Can't set video stream time_base

Created by: hubeichenxing

Hi, i have try to set video stream time_base with code bellow:

output = av.open("out.mp4", 'w')
stream = output.add_stream('h264', 24)
stream.pix_fmt = 'yuv420p'
stream.height = im_height
stream.width = im_width
stream.time_base = Fraction(1, 24)
print(stream.time_base)
print(stream.codec_context.time_base)

The command line output:

None
1/24

It means that the stream.time_base is not be assigned but stream.codec_context.time_base get it. I use the container output to save a video, the time_base of packets is 1/12288 or other number, not 1/24. I want to save a video that the time_base of AVStream set with me, how can i do ?