Never call readlines() on a file

Calling readlines() makes your code slower, less explicit, less concise, for absolutely no benefit.

There are hundreds of questions on places like StackOverflow about the readlines method, and in every case, the answer is the same.

"My code is takes forever before it even gets started, but it's pretty fast once it gets going."

That's because you're calling readlines.

"My code seems to be worse than linear on the size of the input, even though it's just a simple loop."

That's because you're calling readlines.

"My code can't handle giant files because it runs out of memory."

That's because you're calling readlines.

"My second call to readlines returns nothing."

That's not directly because you're calling readlines; it's because you're trying to read from a file whose read pointer is at the end. But the reason it's not obvious why this can't work is that you're using readlines.

In fact, even if you don't have any of these problems, you should not use readlines, because it never gives you any advantage.

What's wrong with readlines()?

So what should I use?

99% of the time, the answer is to just use the file itself. As the documentation says:

Note that it’s already possible to iterate on file objects using for line in file:... without calling file.readlines().

The reason you're calling readlines is to get an iterable full of lines, right? A file is already an iterable full of lines. And it's a smart iterable, reading lines as you need them, with some clever buffering under the covers.

This following two blocks of code do almost the same thing:

    with open('foo.txt') as f:
        for line in f.readlines():
            dostuff(line)

    with open('foo.txt') as f:
        for line in f:
           dostuff(line)

Both of them call dostuff on each line in foo.txt. The only difference is that the first one reads all of foo.txt into memory before starting to loop, while the second one just reads a buffer at a time, automatically, while looping.

What if I actually need a list rather than just some arbitrary iterable?

Make a list out of it:

    with open('foo.txt') as f:
        lines = list(f)

This has exactly the same effect as calling f.readlines(), but it makes it explicit that you wanted a list, in exactly the same way you make that explicit anywhere else (e.g., calling an itertools function, or Python 3.x's map or filter).

What about calling readlines with a sizehint?

There's nothing wrong with that. It's often a useful optimization or simplification.

For example, consider this code using a multiprocessing.Pool:

    with open('foo.txt') as f:
        pool.map(func, f, chunksize=104)

It's a bit silly to break the file down into lines just to chunk them back up together. Also, this won't give you chunks of about-equal size unless your lines are of about-equal length. So, this may turn out to be a lot better:

    with open('foo.txt') as f:
        pool.map(func, iter(partial(f.readlines, 8192), []), chunksize=1)

Of course I'd probably wrap up that iterable to make it more readable—many Python programmers need to think through both partial and two-argument iter to understand them, much less both of them together. But the idea is that, instead of reading line by line and building chunks of 104 lines in hopes that will often be around 8K, we just read 8K worth of lines at a time.

What if I need to be compatible with older Python?

Files have been iterable since 2.3. That's over a decade old. That's the version that RHEL 4 came with.

If you really have to work with all of Python 2.1-2.7 (and don't mind breaking 3.x), you can use f.xreadlines instead. (Note that in 2.3+, f.xreadlines() just returns f, so there's no real harm in calling it—it's just silly to do so if you don't need to.) If you have to work with 2.0 or 1.x, you'll need to write your own custom buffer-and-splitlines code.

But you really don't. Nobody's going to be running your new script on a system from the last century.

What about that other 1% of the time?

There are various other possibilities that come up sometimes (besides the ones described above), where using the file as an iterator is not the answer:

Often you're trying to parse a CSV file or XML or something, and the right fix is to use csv or ElementTree or whatever in the first place instead of trying to do it yourself.

Sometimes, you need to call readline() in a loop (or, equivalently, use something like iter(f.readline, '')). However, this most often comes up with sys.stdin, in which case you're probably doing it wrong in the first place—maybe input is what you wanted here?

Sometimes, you really need to mmap the whole file (or, worse, use a sliding mmap window because you've got a 32-bit Python) and find the line breaks explicitly.

But in none of these cases is f.readlines() going to be any better than f. It's going to have the exact same problems, plus another problem on top.