read from one file to another missing certain lines

Question

I have a text file (file1.txt) with 1 string per line over many lines. I'm trying to read in the file and write certain lines to a new file.(file2.txt)

My text file looks something like this.

foo1
foo2
foo3
foo4
foo5
foo6

etc..

for example, i want to write foo1,foo2,foo4,foo6 to my new file and miss out foo3 and foo5.

foo1
foo2
foo4
foo6

I wish to preserve the original file.

My code looks like this...

with open("file1.txt","r") as r:
    lines=r.read()
    lines =lines.replace("foo3","")
    lines = lines.replace("foo5","")

r.close()
with open("file2.txt","a") as w:
    w.write(lines)
w.close

The problem is I end up with this output..

foo1
foo2

foo4

foo6

I think this is because i am replacing foo with "" how do I get rid of the white space?

TIA,

Paul.

Answer 1

The minimal change is to also replace the line separators by changing the replace calls to:

lines =lines.replace("foo3\n","")
lines = lines.replace("foo5\n","")

Answer 2

Presuming that the exclusions are variable:

def rwfile(infile, outfile, exceptions=[]):

    o = open(outfile, "w")

    for line in open(infile):
        if line.rstrip() not in exceptions:
            o.write(line)

    o.close()


rwfile("in", "out", ['foo3', 'foo5'])

In:

foo1
foo2
foo3
foo4
foo5
foo6
foo7
foo8
foo9

Out:

foo1
foo2
foo4
foo6
foo7
foo8
foo9

Following on from the OP's comments - here's a version using a predicate function to decide which lines should be included.

def rwfilep(infile, outfile, predicate=lambda x: True):

    o = open(outfile, "w")

    for line in open(infile):
        if predicate(line):
            o.write(line)

    o.close()

def ignore_some(line):
    """return True to include"""
    return line.rstrip() not in ['foo3', 'foo5']

def ignore_comments(line):
    """return True to include"""
    return not line.startswith("#")

rwfilep("in", "out2", ignore_some)

rwfilep("in", "out3", ignore_comments)