More pythonic way of skipping header lines

Question

Is there a shorter (perhaps more pythonic) way of opening a text file and reading past the lines that start with a comment character?

In other words, a neater way of doing this

fin = open("data.txt")
line = fin.readline()
while line.startswith("#"):
    line = fin.readline()

Answer 1

At this stage in my arc of learning Python, I find this most Pythonic:

def iscomment(s):
   return s.startswith('#')

from itertools import dropwhile
with open(filename, 'r') as f:
    for line in dropwhile(iscomment, f):
       # do something with line

to skip all of the lines at the top of the file starting with #. To skip all lines starting with #:

from itertools import ifilterfalse
with open(filename, 'r') as f:
    for line in ifilterfalse(iscomment, f):
       # do something with line

That's almost all about readability for me; functionally there's almost no difference between:

for line in ifilterfalse(iscomment, f))

and

for line in (x for x in f if not x.startswith('#'))

Breaking out the test into its own function makes the intent of the code a little clearer; it also means that if your definition of a comment changes you have one place to change it.

Answer 2

for line in open('data.txt'):
    if line.startswith('#'):
        continue
    # work with line

of course, if your commented lines are only at the beginning of the file, you might use some optimisations.

Answer 3

from itertools import dropwhile
for line in dropwhile(lambda line: line.startswith('#'), file('data.txt')):
    pass