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

Ethan Furman ethan at stoneleaf.us
Thu Sep 7 09:25:22 EDT 2017

On 09/06/2017 11:57 PM, Yury Selivanov wrote:

On Wed, Sep 6, 2017 at 11:39 PM, Greg Ewing wrote:

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?

That using a CM is not required, and tracking down a bug caused by not using a CM can be difficult.

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

Because PEP 550 makes a CM unnecessary in the simple (common?) case, hiding the need for a CM in not-so-simple cases. For comparison: in Python 3 we are now warned about files that have been left open (because explicitly closing files was unnecessary in CPython due to an implementation detail) -- the solution? make files context managers whose exit closes the file.

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.

I appreciate that the scientific and number-crunching communities have been a major driver of enhancements for Python (such as rich comparisons and, more recently, matrix operators), but I don't think an enhancement for them that makes life more difficult for the rest is a net win.

-- Ethan

More information about the Python-Dev mailing list