ffmpy is the simplest ffmpeg wrapper one can imagine. Under the hood it uses Python's subprocess module to run ffmpeg executable.

You guessed it:

pip install ffmpy

import ffmpy
ff = ffmpy.FFmpeg(
    inputs={'input.mp4': None},
    outputs={'output.avi': None}
)
ff.run()

This will take input.mp4 file in the current directory as the input, change the video container from MP4 to AVI without changing any other video parameters and create a new output file output.avi in the current directory.

http://ffmpy.readthedocs.io/en/latest