How can I draw more lines?(python sympy)

Question

when I type,

from sympy import *
from sympy.plotting import *
from sympy.plotting.plot import *


x, y = symbols('x y')
f = Function('f')
g = Function('g')

f = 1/((x+0.3)**2 + y**2) - 1/((x-0.3)**2 + y**2 )
g = (x+0.3)/sqrt((x+0.3)**2 + y**2) - (x-0.3)/sqrt((x-0.3)**2 + y**2)


p0 = Plot(ContourSeries(f,(x,-1.5,1.5),(y,-1.5,1.5)))
p1 = Plot(ContourSeries(g,(x,-1.5,1.5),(y,-1.5,1.5)))

p0.show()
p1.show()

p0 shows like first picture. The number of line is few.

I want to draw more line like second picture.

What is the solution?

Answer 1

The ContourSeries class doesn't expose the right info to do this but it is easy to extend. I called the parameter levels, is passed directly to matplotlib.

class MyContourSeries(ContourSeries):

    def __init__(self, expr, var_start_end_x, var_start_end_y, **kwargs):
        super(MyContourSeries, self).__init__(expr, var_start_end_x, var_start_end_y)
        self.nb_of_points_x = kwargs.get('nb_of_points_x', 50)
        self.nb_of_points_y = kwargs.get('nb_of_points_y', 50)
        self.levels = kwargs.get('levels', 5)

    def get_meshes(self):
        mesh_x, mesh_y, f = super().get_meshes()
        return (mesh_x, mesh_y, f, self.levels)

This should work with sympy 1.2 and 1.3 at least. Sympy plotting is using matplotlibs countour for this serie (https://github.com/sympy/sympy/blob/master/sympy/plotting/plot.py#L909)

    elif s.is_contour:
            self.ax.contour(*s.get_meshes())

which signature is matplotlib.pyplot.contour([X, Y,] Z, [levels], **kwargs)

As with the matplotlib contour function, you can used a fixed number of levels

p0 = Plot(MyContourSeries(f, (x, -1.5, 1.5), (y, -1.5, 1.5),
                      nb_of_points_x=50, nb_of_points_y=50, levels=100))
p0.show()

or pass the levels directly

p0 = Plot(MyContourSeries(f, (x, -1.5, 1.5), (y, -1.5, 1.5),
                      nb_of_points_x=50, nb_of_points_y=50, levels=np.arange(-50,50)))
p0.show()