[Numpy-discussion] runtime warning for where

[David Pine]

The program at the bottom of this message returns the following runtime warning:

python test.py
test.py:5: RuntimeWarning: invalid value encountered in divide
 return np.where(x==0., 1., np.sin(x)/x)

The function works correctly returning
x = np.array([  0.,   1.,   2.,   3.,   4.,   5.,   6.,   7.,   8.,   9.,  10.])
y = np.array([ 1.        ,  0.84147098,  0.45464871,  0.04704   , -0.18920062,
      -0.19178485, -0.04656925,  0.09385523,  0.12366978,  0.04579094,
      -0.05440211])

The runtime warning suggests that np.where evaluates np.sin(x)/x at all x, including x=0, even though the np.where function returns the correct value of 1. when x is 0.  This seems odd to me.  Why issue a runtime warning? Nothing is wrong.  Moreover, I don't recall numpy issuing such warnings in earlier versions.

import numpy as np
import matplotlib.pyplot as plt

def sinc(x):
   return np.where(x==0., 1., np.sin(x)/x)

x = np.linspace(0., 10., 11)
y = sinc(x)

plt.plot(x, y)
plt.show()