Python ffmpeg: overlaying videos is dropping all audio

Question

I am using ffmpeg-python (source) to create the effect where you add a blurred background to fill in the sides of a tall vertical video as shown below:

The problem is that the output has no audio attached. Since the clips are the same, I want to keep the audio from one of the clips in the final output. How can I keep the audio? (I don't want to overlay the audio from both and get an echo effect, however!)

This is the function I'm using:

import ffmpeg
def add_blurred_bg():
    HEIGHT = 720 
    WIDTH = 1280
    in_file = ffmpeg.input('input.mp4')
    probe = ffmpeg.probe('input.mp4')
    video_stream = next((stream for stream in probe['streams'] if stream['codec_type'] == 'video'), None)
    iw=int(video_stream['width'])
    ih=int(video_stream['height'])
    nw = HEIGHT*iw/ih
    (
        ffmpeg
        .overlay(
            in_file.filter('scale', WIDTH, -2).crop(0,(WIDTH*HEIGHT/nw-HEIGHT)/2,WIDTH,HEIGHT).filter('gblur', sigma=40),
            in_file.filter('scale', -2, HEIGHT),
            x=(WIDTH-nw)/2
        )
        .output('output.mp4')
        .run()
    )

Answer 1

Do you have to do it keep the audio style? Can you mix the audio and video? If so, this is a messy, but functioning example:

import ffmpeg
import os

def add_blurred_bg():
    HEIGHT = 720 
    WIDTH = 1280
    inp = 'input.mp4'
    os.system("ffmpeg -i " + inp + " -f mp3 -ab 192000 -vn music.mp3")
    print("extracting audio...")
    in_file = ffmpeg.input(inp)
    probe = ffmpeg.probe('input.mp4')
    video_stream = next((stream for stream in probe['streams'] if stream['codec_type'] == 'video'), None)
    iw=int(video_stream['width'])
    ih=int(video_stream['height'])
    nw = HEIGHT*iw/ih
    (
       ffmpeg
       .overlay(
            in_file.filter('scale', WIDTH, -2).crop(0,(WIDTH*HEIGHT/nw-HEIGHT)/2,WIDTH,HEIGHT).filter('gblur', sigma=40),
            in_file.filter('scale', -2, HEIGHT),
            x=(WIDTH-nw)/2
        )
        .output('outputPartial.mp4')
        .run()
    )
    print("bluring...")
    os.system("ffmpeg -i outputPartial.mp4 -i music.mp3 -shortest -c:v copy -c:a aac -b:a 256k output.mp4")
    print("mixing...")
    os.remove("outputPartial.mp4")
    os.remove("music.mp3")
    print("cleaning up...")
    print("done!")

I don't know why you have that problem, but here is a workaround.

STEP 1: Extract the music

STEP 2: Blur the video

STEP 3: Mix the audio and the video

STEP 4: Clean-Up