python分段函数如何编写？

#!/usr/bin/python

# -*- coding:utf-8 -*-

import numpy as np

import matplotlib.pyplot as plt

def sgn(value):

    if value < 4:

        return 20

    else:

        return 15

plt.figure(figsize=(6, 4))

x = np.linspace(0, 8, 100)

y = np.array([])

for v in x:

    y = np.append(y, np.linspace(sgn(v), sgn(v), 1))

l = plt.plot(x, y, 'b', label='type')

plt.legend()

plt.show()

#!/usr/bin/python

# -*- coding:utf-8 -*-

import numpy as np

import matplotlib.pyplot as plt

def triangle_wave(x, c, c0, hc):

   x = x - int(x)  #三角波周期为1 因此只取小数部分进行计算

   if x < c0:

       return x / c0 * hc

   elif x >= c:

       return 0.0

   else:

       return (c-x)/(c-c0)*hc

x = np.linspace(0, 2, 1000)

y = np.array([triangle_wave(t, 0.6, 0.4, 1.0) for t in x])

plt.figure()

plt.plot(x, y)

plt.ylim(-0.2, 1.2)   #限制y的范围

plt.show()