Path: csiph.com!v102.xanadu-bbs.net!xanadu-bbs.net!feeder.erje.net!eu.feeder.erje.net!xlned.com!feeder1.xlned.com!news2.euro.net!newsgate.cistron.nl!newsgate.news.xs4all.nl!post.news.xs4all.nl!not-for-mail
To: python-list@python.org
From: Peter Otten <__peter__@web.de>
Subject: Re: lstrip problem - beginner question
Date: Tue, 04 Jun 2013 17:52:37 +0200
Organization: None
References: <1829efca-935d-4049-ba61-7138015a2806@googlegroups.com>
Mime-Version: 1.0
Content-Type: text/plain; charset="ISO-8859-1"
Content-Transfer-Encoding: 7Bit
User-Agent: KNode/4.7.3
Precedence: list
Newsgroups: comp.lang.python
Message-ID: <mailman.2660.1370361157.3114.python-list@python.org>
Lines: 50
NNTP-Posting-Host: 2001:888:2000:d::a6
Xref: csiph.com comp.lang.python:46927

mstagliamonte wrote:

> Hi everyone,
> 
> I am a beginner in python and trying to find my way through... :)
> 
> I am writing a script to get numbers from the headers of a text file.
> 
> If the header is something like:
> h01 = ('>scaffold_1')
> I just use:
> h01.lstrip('>scaffold_')
> and this returns me '1'
> 
> But, if the header is:
> h02: ('>contig-100_0')
> if I use:
> h02.lstrip('>contig-100_')
> this returns me with: ''
> ...basically nothing. What surprises me is that if I do in this other way:
> h02b = h02.lstrip('>contig-100')
> I get h02b = ('_1')
> and subsequently:
> h02b.lstrip('_')
> returns me with: '1' which is what I wanted!
> 
> Why is this happening? What am I missing?

"abba".lstrip("ab")

does not remove the prefix "ab" from the string "abba". Instead it removes 
chars from the beginning until it encounters one that is not in "ab". So

t = s.lstrip(chars_to_be_removed)

is roughly equivalent to

t = s
while len(t) > 0 and t[0] in chars_to_be_removed:
    t = t[1:]

If you want to remove a prefix use

s = "abba"
prefix = "ab"
if s.startswith(prefix):
    s = s[len(prefix):]