how to pass key password to password prompt with gpg2

Question

We were initially using following code to sign gpg packages with gpg v1.41.x.

def RunSubprocessWithPexpect(cmd, passphrase, prompt_passphrase, prompt_succeeded, prompt_failed):
errmsg = None
answers = []
answer = None
if passphrase is None:
  errmsg = "The passphrase is null. Skipping the execution of the signing command."
  return errmsg

try:
    with pexpect.spawn(cmd[0], cmd[1:], timeout=600) as child:
        logging.debug(' '.join(cmd))

        child.expect(prompt_passphrase)
        child.sendline(passphrase)

        answers = [prompt_succeeded, prompt_failed, pexpect.EOF, pexpect.TIMEOUT]
        answer = child.expect(answers[:2])

        child.expect(pexpect.EOF)
        child.close()  # Close the child process
        if child.exitstatus == 0 and answer == 0:
            logging.debug("Command succeeded")
        else:
            logging.error("Command failed: %s" % ' '.join(cmd))
            logging.error("\tanswer='%s', status='%d', output='%s'" %
                  (answers[answer], child.status, child.before))
            errmsg = 'An unexpected error occurred: ' + str(answers[answer])

except (pexpect.EOF, pexpect.TIMEOUT) as e:
    logging.exception("Command failed: %s" % ' '.join(cmd))
    errmsg = 'An unexpected error occurred: ' + str(e)
return errmsg

This was working fine earlier as we were getting following Password prompt on CLI:

"Enter passphrase:" 

and the pexpect library use to pass the password and signing used to succeed.

But now we are upgrading gpg from version 1.41.x to 2.x and so a lot of things have changed...

With gpg2, we can password prompt in some king of GUI instead of cli... so I tried passing --batch --passphrase-fd 0 to the command:

/bin/gpg2 --batch --passphrase-fd 0 -sa --homedir /build/mts/apps/signing/signserver/key/test/test_gpg_AAAAAAA --default-key AAAAAAA --no-random-seed-file --lock-never --y --verbose --detach-sign --digest-algo SHA1 --output /tmp/hello_2.10-2ubuntu2_amd64.deb.sig /build/apps/signing/signserver/test/test_signserver/files/hello_2.10-2ubuntu2_amd64.deb

This prompts output without any text... (no password prompt but just will wait for password in new line and press enter).

This existing code now fails with following error:

Traceback (most recent call last):
  File "/build/toolchain/noarch/pexpect-4.0.1/lib/python3.3/site-packages/pexpect/spawnbase.py", line 144, in read_nonblocking
    s = os.read(self.child_fd, size)
OSError: [Errno 5] Input/output error

During handling of the above exception, another exception occurred:

Can anyone please help how to handle this case?