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

Greg Ewing greg.ewing at canterbury.ac.nz
Thu Sep 7 06:37:58 EDT 2017

Yury Selivanov wrote:

I understand what Koos is talking about, but I really don't follow you. Using the "with-statements to be skipped" language is very confusing and doesn't help to understand you.

If I understand correctly, instead of using a context manager, your fractions example could be written like this:

def fractions(precision, x, y): ctx = decimal.getcontext().copy() decimal.setcontext(ctx) ctx.prec = precision yield MyDecimal(x) / MyDecimal(y) yield MyDecimal(x) / MyDecimal(y ** 2)

and it would work without leaking changes to the decimal context, despite the fact that it doesn't use a context manager or do anything else to explicitly put back the old context.

Am I right about that?

This is what I mean by "skipping context managers" -- that it's possible in some situations to get by without using a context manager, by taking advantage of the implicit local context push that happens whenever a generator is started up.

Now, there are two possibilities:

  1. You take advantage of this, and don't use context managers in some or all of the places where you don't need to. You seem to agree that this would be a bad idea.

  2. You ignore it and always use a context manager, in which case it's not strictly necessary for the implicit context push to occur, since the relevant context managers can take care of it.

So there doesn't seem to be any great advantage to the automatic context push, and it has some disadvantages, such as yield-from not quite working as expected in some situations.

Also, it seems that every generator is going to incur the overhead of allocating a logical_context even when it doesn't actually change any context vars, which most generators won't.

-- Greg

More information about the Python-Dev mailing list