Introduction to Pexpect

Pexpect is a Python module that allows you to spawn child processes, control them, and respond to expected patterns in their output. It is a scripting tool for automating interactive applications like ssh, ftp, passwd, and others. Pexpect can be used for automating software installations, testing, and various administrative tasks.

Getting Started with Pexpect

First, install Pexpect using pip:

  pip install pexpect

Basic Usage of Pexpect

The following example shows how to use Pexpect to connect to a remote system using ssh:

import pexpect

child = pexpect.spawn('ssh user@hostname')
child.expect('password:')
child.sendline('mypassword')
child.expect('#')
child.sendline('ls -l')
child.expect('#')
print(child.before)

Common Pexpect APIs

pexpect.spawn

Starts a child application and wraps its input and output. Here is a code snippet for running a simple shell command:

import pexpect

child = pexpect.spawn('ls')
child.expect(pexpect.EOF)
print(child.before.decode())

expect

Waits for the child process to return the expected string. Wildcards are allowed in the expected string:

import pexpect

child = pexpect.spawn('ftp')
child.expect('ftp>')
child.sendline('open ftp.example.com')

sendline

Sends a string response to the child process with a newline character:

child.sendline('myresponse')

before and after

Strings that match the expected pattern before and after the pattern respectively:

child.expect('pattern')
print(child.before)
print(child.after)

Pexpect in Action: An Application Example

In this example, Pexpect will be used to automate the process of logging into an email server and checking the content of the inbox:

import pexpect

def check_email():
    child = pexpect.spawn('telnet mail.example.com 110')
    child.expect('username:')
    child.sendline('your_email@example.com')
    child.expect('password:')
    child.sendline('yourpassword')
    child.expect('mail.example.com>')
    child.sendline('LIST')
    child.expect('mail.example.com>')
    print(child.before)

if __name__ == '__main__':
    check_email()

This script logs into a mail server, lists the emails, and prints the raw output of the command.

Conclusion

Pexpect is a powerful tool for Python developers looking to automate command line tasks, control interactive applications, and streamline processes securely and efficiently.

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