Pythonic way to yield alternate lines of a textfile

Question

I have a textfile as below and I want to either yield the odd/even lines:

this is a blah 
 I don't care,
whatever foo bar
ahaha

I have tried checking for the odd/even-ness of the enumerate index but what is the pythonic way to do yield alternate lines in a textfile? I've tried:

text = "this is a blah \n I don't care,\nwhatever foo bar\nahaha"
with open('test.txt', 'w') as fout:
    for i in text.split('\n'):
        print>>fout, i

def yield_alt(infile, option='odd'):
    with open(infile,'r') as fin:
        for i,j in enumerate(fin):
            if option=='odd':
                if i+1 & 0x1:
                    yield j
            elif option=='even':
                if i & 0x1:
                    yield j

for i in yield_alt('test.txt'):
    print i

[out]:

this is a blah 

whatever foo bar

Lastly, what does i & 0x1 mean? I know it checks for even int, is there other ways of checking for even integers?

Answer 1

Another way to do this is to use iterools.islice to slice the file object:

>>> from itertools import islice
>>> def yield_alt(f, option='odd'):
        if option == 'odd':
            return islice(f, 0, None, 2)
        return islice(f, 1, None, 2)
... 
>>> with open('abc1') as f:
    for line in yield_alt(f):      
        print line,
...         
this is a blah 
whatever foo bar

>>> with open('abc1') as f:
    for line in yield_alt(f, 'even'):
        print line,
...         
 I don't care,
ahaha

Answer 2

i & 0x1 is bitmasking the last bit, to check if i is even or odd. I generally do i % 2 because it feels clearer to me, but not sure which one has better performance.

---

Edit: timeit and dis:

>>> timeit(lambda:5 & 0x1, number=1000000)
0.10139431793770104

>>> timeit(lambda:5 % 2, number=1000000)
0.10143369172932282

>>> dis.dis(lambda: 5 & 0x1)
  1           0 LOAD_CONST               2 (1)
              3 RETURN_VALUE

>>> dis.dis(lambda: 5 % 2)
  1           0 LOAD_CONST               2 (1)
              3 RETURN_VALUE

They are almost exactly identical. Whichever is clearer to you is the one you should use.

Answer 3

islice is probably your best choice, but for completeness I'll add another option using next to skip the lines you don't want.

def yield_alt(filename, option='odd'):
    with open(filename) as ifile:
        if option.lower() == 'odd':
            # Skip first line
            next(ifile)
        for line in ifile:
            yield line
            # Skip next line
            next(ifile)