The WebAgg backend requires Tornado - python-3.x

Question

I have found this python code for bezier_curves in Github and the link of the code is shown below:
https://gist.github.com/Juanlu001/7284462
When I run the code I always get this error as shown below:
Traceback (most recent call last):
File "C:\Users\raineen\AppData\Local\Programs\Python\Python37\lib\site-packages\matplotlib\backends\backend_webagg.py", line 27, in <module>
import tornado
ModuleNotFoundError: No module named 'tornado'
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "C:\Users\raineen\Desktop\Bezier.py", line 8, in <module>
import matplotlib.pyplot as plt
File "C:\Users\raineen\AppData\Local\Programs\Python\Python37\lib\site-packages\matplotlib\pyplot.py", line 2372, in <module>
switch_backend(rcParams["backend"])
File "C:\Users\raineen\AppData\Local\Programs\Python\Python37\lib\site-packages\matplotlib\pyplot.py", line 207, in switch_backend
backend_mod = importlib.import_module(backend_name)
File "C:\Users\raineen\AppData\Local\Programs\Python\Python37\lib\importlib\__init__.py", line 127, in import_module
return _bootstrap._gcd_import(name[level:], package, level)
File "C:\Users\raineen\AppData\Local\Programs\Python\Python37\lib\site-packages\matplotlib\backends\backend_webagg.py", line 29, in <module>
raise RuntimeError("The WebAgg backend requires Tornado.")
RuntimeError: The WebAgg backend requires Tornado.
I used to install this Tornado by the line below and still gives the same error:
python -m pip install tornado
So how could I fix the above error?
The code below:
import matplotlib
matplotlib.use('webagg')
import numpy as np
from scipy.special import binom
import matplotlib.pyplot as plt
from matplotlib.lines import Line2D
class BezierBuilder(object):
"""Bézier curve interactive builder.
"""
def __init__(self, control_polygon, ax_bernstein):
"""Constructor.
Receives the initial control polygon of the curve.
"""
self.control_polygon = control_polygon
self.xp = list(control_polygon.get_xdata())
self.yp = list(control_polygon.get_ydata())
self.canvas = control_polygon.figure.canvas
self.ax_main = control_polygon.axes
self.ax_bernstein = ax_bernstein
# Event handler for mouse clicking
self.cid = self.canvas.mpl_connect('button_press_event', self)
# Create Bézier curve
line_bezier = Line2D([], [],
c=control_polygon.get_markeredgecolor())
self.bezier_curve = self.ax_main.add_line(line_bezier)
def __call__(self, event):
# Ignore clicks outside axes
if event.inaxes != self.control_polygon.axes:
return
# Add point
self.xp.append(event.xdata)
self.yp.append(event.ydata)
self.control_polygon.set_data(self.xp, self.yp)
# Rebuild Bézier curve and update canvas
self.bezier_curve.set_data(*self._build_bezier())
self._update_bernstein()
self._update_bezier()
def _build_bezier(self):
x, y = Bezier(list(zip(self.xp, self.yp))).T
return x, y
def _update_bezier(self):
self.canvas.draw()
def _update_bernstein(self):
N = len(self.xp) - 1
t = np.linspace(0, 1, num=200)
ax = self.ax_bernstein
ax.clear()
for kk in range(N + 1):
ax.plot(t, Bernstein(N, kk)(t))
ax.set_title("Bernstein basis, N = {}".format(N))
ax.set_xlim(0, 1)
ax.set_ylim(0, 1)
def Bernstein(n, k):
"""Bernstein polynomial.
"""
coeff = binom(n, k)
def _bpoly(x):
return coeff * x ** k * (1 - x) ** (n - k)
return _bpoly
def Bezier(points, num=200):
"""Build Bézier curve from points.
"""
N = len(points)
t = np.linspace(0, 1, num=num)
curve = np.zeros((num, 2))
for ii in range(N):
curve += np.outer(Bernstein(N - 1, ii)(t), points[ii])
return curve
if __name__ == '__main__':
# Initial setup
fig, (ax1, ax2) = plt.subplots(1, 2, figsize=(12, 5))
# Empty line
line = Line2D([], [], ls='--', c='#666666',
marker='x', mew=2, mec='#204a87')
ax1.add_line(line)
# Canvas limits
ax1.set_xlim(0, 1)
ax1.set_ylim(0, 1)
ax1.set_title("Bézier curve")
# Bernstein plot
ax2.set_title("Bernstein basis")
# Create BezierBuilder
bezier_builder = BezierBuilder(line, ax2)
plt.show()

Answer 1

You can change your backend to one that is supported in Windows without using a third-party library such as Tornado. Change matplotlib.use('webagg') to matplotlib.use('tkagg').

Answer 2

Looks like you are missing an import statement for tornado after installing it with pip.
But you may have another issue as well. According to the tornado documentation here, it requires the following versions of python:
Prerequisites: Tornado runs on Python 2.6, 2.7, 3.2, and 3.3
You are running python 3.7 so that could be the reason. Try running it in one of the versions above.