[Python] How can I convert audio to WAVE_FORMAT_PCM using FFmpeg?

I am using Python's wave module to read audio, and using FFmpeg to convert audio from other types to wav. However, I am encountering some problem.

I wrote v.py to generate an silence audio file a.wav

import sys, wave, math
import numpy as np

wave_data = np.zeros(44100).astype(np.short)

f = wave.open('a.wav', 'wb')
f.setnchannels(1)
f.setsampwidth(2)
f.setframerate(96000)
f.writeframes(wave_data.tostring())
f.close()


Then I used FFmpeg to "copy" a.wav to b.wav (though it seems to encode / decode the file), but I can only read a.wav with Python; b.wav cannot be opened.

[user@localhost tmp]$ ffmpeg -i a.wav b.wav
Guessed Channel Layout for Input Stream #0.0 : mono
Input #0, wav, from 'a.wav':
Duration: 00:00:00.46, bitrate: 1536 kb/s
Stream #0:0: Audio: pcm_s16le ([1][0][0][0] / 0x0001), 96000 Hz, mono, s16, 1536 kb/s
Stream mapping:
Stream #0:0 -> #0:0 (pcm_s16le (native) -> pcm_s16le (native))
Press [q] to stop, [?] for help
Output #0, wav, to 'b.wav':
Metadata:
ISFT : Lavf57.71.100
Stream #0:0: Audio: pcm_s16le ([1][0][0][0] / 0x0001), 96000 Hz, mono, s16, 1536 kb/s
Metadata:
encoder : Lavc57.89.100 pcm_s16le
size= 86kB time=00:00:00.45 bitrate=1537.8kbits/s speed= 706x
video:0kB audio:86kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.115646%
[user@localhost tmp]$ python3
Python 3.6.4 (default, Jan 23 2018, 22:25:37)
[GCC 7.2.1 20170915 (Red Hat 7.2.1-2)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import wave
>>> wave.open('a.wav')
<wave.Wave_read object at 0x7efea1c5e550>
>>> wave.open('b.wav')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/usr/lib64/python3.6/wave.py", line 499, in open
return Wave_read(f)
File "/usr/lib64/python3.6/wave.py", line 163, in __init__
self.initfp(f)
File "/usr/lib64/python3.6/wave.py", line 143, in initfp
self._read_fmt_chunk(chunk)
File "/usr/lib64/python3.6/wave.py", line 260, in _read_fmt_chunk
raise Error('unknown format: %r' % (wFormatTag,))
wave.Error: unknown format: 65534
>>>


How should I change the command of FFmpeg to convert the file to WAVE_FORMAT_PCM, so that I can read b.wav with Python?

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