Question regarding lists and regex

[Paul McGuire]

"Prabhu Gurumurthy" <pgurumur at gmail.com> wrote in message 
news:mailman.1959.1163053812.11739.python-list at python.org...
> Here is a simple program, which queries /var/log/daemon on my OpenBSD box 
> and gets the list of valid ntp peers.
>
> Questions:
> what is the easiest way for me to create lists on the fly, by that I mean 
> like perl
>
> push my @foo, something_from_say_stderr. The reason is as you can ip = 
> [""] statement before the for loop, I want to avoid that and use list 
> within the second ip loop, where I extract the ip address. Am I confusing?
>

Typically, one initializes a list to be empty, that is [], not [""].  Python 
will not read your mind at append time and think "oh! we're appending to a 
list and we forgot to create one in the first place, let's make one now."  I 
guess Perl allows this, but the clarity of including the initialization 
statement overrules the convenience of leaving it out.

> regex: I presume this is rather a dumb question, anyways here it comes! as 
> you can see from my program, pattIp = r\d{1,3}\.... etc, is there any 
> other easy way to group the reptitions, instead of typing the same regex 4 
> times.
>

Here's one way, tested at the Python command line:
>>> print r'\.'.join( [r'\d{1,3}']*4 )
\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}

This avoids the pattern duplication, but I think using join is much less 
easily recognized as a pattern for an IP address.

> TIA
> Prabhu
>

Some other comments/free advice:
1. I was curious about this line:
    pid = int(re.sub(r'\[|\]', "", pidMatch.group()))
You already know pidMatch.group() is going to start with a '[', followed by 
an integer string, and end with a ']', otherwise it wouldn't have matched 
pidPatt.  Instead of whacking this with another re-type call, how about just 
some simple string slicing:
    pid = pidMatch.group()[1:-1]

2. No real need to keep count of the found ip's, just use len(ip) to tell 
you how many entries there are in the list (especially once you convert to 
intializing with an empty list).

3. Similarly, you'll be able to remove the 'if len(x)' test when printing 
out the contents of the ip list if you init with [] instead of [""].  Also, 
the Python idiom for testing if x is the empty string is usually just 'if 
x', not 'if len(x)'.

-- Paul