bismuth

Looks like there are two different problems here:

• How to use process substitution in Python
• How to write pipelines in a more compact way like in the shell

For the first one, I would rewrite your shell code as follows:

from subprocess import Popen, PIPE
dl1 = Popen(["aws", "s3", "cp", path1, "-"], stdout=PIPE)
gunzip1 = Popen(["gunzip"], stdin=dl1.stdout, stdout=PIPE)
gunzip1_fd = gunzip1.stdout.fileno()
dl2 = Popen(["aws", "s3", "cp", path2, "-"], stdout=PIPE)
gunzip2 = Popen(["gunzip"], stdin=dl2.stdout, stdout=PIPE)
gunzip2_fd = gunzip2.stdout.fileno()
cmd = Popen(["cmd", "-1", f"/dev/fd/{gunzip1_fd}", "-2", f"/dev/fd/{gunzip2_fd}"],
    stdout=PIPE, pass_fds=[gunzip1_fd, gunzip2_fd])
gzip = Popen(["gzip"], stdin=cmd.stdout)
upload = Popen(["aws", "s3", "cp", "-", pathOut], stdin=gzip.stdout)
for p in dl1, gunzip1, dl2, gunzip2, cmd, gzip:
   p.stdout.close()
outs, errs = upload.communicate()

This assumes /dev/fd support on your platform. Shells fall back... (read more)