python – 为什么numpy fft会返回错误的相位信息？

Matlab的：

fs = 1e4;
dt = 1 / fs;
t = 0:dt:0.5;
F = 1e3;

y = cos(2*pi*F*t);
S = fftshift(fft(y) / length(y));
f_scale = linspace(-1,1,length(y)) * (fs / 2);

a = abs(S);
phi = (angle(S));

subplot(2,1)
plot(f_scale,a)
title('amplitude')

subplot(2,2)
plot(f_scale,phi)
title('phase')

Python：

import numpy as np
import matplotlib.pyplot as plt

fs = 1e4
dt = 1 / fs
t = np.arange(0,0.5,dt)
F = 1e3

y = np.cos(2*np.pi*F*t)
S = np.fft.fftshift(np.fft.fft(y) / y.shape[0])
f_scale = np.linspace(-1,y.shape[0]) * (fs / 2)

a = np.abs(S)
phi = np.angle(S)

plt.subplot(2,title="amplitude")
plt.plot(f_scale,a)

plt.subplot(2,2,title="phase")
plt.plot(f_scale,phi)

plt.show()

matlab output

numpy output