Monday, December 22, 2014

A tale of two scripts redux: joining Python and expect-lite

Python wrapper
As I mentioned earlier, expect-lite interpreter is quite basic. The advantage of such simpleness is that it ignores anything it doesn't understand. In a tale of two scripts I explained how to join bash and expect-lite in the same script. But the technique can also be applied to other languages.

In this post, I'll show how expect-lite can be embedded in a Python script allowing you to get all the goodness of Python and expect-lite.

First we start with a simple Python script which just makes a call to subprocess (used to execute an external application), prints the output of the expect-lite script in real time and then checks the return code of the subprocess.

#!/usr/bin/env python
"""
Example script: embedding expect-lite in python
    22 December 2014 -- Craig Miller
"""

import subprocess, inspect, os

def embed():
    script_name = inspect.getfile(inspect.currentframe())
    process = subprocess.Popen('expect-lite ' + script_name, 
                                shell=True, stdout=subprocess.PIPE)
    # print stdout in real time
    line = None
    while line != '':
        line = process.stdout.readline()
        print "EL:", line.rstrip()
    out, err = process.communicate()
    if process.returncode > 0:
        print("========= ERROR:%s" % process.returncode)
    else:
        print("========= GOOD:%s" % process.returncode)

#############################
# beginning of python script

if __name__ == '__main__':
    try:
        embed()
    except KeyboardInterrupt:
        print "Detected ^C"
        os._exit(1)

If you are familiar with Python, then you know that indentation is more than just a a good idea, it is required by the language. The script above has the basic components of a Python script, but the important part is the function embed()

In embed(), it figures out the name of the script using inspect.getfile() and saves it in the variable script_name. Then subprocess.Popen() is called with expect-lite and the script_name, to recursively run the script again, this time using expect-lite as the interpreter. 

However, before we do that, we need to add the expect-lite part of the script. At the bottom of the Python script add:
if False:'''
#############################
# beginning of expect-lite code

*EXP_INFO
$count=3
; === test of EL
>echo "inside expect-lite! whoo-hoo"
<inside
@5
;purple === ping loopback
>ping6 -c $count ::1
<packets transmitted
>echo "Continue"
>
; === pau
'''

The trick is to protect the expect-lite lines from the Python interpreter. The Python interpreter is a bit smarter than the bash interpreter, which does not read the entire file before executing. So to make python happy, we must shield the expect-lite lines inside an if false statement. The triple quotes is a Python mechanism that allows anything to be entered, even expect-lite. Save the completed script as two_scripts.py

The expect-lite part of the script is a simple one, showing that we are actually inside the expect-lite script, and a simple ping6 of the IPv6 loopback address.

Because of the dual nature of the script, it is possible to run (this example) without any Python, by running the script directly from expect-lite.
expect-lite two_scripts.py

Or run it using Python:
python two_scripts.py

The expect-lite part of the script can be located anywhere in the python script, as long as if false is used to protect it. Of course, you can also have Python pass parameters to expect-lite using CLI constants, making the script even more flexible and useful. 

There you have it, two languages, one script, all the power of Python, all the simplicity of expect-lite, automation for the rest of us.

Happy Holidays!

2 comments:

  1. I simply wanted to write down a quick word to say thanks to you for those wonderful tips and hints you are showing on this site.
    It’s great to come across a blog every once in a while that isn’t the same out of date rehashed material. Fantastic read.

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