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Thu Sep 7 02:39:48 EDT 2017 · https://mail.python.org/pipermail/python-dev/2017-September/149271.html

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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[Python-Dev] PEP 550 v4