Is it possbile to let pexpect output the texts it matches?

Question

I am familiar with expect script so I feel a bit odd when I first use pexpect. Take this simple script as an example,

#!/usr/bin/expect
set timeout 10
spawn npm login
expect "Username:"
send "qiulang2000\r"
expect "Password:"
send "xxxxx\r"
expect "Email:"
send "qiulang@gmail.com\r"
expect "Logged in as"
interact

When run it I will get the following output. It feels natural because that is how I run those commands

spawn npm login
Username: qiulang2000
Password:
Email: (this IS public) qiulang@gmail.com
Logged in as qiulang2000 on https://registry.npmjs.com/.

But when I use pexpect, no matter how I add print(child.after)or print(child.before) I just can't get output like expect, e.g. when I run following command,

#! /usr/bin/env python3
import pexpect
child = pexpect.spawn('npm login')
child.timeout = 10
child.expect('Username:')
print(child.after.decode("utf-8"))
child.sendline('qiulang2000')
child.expect('Password:')
child.sendline('xxxx')
child.expect('Email:')
child.sendline('qiulang@gmail.com')
child.expect('Logged in as')
print(child.before.decode("utf-8"))
child.interact()

I got these output, it feels unnatural because that is not what I see when I run those commands.

Username:
 (this IS public)       qiulang@gmail.com

 qiulang2000 on https://registry.npmjs.com/.

So is it it possbile to achieve the expect script output?

--- update ---

With the comment I got from @pynexj I finally make it work, check my answer below.

Thanks! I finally make it work. A final question, I feel I don't need to call child.interact() at all(unlike expect). Is that right ? — Qiulang, Sep 10 '20 at 09:19
must be py3 related. what the version of your python and pexpect? — pynexj, Sep 10 '20 at 13:20
and, `interact()` is necessary if you want to manually interact with the spawned process. otherwise, you can `expect(EOF)` (just like Expect's `expect eof`). — pynexj, Sep 10 '20 at 13:53
child.logfile_read = sys.stdout.buffer didn't work either and I gave up. I just need a simple ssh login script and didn't expect to hit so much trouble. — Qiulang, Sep 11 '20 at 02:21
take a look at my [Expect for Shells](https://github.com/clarkwang/sexpect) if you *just need a simple ssh login script*. — sexpect - Expect for Shells, Sep 11 '20 at 02:42
@Qiulang what's the error? (i updated the answer at https://stackoverflow.com/a/46015727/900078 which now works for both py2 and py3.) — pynexj, Sep 11 '20 at 02:43
@sexpect_Expect.for.Shells thanks. But as I said I know expect pretty well, I am using pexpect just for the sake of learning it. But if it causes too many troubles I just give up. — Qiulang, Sep 11 '20 at 02:45
ok. give it a try when you are interested. it by design avoids quite some Expect's pitfalls. btw, based on my experience pexpect's behavior is kind of counter intuitive. :) — sexpect - Expect for Shells, Sep 11 '20 at 02:48
@pynexj I finally make it work. The error was because I set encoding as your answer suggested. Not sure why. Check my answer. — Qiulang, Sep 11 '20 at 03:40

Qiulang · Answer 1 · 2020-09-11T03:52:56.717

With the comment I got I finally made it work

#! /usr/bin/env python3

import pexpect
import sys
print('npm login',timeout = 10)
child = pexpect.spawn('npm login')
child.logfile_read = sys.stdout.buffer // use sys.stdout.buffer for python3
child.expect('Username:')
child.sendline('qiulang2000')
child.expect('Password:')
child.sendline('xxxx')
child.expect('Email:')
child.sendline('qiulang@gmail.com')
child.expect('Logged in as')

If I need to call child.interact(), then it is important that I call child.logfile_read = None before it, otherwise sys.stdout will echo everything I type.

The answer here How to see the output in pexpect? said I need to pass an encoding for python3, but I found that if I use encoding='utf-8' it will cause TypeError: a bytes-like object is required, not 'str' If I don't set encoding at all, everything works fine.

So a simple ssh login script looks like this

#!/usr/bin/env python3

import pexpect
import sys
child = pexpect.spawn('ssh qiulang@10.0.0.32')
child.logfile_read = sys.stdout.buffer
child.expect('password:')
child.sendline('xxxx')
#child.expect('Last login')  don't do that
child.logfile_read = None # important !!!
child.interact()

One problem remains unresolved, I had added one last expect call to match the ssh login output after sending the password, e.g. child.expect('Last login')

But if I added that, that line would show twice. I have gave up trying, like one comment said "pexpect's behavior is kind of counter intuitive".

Welcome to Ubuntu 16.04 LTS (GNU/Linux 4.4.0-141-generic x86_64)

 * Documentation:  https://help.ubuntu.com/

33 packages can be updated.
0 updates are security updates.

Last login: Fri Sep 11 11:44:19 2020 from 10.0.0.132
: Fri Sep 11 11:44:19 2020 from 10.0.0.132

for "that line would show twice", try `child.expect('.*')` before `child.logfile_read = None`. — sexpect - Expect for Shells, Sep 11 '20 at 04:00
the `encoding` param is suggested by the official pexpect doc which i think is not necessary unless you know exactly of the encoding of the spawned process' output (and input). — sexpect - Expect for Shells, Sep 11 '20 at 04:12

Is it possbile to let pexpect output the texts it matches?

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