Is a template for defining a class inside a function acceptable?

Question

Here is an example of creating a radar chart in the documentation for Matplotlib, and I'm trying to understand the design. http://matplotlib.org/examples/api/radar_chart.html

""" Example of creating a radar chart (aka a spider or star chart) [1]_. Although this example allows a frame of either 'circle' or 'polygon', polygon frames don't have proper gridlines (the lines are circles instead of polygons). It possible to get a polygon grid by setting GRIDLINE_INTERPOLATION_STEPS in matplotlib.axis to the desired number of vertices, but the orientation of the polygon is not aligned with the radial axes. .. [1] http://en.wikipedia.org/wiki/Radar_chart """ import numpy as np import matplotlib.pyplot as plt from matplotlib.path import Path from matplotlib.spines import Spine from matplotlib.projections.polar import PolarAxes from matplotlib.projections import register_projection def radar_factory(num_vars, frame='circle'): """Create a radar chart with `num_vars` axes. This function creates a RadarAxes projection and registers it. Parameters ---------- num_vars : int Number of variables for radar chart. frame : {'circle' | 'polygon'} Shape of frame surrounding axes. """ # calculate evenly-spaced axis angles theta = np.linspace(0, 2*np.pi, num_vars, endpoint=False) # rotate theta such that the first axis is at the top theta += np.pi/2 def draw_poly_patch(self): verts = unit_poly_verts(theta) return plt.Polygon(verts, closed=True, edgecolor='k') def draw_circle_patch(self): # unit circle centered on (0.5, 0.5) return plt.Circle((0.5, 0.5), 0.5) patch_dict = {'polygon': draw_poly_patch, 'circle': draw_circle_patch} if frame not in patch_dict: raise ValueError('unknown value for `frame`: %s' % frame) class RadarAxes(PolarAxes): name = 'radar' # use 1 line segment to connect specified points RESOLUTION = 1 # define draw_frame method draw_patch = patch_dict[frame] def fill(self, *args, **kwargs): """Override fill so that line is closed by default""" closed = kwargs.pop('closed', True) return super(RadarAxes, self).fill(closed=closed, *args, **kwargs) def plot(self, *args, **kwargs): """Override plot so that line is closed by default""" lines = super(RadarAxes, self).plot(*args, **kwargs) for line in lines: self._close_line(line) def _close_line(self, line): x, y = line.get_data() # FIXME: markers at x[0], y[0] get doubled-up if x[0] != x[-1]: x = np.concatenate((x, [x[0]])) y = np.concatenate((y, [y[0]])) line.set_data(x, y) def set_varlabels(self, labels): self.set_thetagrids(np.degrees(theta), labels) def _gen_axes_patch(self): return self.draw_patch() def _gen_axes_spines(self): if frame == 'circle': return PolarAxes._gen_axes_spines(self) # The following is a hack to get the spines (ie the axes frame) # to draw correctly for a polygon frame. # spine_type must be 'left', 'right', 'top', 'bottom', or `circle`. spine_type = 'circle' verts = unit_poly_verts(theta) # close off polygon by repeating first vertex verts.append(verts[0]) path = Path(verts) spine = Spine(self, spine_type, path) spine.set_transform(self.transAxes) return {'polar': spine} register_projection(RadarAxes) return theta 

What is the reason for this pattern?