[Python-Dev] PEP 550 v4 (original) (raw)

Yury Selivanov yselivanov.ml at gmail.com
Thu Sep 7 02:57:34 EDT 2017

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(): calculatesomething() #in a coroutine, so we can be lazy and not use a cm

Where exactly does PEP 550 encourage users to be "lazy and not use a cm"? PEP 550 provides a mechanism for implementing context managers! What is this example supposed to show?

ctx = decimal.getcontext().copy() ctx.prec = 5 decimal.setcontext(ctx) calculatesomethingelse()

And factor part of it out (into an ordinary function!) async def foo(): calculatesomething() calculatesomethingelsewith5digits() def calculatesomethingelsewith5digits(): ctx = decimal.getcontext().copy() ctx.prec = 5 decimal.setcontext(ctx) calculatesomethingelse() Now we add some more calculation to the end of foo(): async def foo(): calculatesomething() calculatesomethingelsewith5digits() calculatemorestuff() Here we didn't intend calculatemorestuff() to be done with prec=5, but we forgot that calculatesomethingelse with5digits() 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.

How is PEP 550 is at fault of somebody being lazy and not using a context manager?

PEP 550 has a hard requirement to make it possible for decimal/other libraries to start using its APIs and stay backwards compatible, so it allows decimal.setcontext(ctx) function to be implemented. We are fixing things here.

When you are designing a new library/API, you can use CMs and only CMs. It's up to you, as a library author, PEP 550 does not limit you.

And when you use CMs, there's no "problems" with 'yield from' or anything in PEP 550.

Summary: I think that skipping context managers in some circumstances is a bad habit that shouldn't be encouraged.

PEP 550 does not encourage coding without context managers. It does, in fact, solve the problem of reliably storing context to make writing context managers possible.

To reiterate: it provides mechanism to set a variable within the current logical thread, like storing a current request in an async HTTP handler. Or to implement decimal.setcontext. But you are free to use it to only implement context managers in your library.

Yury

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