parent figure is not managed by pyplot")
      **# !!! ax.figure.canvas.manager is None!**
    _pylab_helpers.Gcf.set_active(ax.figure.canvas.manager)
    ax.figure.sca(ax)

def sca(ax):
    """
    Set the current Axes instance to *ax*.

    The current Figure is updated to the parent of *ax*.
    """
    managers = _pylab_helpers.Gcf.get_all_fig_managers()
    for m in managers:
        if ax in m.canvas.figure.axes:
            _pylab_helpers.Gcf.set_active(m)
            m.canvas.figure.sca(ax)
            return
    raise ValueError("Axes instance argument was not found in a figure")