IPython：音频：连接列表以添加和弦会导致一个和弦而不是几个一个一个地演奏：`+=` vs `append`

Question

I am completetly new to IPython hence I am sorry in case this is totally obvious:

import IPython.display as ipd
import scipy
from scipy import signal as sp
import math
import numpy as np

I defined two functions which should help me generate chords:

def f_k(f_0, k):
    return f_0*2**((k%12)/12)

def h_k(k, f0, t):
    return math.sin(2*math.pi*f_k(f0, k)*t)

F = 44000
T = 2
f0 = 440
N = F*T

I define H0 :

H0 = []
for k in [0,4,7]:
    H0.append([h_k(k, f0, t) for t in np.linspace(0, T, N)])
ipd.Audio(H0, rate=F)

and it plays 2 seconds as expected because of the discretization through linspace.

I defined a few several chords and I wanted to concat the lists to get several chords (I expected the sound to be 8 secs long)

H5 = []
for k in [5,9,12]:
    H5.append([h_k(k, f0, t) for t in np.linspace(0, T, N)])
    

H7 = []
for k in [7, 11, 14]:
    H7.append([h_k(k, f0, t) for t in np.linspace(0, T, N)])


H9 = []
for k in [5,9,16]:
    H9.append([h_k(k, f0, t) for t in np.linspace(0, T, N)])

added_sample = []
for h in [H0, H7, H9, H5]:
    added_sample += h

ipd.Audio(added_sample, rate=F)

Yet the sound is somehow 2secs long. Could someone explain how to add chords insted of 'layering' them? Any hint would be greatly appreciated!

Answer 1

Maybe this small example will help you see the different between append and += :

For each we run:

In [54]: alist = [1,2,3]; blist = [4,5]

append adds one item to the list:

In [55]: alist.append(blist); alist
Out[55]: [1, 2, 3, [4, 5]]

+= is a list join:

In [57]: alist += blist; alist
Out[57]: [1, 2, 3, 4, 5]

and is the equivalent of extend :

In [59]: alist.extend(blist); alist
Out[59]: [1, 2, 3, 4, 5]

The non-in-place version of extend:

In [61]: alist + blist
Out[61]: [1, 2, 3, 4, 5]