[Python-Dev] PEP 550 v4

[Yury Selivanov]

On Wed, Sep 6, 2017 at 11:39 PM, Greg Ewing <greg.ewing at canterbury.ac.nz> wrote:
> Yury Selivanov wrote:
>>
>> It would be great if you or Greg could show a couple of real-world
>> examples showing the "issue" (with the current PEP 550
>> APIs/semantics).
>
>
> Here's one way that refactoring could trip you up.
> Start with this:
>
>    async def foo():
>       calculate_something()
>       #in a coroutine, so we can be lazy and not use a cm
>       ctx = decimal.getcontext().copy()
>       ctx.prec = 5
>       decimal.setcontext(ctx)
>       calculate_something_else()
>
> And factor part of it out (into an *ordinary* function!)
>
>    async def foo():
>       calculate_something()
>       calculate_something_else_with_5_digits()
>
>    def calculate_something_else_with_5_digits():
>       ctx = decimal.getcontext().copy()
>       ctx.prec = 5
>       decimal.setcontext(ctx)
>       calculate_something_else()
>
> Now we add some more calculation to the end of foo():
>
>    async def foo():
>       calculate_something()
>       calculate_something_else_with_5_digits()
>       calculate_more_stuff()
>
> Here we didn't intend calculate_more_stuff() to be done
> with prec=5, but we forgot that calculate_something_else_
> with_5_digits() changes the precision and *doesn't restore
> it* because we didn't add a context manager to it.
>
> If we hadn't been lazy and had used a context manager in the
> first place, that wouldn't have happened.
>
> Summary: I think that skipping context managers in some
> circumstances is a bad habit that shouldn't be encouraged.
>
>
> --
> Greg
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