plot a donut with fill or fill_between use pyplot in matplotlib

Question

I want to plot a donut and my script is

import numpy as np
import matplotlib.pyplot as plt
pi,sin,cos = np.pi,np.sin,np.cos

r1 = 1
r2 = 2

theta = np.linspace(0,2*pi,36)

x1 = r1*cos(theta)
y1 = r1*sin(theta)

x2 = r2*cos(theta)
y2 = r2*sin(theta)

How to get a donut with red filled area ?

Answer 1

You can traverse the boundaries of the area in closed curve, and use fill method to fill the area inside this closed area:

import numpy as np
import matplotlib.pyplot as plt

n, radii = 50, [.7, .95]
theta = np.linspace(0, 2*np.pi, n, endpoint=True)
xs = np.outer(radii, np.cos(theta))
ys = np.outer(radii, np.sin(theta))

# in order to have a closed area, the circles
# should be traversed in opposite directions
xs[1,:] = xs[1,::-1]
ys[1,:] = ys[1,::-1]

ax = plt.subplot(111, aspect='equal')
ax.fill(np.ravel(xs), np.ravel(ys), edgecolor='#348ABD')

plt.show()

This can easily be applied to any shape, for example, a pentagon inside or outside of a circle:

Answer 2

You can do this by plotting the top and bottom halves separately:

import numpy as np
import matplotlib.pyplot as plt

inner = 5.
outer = 10.

x = np.linspace(-outer, outer, 1000, endpoint=True)

yO = outer*np.sin(np.arccos(x/outer)) # x-axis values -> outer circle
yI = inner*np.sin(np.arccos(x/inner)) # x-axis values -> inner circle (with nan's beyond circle)
yI[np.isnan(yI)] = 0.                 # yI now looks like a boulder hat, meeting yO at the outer points

ax = plt.subplot(111)
ax.fill_between(x, yI, yO, color="red")
ax.fill_between(x, -yO, -yI, color="red")

plt.show()

Or you can use polar coordinates, though whether this is beneficial depends on the broader context:

import numpy as np
import matplotlib.pyplot as plt

theta = np.linspace(0., 2.*np.pi, 80, endpoint=True)
ax = plt.subplot(111, polar=True)
ax.fill_between(theta, 5., 10., color="red")

plt.show()