Python pexpect

last modified January 29, 2024

In this article we show how to manage interactive programs in Python using the pexpect module.

The pexpect module is used to manage interactive programs in Python. It spawns child applications, controls them; and responds to expected patterns in their output.

The pexpect module is used to automate interactive applications such as ssh, ftp, or passwd. It can be used to automate setup scripts for software package installations on different servers.

There are two basic functions: run and spawn. The run function executes a command and returns the output. It is useful for simple interatctions.

The spawn function is a more powerful option. It spawns an external child command and then interacts with the child by sending lines and expecting responses.

Python pexpect run

The run function runs the given command; waits for it to finish; then returns all output as a string. The standard error output is included in output.

main.py

#!/usr/bin/python

import pexpect

r = pexpect.run(’echo hello’) print(str(r, ‘UTF-8’))

In the program, we run the echo command.

r = pexpect.run(’echo hello’)

We run the echo command. The output is assigned to the r variable.

print(str(r, ‘UTF-8’))

We transform the output into a string with the str function.

$ ./main.py hello

We can use the bytes.decode function to convert the bytes to a str.

main.py

#!/usr/bin/python

import pexpect

r = pexpect.run(‘pwd’).decode(“utf-8”)

print(f"The current working directory: {r.rstrip()}")

The programs runs the pwd program, which returns the current working directory.

$ ./main.py The current working directory: /home/jano/Documents/prog/python/pexpect

Python pexpect logfile

It is possible to redirect the output into a log file. Both run and spawn functions have the logfile parameter.

main.py

#!/usr/bin/python

import pexpect

f = open(‘mylog.txt’, ‘wb’)

pexpect.run(’ls -l’, logfile=f)

f.close()

In the program, we run the ls -l command and write the output into the logfile.

$ ./main.py $ head -3 mylog.txt total 60 -rw-rw-r– 1 jano jano 120 dec 23 10:02 doc.txt -rwxrwxr-x 1 jano jano 418 dec 23 14:52 ftp2.py

Python pexpect spawn

The spawn function executes a child program and allows to interact with it.

The expect function of the child seeks through the stream until a pattern is matched. The pattern is overloaded and may take several types. The pattern can be a string, EOF, a compiled regex, or a list of any of those types. The strings will be compiled to regex types.

main.py

#!/usr/bin/python

import pexpect

child = pexpect.spawn(‘date’) child.expect(pexpect.EOF) output = child.before

print(f’Today is : {output.decode(“utf-8”)}’) child.close()

In the program, we run the date command to get the current datetime.

child = pexpect.spawn(‘date’)

We spawn the date command.

child.expect(pexpect.EOF)

With pexpect.EOF we expect that the child has exited. After each call to expect the before and after properties will be set to the text printed by child application.

output = child.before

The before attribute contains all text up to the expected pattern.

$ ./main.py Today is : Št 29. december 2022, 11:39:30 CET

Python pexpect FTP example

In the next example, we interact with an FTP command.

main.py

#!/usr/bin/python

import pexpect child = pexpect.spawn(‘ftp ftp.freebsd.org’, encoding=‘utf-8’)

child.expect(‘Name .*: ‘) child.sendline(‘anonymous’) child.expect(‘Password:’) child.sendline(’’) child.expect(‘ftp> ‘) child.sendline(’ls /pub/FreeBSD/’) child.expect(‘ftp> ‘)

print(child.before)

child.close()

In the program we connect to an anonymous FTP server and issue the ls command.

child = pexpect.spawn(‘ftp ftp.freebsd.org’, encoding=‘utf-8’)

We spawn the ftp command. With this command, we connect to the ftp.freebsd.org server. We also provide the encoding.

child.expect(‘Name .*: ‘)

From the server, we expect a line starting with the Name string pattern followed by some characters. Note that the input is a regular expression.

child.sendline(‘anonymous’)

With sendline, we send the login name.

child.expect(‘Password:’) child.sendline(’’)

We expect the password promt and send an empty password, since we log in as anonymous.

child.sendline(’ls /pub/FreeBSD/’)

We issue the ls command.

print(child.before)

We print the output of the ls command.

$ ./main.py ls /pub/FreeBSD/ 229 Entering Extended Passive Mode (|||51545|) 150 Here comes the directory listing. -rw-r–r– 1 ftp ftp 4259 May 07 2015 README.TXT -rw-r–r– 1 ftp ftp 35 Dec 29 10:45 TIMESTAMP drwxr-xr-x 9 ftp ftp 10 Dec 29 10:45 development -rw-r–r– 1 ftp ftp 2995 Dec 29 10:00 dir.sizes drwxr-xr-x 22 ftp ftp 28 Dec 26 23:00 doc drwxr-xr-x 6 ftp ftp 6 Nov 10 15:01 ports drwxr-xr-x 12 ftp ftp 14 Dec 29 10:45 releases drwxr-xr-x 12 ftp ftp 14 Dec 09 20:25 snapshots 226 Directory send OK.

Python pexpect SSH

In the following example, we interact with the ssh command.

For this example, we must have a ssh account with a password authentication enabled.

main.py

#!/usr/bin/python

import pexpect

username = ‘user7@192.168.0.25’ passwd = ‘passwd’

child = pexpect.spawn(f’ssh {username}’) f = open(‘mylog.txt’, ‘wb’) child.logfile = f

child.expect(‘password:’) child.sendline(passwd)

i = child.expect([‘Permission denied’, ‘[#$] ‘])

if i == 0:

print('failed to login. permission denied')
child.close()

elif i == 1:

print('connected')

child.sendline('uname -a')
child.expect('[#\$] ')
r = child.before.decode('utf-8')

child.write('exit')
child.close()

f.close()

We connect to a computer on a local network and issue the uname command on the host.

username = ‘user7@192.168.0.25’ passwd = ‘passwd’

We provide the username and the password.

child = pexpect.spawn(f’ssh {username}’)

We log into into the host with the ssh command.

f = open(‘mylog.txt’, ‘wb’) child.logfile = f

The output is directed to a log file.

child.expect(‘password:’) child.sendline(passwd)

The computer sends a prompt and we send the password.

i = child.expect([‘Permission denied’, ‘[#$] ‘])

We expect two outcomes: either a permission denied message or a standard prompt. The expect function returns the index to the returned option.

elif i == 1:

print('connected')

child.sendline('uname -a')
child.expect('[#\$] ')

child.write('exit')
child.close()

In case the login was successful, we issue the uname command. Later, we issue the exit command to terminate the connection.

Source

Python pexpect documentation

In this article we have covered the Python pexpect module.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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