lstrip problem - beginner question

[MRAB]

On 04/06/2013 16:21, 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?
>
The methods 'lstrip', 'rstrip' and 'strip' don't strip a string, they
strip characters.

You should think of the argument as a set of characters to be removed.

This code:

h01.lstrip('>scaffold_')

will return the result of stripping the characters '>', '_', 'a', 'c',
'd', 'f', 'l', 'o' and 's' from the left-hand end of h01.

A simpler example:

 >>> 'xyyxyabc'.lstrip('xy')
'abc'

It strips the characters 'x' and 'y' from the string, not the string
'xy' as such.

They are that way because they have been in Python for a long time,
long before sets and such like were added to the language.