lines = (line for line in file if line)

    def gen():

        for line in file:

            if line:

                yield line

    lines = gen()

    def gen(file):

        for line in file:

            if line:

                yield line

    lines = gen(file)

    lines = (line with open(path) as file for line in file)

    def gen(file):

        for line in file:

            if line:

                yield line

    with open(path) as file:

        lines = gen(file)

    upperlines = (line.upper() with open('foo', 'r') as file for line in file)

    def foo():
        with open('foo', 'r') as file:
            for line in file:
                yield line.upper()
    upperlines = foo()

Most people don't notice this feature is missing—but they work around it anyway. If you look over a few tutorials for modules on PyPI, questions and answers at StackOverflow, etc., you find people doing this:

    upperlines = (line.upper() for line in open('foo', 'r'))

Everyone knows this is wrong, because it relies on refcounting, and therefore only works in CPython. In fact, it's even wrong in CPython, because the file isn't closed until the last reference to the generator goes away, instead of when it finishes iterating over the file. And yet, people do it all the time.

This is why with statements were added to Python. Except here, they don't work. Compare:

    with open('foo', 'r') as file:

        contents = file.read()

    doStuff(contents) # works

    with open('foo', 'r') as file:

        contents = (line.upper() for line in file)

    doStuff(contents) # raises a ValueError

Of course in this trivial case, you can just indent the doStuff call, but in general, there is no inherent static scope for generator objects—they may get stored and used later (especially when they're passed into other generators). And even in the trivial case, the with statement isn't quite right—doStuff might iterate over contents, then do a bunch of other stuff before returning, and the file will be left open unnecessarily the whole time.

Here's a slightly larger example. Imagine a mail server built around a traditional select reactor, where send_async pushes a file onto a queue that will be sent as the socket is ready, and closed when it's done:

    class Connection:

        def handle_get(self, message_id):
            path = os.path.join(mailbox_path, message_id)
            self.send_async(open(path, 'r'))

Now imagine that you need to process each line of that message—e.g., to de-activate HTTP URLs, censor bad words, recognize phone numbers and turn them into links, whatever:

    class Connection:
        def handle_get(self, message_id):
            path = os.path.join(mailbox_path, message_id)
            self.send_async(self.process(line) for line in open(path, 'r'))

Oops, now we're hosed—the reactor may close the generator, but that won't cause the file to get closed.

The only way to do this correctly today is to write the explicit generator function instead of using a generator expression.

    class Connection:
        def handle_get(self, message_id):
            path = os.path.join(mailbox_path, message_id)

            def processed_file():

                with open(path, 'r') as f:

                    for line in f:

                        yield self.process(line)
            self.send_async(processed_file())

That's more verbose, less transparent even for experts, and impenetrable for new users—the same reason generator expressions were added to the language in the first place.

But it's even worse here. In my experience, even people who know better rarely think to write the generator function. When you point out the leaky-open problem in their code, after trying the with statement and finding it doesn't work, they either figure out the right place to put in an explicit close, or they write a class to manage the file and the iteration together. In other words, they treat Python like C++, because Python doesn't offer any obvious right way to do it.

But it could be this simple:

    class Connection:
        def handle_get(self, message_id):
            path = os.path.join(mailbox_path, message_id)
            self.send_async(self.process(line) 

                            with open(path, 'r') as f for line in f)

Syntax

The name "with" is pretty obvious; the question is, where to put it.

Adding with to the expression the same way as "if" means the right place for it is to the left of the "for" statement, so the top-down order of the nested statements in the equivalent generator function is reflected in the left-right order of the nested clauses in the expression.

A number of people have suggested that it would look more readable if the with statement came at the end:

    upperlines = (line.upper() for line in file with open('foo', 'r') as file)

    upperlines = (line.upper() with open('foo', 'r') as file for line in file)

However, the latter is more consistent, and easier to add to the parser or explain as a rule to new users, so that's what I'm proposing, even if it doesn't look as nice in the simplest cases.

Either way, looking over the syntax, it's obvious that this change would not affect the meaning of any valid generator expression; it would only make certain lines (that no one would ever have written) that are currently SyntaxErrors into valid expressions. And there shouldn't be anything tricky about adding it into the parser.

This feature isn't actually necessary in list comprehensions and dictionary comprehensions, because the iteration is finished as soon as the expression completes. But it doesn't hurt, and it might make the language more consistent, easier to learn, and even easier to parse.

If so, I suggest adding it to comprehensions as well, but I don't really care strongly either way.

Nearly every person I've suggested this to has said, "Why not just a general with expression instead"?

At first glance, that sounds nice, because then you could also write this:

    contents = file.read() with open('foo', 'r') as file

That's the motivating example for every suggestion for with expressions ever.

But anything that can usefully written using a with expression can be rewritten using a with statement:

This is clearly going to raise a ValueError as soon as you try to iterate contents. But it's just as true for generator expressions as for anything else. Consider what the following would mean if Python had a with expression:

A few people I've mentioned this idea to have objected on the basis that (line with open('foo', 'r') as file for line in file) makes me repeat both line and file twice.

The case for the repetition of file here is exactly the same as for the repetition of line. Yes, it's mildly annoying in completely trivial cases, but there's no way around it if you ever want to write non-trivial cases. Compare:

    (line[2:] with open('foo', 'r') as file for line in file)

    (line with open('foo', 'r') as file for line in decompress(file))

Besides, most of the people who make this argument are the kind of people who already avoid generator expressions because they believe (line[2:] for line in file) can be better written as:

I don't think there's any point catering to these people, whose real objection is usually "Lisp is a perfectly good language without the need for these new-fangled comprehensions and generator expressions"; the only reasonable answer is that Lisp was not actually outlawed when Haskell was invented.