为什么是 int 类型错误？类型错误

我收到一个错误，TypeError: slice indices must be integers or None or have an index method 。追溯就像

Traceback (most recent call last):
  File "test.py", line 24, in <module>
    wavdata = wav[z:q]
TypeError: slice indices must be integers or None or have an __index__ method

我的代码是

#coding:utf-8
import wave
import numpy as np
from pylab import *

def wavread(filename):
    wf = wave.open(filename, "r")
    fs = wf.getframerate()
    x = wf.readframes(wf.getnframes())
    x = np.frombuffer(x, dtype="int16") / 32768.0  # (-1, 1)に正規化
    wf.close()
    return x, float(fs)

if __name__ == "__main__":
    wav, fs = wavread("3_7_9.wav")
    t = np.arange(0.0, len(wav) / fs, 1/fs)

    center = len(wav) // 2  
    cuttime = 0.04         
    z = center-cuttime // 2*fs
    q= center + cuttime // 2*fs
    wavdata = wav[z:q]
    time = t[z:q]

    plot(time * 1000, wavdata)
    xlabel("time [ms]")
    ylabel("amplitude")
    savefig("waveform.png")
    show()

我认为 center & z & q 的变量是 int 类型，所以我真的不明白为什么会发生这个错误。首先，我写了这部分

if __name__ == "__main__":
    wav, fs = wavread("3_7_9.wav")
    t = np.arange(0.0, len(wav) / fs, 1/fs)

    center = len(wav) / 2  
    cuttime = 0.04         
    z = center-cuttime / 2*fs
    q= center + cuttime / 2*fs
    wavdata = wav[z:q]
    time = t[z:q]

    plot(time * 1000, wavdata)
    xlabel("time [ms]")
    ylabel("amplitude")
    savefig("waveform.png")
    show()

所以这些除法部分不是int类型而是float类型，因此我可以理解为什么会发生这个错误。但现在我把这些部分改成了//，所以这些部分是int。那么，我该如何解决这个错误？