from pylab import *  xpoints = arange(-6,6,0.05) ypoints = arange(-6,6,0.05) X,Y = meshgrid(xpoints, ypoints)  circularMask = False  areaRadius = 4  # order of the magnet n = 3  def func(x,y):     # the function to draw     return ((x + 1j * y)**(n)).real  func = vectorize(func)  V = func(X,Y)   # mask points which we don't want to draw if circularMask:     # circular mask     distance = sqrt(X**2 + Y**2)     V = ma.masked_where(distance > areaRadius, V) else:     # polygonal mask      # principal directions are at  (i + 0.5) / (2n) * 2pi     #      for i in range(2*n):         angle = (i + 0.5) / float(2*n) * 2*pi              # define a straight angle perpendicular to angle         # mask all points on one side of this line         anchor_x = areaRadius * cos(angle)         anchor_y = areaRadius * sin(angle)          normal_x = cos(angle)         normal_y = sin(angle)                  def acceptFunc(x,y):             value = (x - anchor_x) * normal_x + (y - anchor_y) * normal_y             return value > 0                  acceptFunc = vectorize(acceptFunc)                  V = ma.masked_where(acceptFunc(X,Y), V)                    if True:     # levels equidistant in function value      V /= V.max()      levels = arange(-2,2,0.05)  else:     # levels equidistant on x and y axis      # determine the levels to draw from values on one of the axes      levels = [ float(func(x,0)) for x in arange(min(xpoints), max(xpoints),0.50) ] + \         [ float(func(0,y)) for y in arange(min(ypoints), max(ypoints),0.50) ]     levels = sorted(list(set(levels)))       figure(figsize=(6,6));  Q = contour(X,Y, V, colors=  'black', linestyles = 'solid',      levels = levels ) axis([-5,5,-5,5]) xlabel("x coordinate") ylabel("y coordinate")  # mask points which we don't want to draw if not circularMask:     # polygonal mask      # principal directions are at  (i + 0.5) / (2n) * 2pi     #      for i in range(2*n):         angle = (i + 0.5) / float(2*n) * 2*pi                  if i % 2:             label = "N"             color = 'red'         else:             label = "S"             color = 'green'                  anchor_x = 1.1 * areaRadius * cos(angle)         anchor_y = 1.1 * areaRadius * sin(angle)          text(anchor_x, anchor_y, label, size = 20, color = color,              horizontalalignment='center',              verticalalignment='center')