How can I run an ffmpeg command in a python script?

Question

I want to overlay a transparent video over top of an image using ffmpeg and python

I am able to do this successfully through terminal, but I cannot get ffmpeg commands to work in python. The following command produces the result that I want in terminal when I am in the directory with the files there.

ffmpeg -i head1.png -i hdmiSpitting.mov -filter_complex "[0:v][1:v] overlay=0:0" -pix_fmt yuv420p -:a copy output3.mov

In python, my code is simple:

import os
import subprocess
command = "ffmpeg -i head1.png -i hdmiSpitting.mov -filter_complex \"[0:v][1:v] overlay=0:0\" -pix_fmt yuv420p -c:a copy output3.mov"
subprocess.call(command,shell=True)

The code runs, there is no indication of an error, but no output is produced.

What am I missing here?

Answer 1

In Windows one line with spaces should work, but in Linux we have to pass the arguments as list.

• We can build the command as a list:

   command = ['ffmpeg', '-i', 'head1.png', '-i', 'hdmiSpitting.mov', '-filter_complex', '[0:v][1:v]overlay=0:0', '-pix_fmt', 'yuv420p', '-c:a', 'copy', 'output3.mov']
  

• We may also use shlex.split:

   import shlex
   command = shlex.split('ffmpeg -i head1.png -i hdmiSpitting.mov -filter_complex "[0:v][1:v] overlay=0:0" -pix_fmt yuv420p -c:a copy output3.mov')
  

---

Adding -y argument:
If the output file output3.mov already exists, FFmpeg prints a message:
File 'output3.mov' already exists. Overwrite? [y/N]
And waits for the user to press y.
In some development environments we can't see the message.
Add -y for overwriting the output if already exists (without asking):

command = shlex.split('ffmpeg -y -i head1.png -i hdmiSpitting.mov -filter_complex "[0:v][1:v] overlay=0:0" -pix_fmt yuv420p -c:a copy output3.mov')

---

Path issues:
There are cases when ffmpeg executable is not in the execution path.
Using full path may be necessary.
Example for Windows (assuming ffmpeg.exe is in c:\FFmpeg\bin):

command = shlex.split('c:\\FFmpeg\\bin\\ffmpeg.exe -y -i head1.png -i hdmiSpitting.mov -filter_complex "[0:v][1:v] overlay=0:0" -pix_fmt yuv420p -c:a copy output3.mov')

In Linux, the default path is /usr/bin/ffmpeg.

---

Using shell=True is not recommended and considered "unsafe".
For details see Security Considerations.
The default is False, so we may use subprocess.call(command).

Note: subprocess.run supposes to replace subprocess.call.
See this post for details.

---

Creating log file by adding -report argument:
In some development environments we can't see FFmpeg messages, which are printed to the console (written to stderr).
Adding -report argument creates a log file with name like ffmpeg-20220624-114156.log.
The log file may tell us what went wrong when we can't see the console.

Example:

import subprocess
import shlex
subprocess.run(shlex.split('ffmpeg -y -i head1.png -i hdmiSpitting.mov -filter_complex "[0:v][1:v] overlay=0:0" -pix_fmt yuv420p -c:a copy output3.mov -report'))

Answer 2

I ended up using os.system() instead of subprocess and got the results I wanted before returning to see answers on this question. The answer from Rotem is incredibly useful and does solve my issue as well, with the added information of -y parameter.

I'll paste my entire code here, as it may be useful to someone in the future.

import os

os.chdir('/Users/Todd/Desktop/ffmpeg')
background = "01Background/Untitled_Artwork7.png"
backgear = "02Backgear/Surfboard-01.png"
head = "03Head/Goldhead_Goldshell1.png"
eye = "04eye/Keye-01.png"
outfit = "05Outfit/Summershirt-01.png"
headgear = "06Headgear/PopejoyHair_Goldshell1.png"
mouth = "hdmiSpitting.mov"
frontgear = "08Frontgear/TreePot-01.png"


# takes a list of 3 or more files and creates ffmpeg command to overlay them in order
# the first element in the list will be the lowest Z element (farthest back)
def generateCommand(files = []):
    command = "ffmpeg"
    i = 0
    count = 0
    alphabet = ["a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m", "n", "o"]
    for file in files:
        command += " -i " + file
        count += 1
    command += " -filter_complex \"[0][1]overlay[a];["
    while i < count-3: 
        command += alphabet[i] + "][" + str(i+2) + "]overlay[" + alphabet[i+1] + "];["
        i += 1
    command += alphabet[i] + "][" + str(i+2) + "]overlay\""
    command += " -pix_fmt yuv420p -c:a copy output3.mov"
    return command

# Takes two files and overlays file1 over file2
# This is a separate function because of the different command syntax for less than 3 files
def overlayTwoLayers(file1, file2):
    command = "ffmpeg -i " + file1 + " -i " + file2 + " -filter_complex \"[0:v][1:v] overlay=0:0\" -pix_fmt yuv420p -c:a copy output3.mov"
    os.system(command)

# Call this function with a list of files you want to be compiled
def generateImage(files):
    command = generateCommand(files)
    os.system(command)

files = []
files.append(background)
files.append(backgear)
files.append(head)
files.append(eye)
files.append(outfit)
files.append(headgear)
files.append(mouth)
files.append(frontgear)

generateImage(files)


print("done")

Answer 3

subprocess.call will get the status code but not output

may be you can use subprocess.Popen

In [1]: import subprocess                                                                                                                              

In [2]: res=subprocess.Popen("ifconfig|grep 192",shell=True,stdout=subprocess.PIPE)
   ...: res.stdout.read()                                                                                                                              
Out[2]: b'          inet addr:192.168.45.134  Bcast:192.168.45.255  Mask:255.255.255.0\n'