Rudy,

I think your proposal may be very specific to iterable context managers; in which case, make a method that makes that assumption:

def iter_with(obj):      with obj as context:          yield from context

and use it

g = (      f.read()      for fn in filenames      for f in iter_with(open(fn)) )

On 2018-07-30 15:15, Rudy Matela wrote:

Hello,

Do you think it would be nice to allow with statements inside genexps or list comprehensions? The functions __enter__ and __exit__ would be automatically called as iterables are traversed. I am thinking of drafting a PEP about this. Examples:

This

g = (f.read() for fn in filenames with open(fn) as f)

would be equivalent to the following use of a generator function:

def __gen(): for fn in filenames: with open(fn) as f: yield f.read() g = __gen()

This

list = [f.read() for fn in filenames with open(fn) as f]

would be equivalent to the following:

list = [] for fn in filenames: with open(fn) as f: list.append(f.read())

-- Rudy _______________________________________________ Python-ideas mailing list Python-ideas@python.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/

On 30 July 2018 at 20:15, Rudy Matela rudy@matela.com.br wrote:

Hello,

Hi Rudy,

Do you think it would be nice to allow with statements inside genexps or list comprehensions? The functions __enter__ and __exit__ would be automatically called as iterables are traversed. I am thinking of drafting a PEP about this. Examples:

This

    g = (f.read() for fn in filenames with open(fn) as f)

would be equivalent to the following use of a generator function:

    def __gen():
            for fn in filenames:
                    with open(fn) as f:
                            yield f.read()
    g = __gen()

Yielding from a with block should be discouraged rather than given special syntax. There is essentially a contradiction between the meaning/purpose of yield (suspend indefinitely) and with (definitely call __exit__).

If I partially iterate over g as in

for line in g: break

then at this point g is suspended and f.__exit__ has not been called, so the file is not closed. I may choose to iterate over g later or not, so it has to remain in suspension just in case. In practice if you do this in CPython then f.__exit__ will *probably* be invoked indirectly by g.__del__ if/when the gc collects g. This defeats the main point of using with-open though which is to avoid depending on the gc for closing files.

-- Oscar

On 30/07/18 20:15, Rudy Matela wrote:

Hello,

Do you think it would be nice to allow with statements inside genexps or list comprehensions? The functions __enter__ and __exit__ would be automatically called as iterables are traversed. I am thinking of drafting a PEP about this. Examples:

I was worried until I twigged that you meant adding a clause rather than a statement per se. How does this interact with the other clauses in a comprehension? I'm thinking of monstrosities like:

g = (f.read() for fn in filenames if fn.endswith(".txt") with open(fn) as f for filenames in get_directory_listing(dir) if .....)