Python - how to parse strings from an array?

Question

I'm a Java/C programming, struggling to learn Python. (I'm writing in Python 2.4.3 on a Linux machine.) I have a simple program which telnets to a Cisco router, logs in, then captures the config. The router's config is stored in a variable named "output":

#!/usr/bin/python

import telnetlib
import datetime
import sys

def telnetFunction( host ):
  print("Telnetting to device "+host+"...\n")
  tn = telnetlib.Telnet(host)
  # ...use tn.read_until() and tn.write() to log into the router...  ...this works...
  tn.write("terminal length 0"+"\n")
  tn.write("show run"+"\n")
  tn.write("exit"+"\n")
  output=tn.read_all()      # router config stored as "output"
  return output

host = "192.168.102.133"
output=telnetFunction( host )

The above code works. From using a bunch of different print() statements, I can see that the router's config is all in "output", which I am assuming is an array of newline-terminated strings....? Not entirely sure about that; the telnetlib documentation doesn't specify.

My problem now is I need my program to step through output again, extracting one string at a time. I need something like:

while(iterating through output)
   tmpString = output.getNextStr()

So if output looked like this:

Current configuration : 34211 bytes\n!\nversion 12.3\nno service pad\n...etc...

I need tmpString to equal the follow in each iteration of the loop above:

tmpString = "Current configuration : 34211 bytes"
tmpString = "!"
tmpString = "version 12.3"
tmpString = "no service pad"

I've been Googling for a few hours, but am stumped. (Part of the problem is I'm not clear that "output" is indeed an array. Could it be a string?) I've played with split() and using []'s but no luck so far. Does anyone see where I'm going wrong? Thanks, -ROA

You are working with lists not arrays, firstly. In any event, output looks like it's a string. — juanpa.arrivillaga
– juanpa.arrivillaga, Commented May 10, 2017 at 19:51
@juanpa.arrivillaga Interesting. Lists, not arrays? I never would have guessed that. Do you know this from experience, or is there some documentation out there that I haven't uncovered? The hardest thing about learning python is that sometimes you just don't know what data type you are working with. Thanks...! — Pete
– Pete, Commented May 11, 2017 at 13:15
@aryamccarthy Correct. The '\n' characters are part of the original chunk of data I read in — Pete
– Pete, Commented May 11, 2017 at 13:16

somesingsomsing · Accepted Answer · 2017-05-10 20:18:25Z

2

As you stated in your example, let say output is defined as

output = 'Current configuration : 34211 bytes\n!\nversion 12.3\nno service pad'

Your next step is to split the string by \n so

output_list = output.split('\n')

this will generate the following list

output_list = ['Current configuration : 34211 bytes', '!', 'version 12.3', 'no service pad']

then you can iterate over this list.

to tie it all together

#!/usr/bin/python

import telnetlib
import datetime
import sys

def telnetFunction( host ):
  print("Telnetting to device "+host+"...\n")
  tn = telnetlib.Telnet(host)
  # ...use tn.read_until() and tn.write() to log into the router...  ...this works...
  tn.write("terminal length 0"+"\n")
  tn.write("show run"+"\n")
  tn.write("exit"+"\n")
  output=tn.read_all()      # router config stored as "output"
  return output

host = "192.168.102.133"
output=telnetFunction( host )
output_list = output.split('\n')
for item in output_list:
    print item

answered May 10, 2017 at 20:18

somesingsomsing

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1 Comment

Pete Over a year ago

Perfect! Your code solved the problem, thanks. I wasn't aware you could convert a large string into a list with split() - that's def a trick for my notebook.

OnNIX · Accepted Answer · 2017-05-10 19:53:16Z

1

Your variable tn is, I'm assuming, treated like a file handle. So when you read this 'file' once, the pointer is at the end of the file and needs to be reset:

tn.seek(0)

Call this before you execute your second loop.

Alternatively, you can iterate over output like so:

for line in output.split("\n"):
    print line

answered May 10, 2017 at 19:53

OnNIX

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1 Comment

Pete Over a year ago

To be honest, I don't think tn is a file handle. It seems to be an array or a string or something saved in local memory. However, your use of split() looks promising, thanks!

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