Remove String From List

Question

I have a list of strings containing IP addresses and DNS names with which I would like to remove the values beginning with "10." only. The sample data is as follows:

['www.example.com','1.2.3.4','4.3.2.1','example.net','10.1.1.10','10.1.1.11',...]

I thought this would be simple and started with the following:

for v in address:   
    test = re.match('(^\d+\.)',v)
    if test:
        if test.group(1) == '10.':
            address.remove(v)

The "10." addresses were not removed although I didn't receive any errors (and did some t-shooting with "print address.remove(v)" which resulted in "None" for each "10." address. Leads me to believe the regex is wrong but it seems to work other than in this capacity.

So I poked around with re.purge() - this didn't help either but don't think it's a factor in my problem. I also tried using del address[...] to no avail.

Where have I gone wrong?

Thanks very much for your attention.

When I test your code, the first address containing 10.1 is removed, but not the other one. — Jeremy D
– Jeremy D, Commented Jan 6, 2012 at 4:57
possible duplicate of Removing Item From List - during iteration - what's wrong with this idiom? — Brooks Moses
– Brooks Moses, Commented Jan 6, 2012 at 5:05

DRH · Accepted Answer · 2012-01-06 05:14:04Z

14

The easy way would be to use list comprehensions:

filtered = [ v for v in address if not v.startswith('10.') ]

edited Jan 6, 2012 at 5:14

DRH

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answered Jan 6, 2012 at 4:54

Reverend Gonzo

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2 Comments

Bit Bucket Over a year ago

This works although as a couple people kindly pointed out the root problem is modifying while iterating. Iterating a slice, modifying the original is the way to go - this approach solved my problem. Refer to link.

johnsyweb Over a year ago

@BitBucket: N.B: The linked solution does, in fact, create a copy of the list and iterates over that. The list comprehension is surely more efficient (not to mention easier to grok).

Community · Accepted Answer · 2017-05-23 10:30:18Z

3

One way is to create a new list using a list comprehension and str.startswith():

>>> [a for a in address if not a.startswith('10.')]
['www.example.com', '1.2.3.4', '4.3.2.1', 'example.net', '...']

This avoids using regular expressions and removing items during iteration, but does create a copy.

edited May 23, 2017 at 10:30

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answered Jan 6, 2012 at 5:05

johnsyweb

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Comments

Community · Accepted Answer · 2017-05-23 12:18:33Z

1

What you've done wrong here is iterating over a list while you're changing the list. That means the iteration gets confused.

See Removing Item From List - during iteration - what's wrong with this idiom? for some suggestions on how to do this correctly.

edited May 23, 2017 at 12:18

CommunityBot

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answered Jan 6, 2012 at 5:02

Brooks Moses

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Community · Accepted Answer · 2017-05-23 12:02:54Z

1

If would probably make sense to test first that there is really an IP address in question.

Otherwise 10.some-cdn.some-mighty-corp.com will be filtered out.

CommunityBot

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answered Jan 6, 2012 at 5:05

Roman Susi

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1 Comment

Bit Bucket Over a year ago

Certainly, and this will be accounted for once the "guts" are working. Knowing my data source, the possibility of this occurring is highly unlikely. Thanks for the input.

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