Easiest way to ignore blank lines when reading a file in Python

I would stack generator expressions:

    with open(filename) as f_in:
            # All lines including the blank ones    
            lines = (line.rstrip() for line in f_in) 
    
            lines = (line for line in lines if line) # Non-blank lines

Now, lines is all of the non-blank lines. This will save you from having to call strip on the line twice. If you want a list of lines, then you can just do:

 with open(filename) as f_in:
    lines = (line.rstrip() for line in f_in)
    # Non-blank lines in a list
    lines = list(line for line in lines if line) 

You can also do it in a one-liner (exluding with statement) but it's no more efficient and harder to read:

with open(filename) as f_in:
    lines = list(line for line in (l.strip() for l in f_in) if line)

Update:

I agree that this is ugly because of the repetition of tokens. You could just write a generator if you prefer:

def nonblank_lines(f):
    for l in f:
            line = l.rstrip()
            if line:
                    yield line

Then call it like:

with open(filename) as f_in:
        for line in nonblank_lines(f_in):
        # Stuff

update 2:

with open(filename) as f_in:
        lines = filter(None, (line.rstrip() for line in f_in))

and on CPython (with deterministic reference counting)

lines = filter(None, (line.rstrip() for line in open(filename)))

In Python 2 use itertools.ifilter if you want a generator and in Python 3,

just pass the whole thing to list if you want a list.