np.vectorize quirk

I was being lazy, and had to code up a piecewise function. Rather than use the proper array tools, I used np.vectorize instead but somehow got weird results:

import numpy as np

def f(x):
    if x < .5:
        return 1
    else:
        return .4

points = np.linspace(0, 1, 5)

points_flipped = np.flip(points)

vec_f = np.vectorize(f)

print(vec_f(points))

print(vec_f(points_flipped))

returns [1 1 0 0 0]
[0.4 0.4 0.4 1. 1. ]


Tuorns out np.vectorize has the property that

The output type is determined by evaluating the first element of the input, unless it is specified

Took me a minute to figure this out. RTFM.