ffmpeg with Popen (python) on Windows - python-3.x

Question

I am trying to use ffmpeg with Popen. The ffmpeg command I am trying works on cmd but gives me error with Popen.
I am using the standalone ffmpeg .exe:
ffmpeg -f gdigrab -offset_x 10 -offset_y 20 -show_region 1 -i desktop -video_size 1536x864 -b:v 2M -maxrate 1M -bufsize 1M -tune fastdecode -crf 15 -preset ultrafast -pix_fmt yuv420p -r 25 <path>/video.mov -qp 1 -y -an
This gives me Invalid argument, but if I remove the last parameters in order to make the output the last thing on the string, I get a different error:
Output file #0 does not contain any stream
I tried to use -f dshow -i video="UScreenCapture" instead of the gdigrab, but both give me the same error with and without the parameters in the end.
Both commands work on command line.
On command line this ffmpeg -list_devices true -f dshow -i dummy returns this:
[dshow # 000001b24fa6a300] DirectShow video devices (some may be both video and audio devices)
[dshow # 000001b24fa6a300] "Integrated Webcam"
[dshow # 000001b24fa6a300] Alternative name "#device_pnp_\\?\usb#vid_1bcf&pid_2b8a&mi_00#6&2c03619a&0&0000#{65e8773d-8f56-11d0-a3b9-00a0c9223196}\global"
[dshow # 000001b24fa6a300] "UScreenCapture"
[dshow # 000001b24fa6a300] Alternative name "#device_sw_{860BB310-5D01-11D0-BD3B-00A0C911CE86}\UScreenCapture"
[dshow # 000001b24fa6a300] DirectShow audio devices
[dshow # 000001b24fa6a300] "Microphone (Realtek Audio)"
[dshow # 000001b24fa6a300] Alternative name "#device_cm_{33D9A762-90C8-11D0-BD43-00A0C911CE86}\wave_{35EBFC89-7B09-4557-8032-85AA0B688FE9}"
But on popen I can't check it:
-list_devices true -f dshow -i dummy: Invalid argument
For the python part of the code I am using this:
p = subprocess.Popen([getPathForFile("windows/ffmpeg").replace('\\','/'), " -f gdigrab -offset_x 10 -offset_y 20 -show_region 1 -i desktop -video_size 1536x864 -b:v 2M -maxrate 1M -bufsize 1M -tune fastdecode -crf 15 -preset ultrafast -pix_fmt yuv420p -r 25 -qp 1 -y -an "+ path.replace('\\\\','/').replace('\\','/')+"video.mov"], shell=True)
The getPathForFile is a custom function that returns the path. this is correct, mainly because the errors I am getting are from the ffmpeg, so...
I am on a Windows 10. FFmpeg 4.0. Python 3.5.
Any ideas why am I getting these errors on Popen but not on command line and how to fix them? (mainly the second error)

Answer 1

Put each argument in its own string and turn off the shell.
Like this:
import subprocess
import os
cmd = ["-f", "gdigrab", "-offset_x", "10", "-offset_y", "20",
"-show_region", "1", "-video_size", "1536x864", "-i", "desktop",
"-b:v", "2M", "-maxrate", "1M", "-bufsize", "1M", "-tune", "fastdecode",
"-preset", "ultrafast", "-pix_fmt", "yuv420p",
"-r", "25", "-qp", "1", "-y", "-an",
os.path.join(path, "video.mov")]
p = subprocess.Popen([getPathForFile("windows/ffmpeg")]+cmd)
p.communicate()