Sorry, I was definitely unclear: I understand that isn't their sole purpose, and what role they play, in both the iteration examples and others, and I don't mean to "object" to anything, nor was I suggesting context-manager current implementation was uneeded, etc.

My point is that, in the examples I listed, the sole purpose of the context-manager is to provide a context the for loop will execute in. 

with manager as context:
    # No statements here...
    for items in context:
       # blah
    # no statements here...

I was suggesting this scenario, wherein the body of a for loop is, in fact, the block of code that acts upon some resource or set of resource being managed by a context manager, might be common enough to warrant a closer look, as the more "pythonic" approach might accept the for loop's body as the with statement's code block context. 

with open(file) as input:
    # why have input exist here, if it's not used, other than...
    for line in input:
       # do something...
       # why have input--the context--exist outside of me?
    # why have input exist here?

And finally, why define the outer with block, for the sole pupose of containing the inner "for in.." loop's block?






Steven D'Aprano
February 2, 2013 4:46 AM
On 02/02/13 22:46, Shane Green wrote:
with open(path) as input:
for line in input:
do(line)

Using with to create reference to opened file returned by open() so it could temporarily be assigned to input for the sole purpose of iterating its contents never sat very well with me.

It's not the *sole* purpose. If all you want it to iterate over the file, you can do this:

for line in open(path):
    ...


and no context manager is created. The context manager is also responsible for closing the file immediately you exit the block, without waiting for the caller to manually close it, or the garbage collector to (eventually) close it. So it is not *solely* for iteration.

File context managers can also be used for more than just iteration:

with open(path) as input:
    text = input.read()


with open(path, 'r+') as output:
    output.write('ZZ')

and so forth.


* The context manager returned by open() exists only to create the
context and return reference "input";
* the context and code block created by the "with" only exists for
inner "for" loop's code block to execute in.


I don't understand that objection. As I see it, that's a bit like saying "the len function exists only to get the length of objects". What did you expect the context manager to exist for if not to do the things you say?

What am I missing?




Shane Green
February 2, 2013 3:46 AM
with open(path) as input:
    for line in input:
        do(line)

Using with to create reference to opened file returned by open() so it could temporarily be assigned to input for the sole purpose of iterating its contents never sat very well with me. 
Now, given a generator function:

def iterwith(cm):
    with cm as context:
        if context is None:
            context = cm
        for item in context:
            yield item

The previous code can be turned into:

for line in iterwith(open(path)):
    do(line)

So, questions:
    - Is there anything inherently wrong with the idea, or does it exist?
    - Is it a generally useful tool, or are the examples limited to files?
    - Is it possible a more general mechanism could have value, such as:

for line in file with open(path) as file:
    do(line)


The preceding could be leveraged to different effect:

for line in file with locked(path):
    write(path + ".out", line)

Or,
    for line in input with nested(open(path),lock,open(opath)) as input,locked,output:
        output.write(line)

To revisit the original purpose of "with", this seems to cleanly address a very common scenario wherein:

resource = create_resource()
try:
    for item in resource:
        do_something(resource, item)
except:
    raise
finally:
    cleanup()

# Standard with approach...
with create_resource() as resource:
    for item in resource:
        do_something(resource, item)

# With for loop as context...
for item in resource with create_resource() as resource:
    do_something(resource, item)

And, given the translation into statements, maybe even crazy stuff... 

[line for line in file with open(path) as file]

J/K, I think.