Issues with Python Popen() (Syntax?)

Question

have a small issue with a program I am trying to launch from a Python script via Popen() (I understand Popen() may not be ideal, but I am working with somewhat of a template used in other instances, and want to follow convention).

I am a bit confused, as I can't seem to get the following to run:

root = os.getcwd()

bin = 'my_executable.exe'

bin_fullpath = os.path.join(root,bin)

params = 'Option C -f Module -y -q'

p = subprocess.Popen([bin_fullpath,params])

out = p.communicate()

The program launches, but exits with error code 1 (I checked with check_call).

However, when I forgo the above method, and simply provide the entire string I need to run, as follows:

subprocess.Popen(r'C:\Users\me\Desktop\path\to\tool\my_executable.exe Option C -f Module -y -q')

The program executes as expected. Obviously I have something wrong with the sytntax, but I can't figure out what . . .

Any insight would be greatly appreciated!

Answer 1

When you use the "list of arguments" format, each one has to be its own string, as a separate member of the list, like this:

params = ['Option', 'C', '-f', 'Module', '-y', '-q']
p = subprocess.Popen([bin_fullpath, *params])

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When you put them all in a single string, you're telling subprocess they're all one big argument.¹ So, rather than being the rough equivalent of this command line:

C:\Users\me\Desktop\path\to\tool\my_executable.exe Option C -f Module -y -q

… it's the rough equivalent of this one:

C:\Users\me\Desktop\path\to\tool\my_executable.exe "Option C -f Module -y -q"

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If it's not clear why those are different, consider these examples instead:

fix.exe "My Pictures\picture1.jpg"
fix.exe My Pictures\picture1.jpg

The first one is fixing one picture, My Pictures\picture1.jpg. The second is fixing two pictures, My, and Pictures\picture1.jpg.

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For more details on the args argument, see Frequently Used Arguments. Notice the "one big string" version is actually not valid without shell=True—even though it happens to usually work on Windows.

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_{1. Things are a little more complicated than this on Windows, because subprocess actually has to take all of the args and work out how to put them together in a string so that they can be parsed back into the actual separate values the way you asked for them. But never mind that.}