xrange not adhering to start, stop, step

Question

My input file looks like:

6
*,b,*
a,*,*
*,*,c
foo,bar,baz
w,x,*,*
*,x,y,z
5
/w/x/y/z/
a/b/c
foo/
foo/bar/
foo/bar/baz/

When I use xrange, why does it not adhere to the start, stop, step method?

with open(sys.argv[1], 'r') as f:
  for _ in xrange(0, 7, 1):
    next(f)
  for listPatterns in f:
    print listPatterns.rstrip()

It outputs the text starting at line 7 when in actuality I want it to print line 1 through 7.

Answer 1

The code you want is

with open(sys.argv[1], 'r') as f:
  for _ in xrange(0, 7, 1):
    print f.next().rstrip()

The first loop you have is advancing through the file.

Answer 2

For each item in the iterable (in this case xrange) you're calling next on the file-obj and ignoring the result - either do something with that result, or better yet, make it much clearer:

from itertools import islice
with open('file') as fin:
    for line in islice(fin, 7):
        # do something

Answer 3

Er, well, because you told it to skip the first 7 lines? Solution: don't do that.

Answer 4

it isn't xrange.

you first loop through all of xrange.

then you exit the loop

then you have another loop, acting on the last element

Answer 5

You can also iterate through the file, without using a proxy iterator

START_LINE = 0
STOP_LINE = 6
with open(sys.argv[1], 'r') as f:
   for i, line in enumerate(f.readlines()):
       if START_LINE <= i <= STOP_LINE:
           print line.rstrip()
       elif i > STOP_LINE: 
           break