OSError: [WinError 193] %1 is not a valid Win32 application

The error is pretty clear. The file hello.py is not an executable file. You need to specify the executable:

subprocess.call(['python.exe', 'hello.py', 'htmlfilename.htm'])

You'll need python.exe to be visible on the search path, or you could pass the full path to the executable file that is running the calling script:

import sys
subprocess.call([sys.executable, 'hello.py', 'htmlfilename.htm'])

I got the same error while I forgot to use shell=True in the subprocess.call.

subprocess.call('python modify_depth_images.py', shell=True)

Running External Command

To run an external command without interacting with it, such as one would do with os.system(), Use the call() function.

import subprocess

Simple command subprocess.call(['ls', '-1'], shell=True)

Python installers usually register .py files with the system. If you run the shell explicitly, it works:

import subprocess
subprocess.call(['hello.py', 'htmlfilename.htm'], shell=True)
# --- or ----
subprocess.call('hello.py htmlfilename.htm', shell=True)

You can check your file associations on the command line with

C:\>assoc .py
.py=Python.File

C:\>ftype Python.File
Python.File="C:\Python27\python.exe" "%1" %*