How to write python piecewise function?

#!/usr/bin/python
# -*- coding:utf-8 -*-
import numpy as np
import matplotlib.pyplot as plt
#绘制分段函数：y=4sin(4πt)-sgn(t-0.3)-sgn(0.72-t)
def sgn(x):
    if x > 0:
        return 1
    elif x < 0:
        return -1
    else:
        return 0
t = np.arange(0, 1, 0.01)
y = []
for i in t:
    y_1 = 4 * np.sin(4 * np.pi * i) - sgn(i - 0.3) - sgn(0.72 - i)
    y.append(y_1)
plt.plot(t, y)
plt.xlabel("t")
plt.ylabel("y")
plt.title("Heavsine")
plt.show()

#!/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, &#39;b&#39;, label=&#39;type&#39;)
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()