On 8 August 2018 at 15:37, Oscar Benjamin <oscar.j.benja...@gmail.com> wrote:
> On 8 August 2018 at 15:22, Ronald Oussoren via Python-ideas
> <python-ideas@python.org> wrote:
>>
>> It is also possible to fix the particular issue by using another with
>> statement, that is use:
>>
>> with contextlib.closing(read_multiple(…)) as chunks:
>> for contents in chunks:
>> …
>
> That's a very good point Ronald. Having seen this a few times I can't
> think of any cases that wouldn't be solved by this.
Thinking about this some more: closing() can ensure finalisation but
it is still generally bad to yield from a with block. Some context
managers are designed to temporarily alter global state between
__enter__ and __exit__ - this can be very confusing if you use them
around a yield block.
As an example the localcontext context manager from the decimal module
does this. So if you do something like
def decimal_generator():
...
with decimal.localcontext() as ctx:
ctx.prec = 4 # Use 4 digits for calculations temporarily
yield x + y
yield y + z
with decimal.localcontext() as ctx:
ctx.prec = 10 # Use 10 digits for calculations temporarily
total = decimal.Decimal(0)
for item in decimal_generator():
total += item
Here the line total += item will end up using 4 digit arithmetic
rather than 10. That's because the yield allows the "local" context to
leak out. The fix for that is something like:
def decimal_generator():
...
ctx = decimal.localcontext()
ctx.prec = 4 # Use 4 digits for calculations temporarily
with ctx:
val = x + y
yield val
with ctx:
val = y + z
yield val
I still think that this is a good idea though:
> If a language change was wanted for this then perhaps it should be to
> do like file objects and add the __exit__ (calls close()) method to
> generator objects so that you can omit the closing and just do:
>
> with read_multiple(…) as chunks:
> for contents in chunks:
> ...
--
Oscar
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