Subprocess.call() and gunzip error

Question

I'm trying to use Python's package subprocess to gunzip and gzip files. I'm using the following command:

subprocess.call(['gunzip', a_path+myfile_gz], shell=True)

Then I get the error message:

gzip: compressed data not read from a terminal. Use -f to force decompression.

So I typed:

subprocess.call(['gunzip', '-f', a_path+myfile_gz], shell=True)

And I get exactly the same error message. If I type shell=False, it's worse.

It's really weird because if I do it from my terminal, it works fine! Does anybody knows what's wrong?

Answer 1

The problem is that you're passing shell=True, so you're executing

sh -c gunzip /path/to/file.gz

(assuming a_path+myfile_gz is "/path/to/file.gz"). In other words, because the shell parameter is True, the first element of the command list is a shell snippet; the other arguments are arguments to that shell snippet (here just $0). This is explained in the documentation.

If you leave out shell=True, you'll get working but not good code. Don't manipulate paths as strings; use the functions in the os.path. That a_path+myfile_gz should probably be os.path.join(a_path, myfile_gz) instead.

In any case, Python has built-in support for gzip. Don't use an external command.