lstrip problem - beginner question

[John Gordon]

In <1829efca-935d-4049-ba61-7138015a2806 at googlegroups.com> mstagliamonte <madmaxthc at yahoo.it> writes:

> 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?

It's happening because the argument you pass to lstrip() isn't an exact
string to be removed; it's a set of individual characters, all of which
will be stripped out.

So, when you make this call:

    h02.lstrip('>contig-100_')

You're telling python to remove all of the characters in '>contig-100_' from
the base string, which leaves nothing remaining.

The reason it "worked" on your first example was that the character '1'
didn't occur in your sample header string 'scaffold_'.

If the underscore character is always the separating point in your headers,
a better way might be to use the split() method instead of lstrip().

-- 
John Gordon                   A is for Amy, who fell down the stairs
gordon at panix.com              B is for Basil, assaulted by bears
                                -- Edward Gorey, "The Gashlycrumb Tinies"