Phillip writes:

...

@do_template def with_extra_precision(places=2): "Performs nested computation with extra digits of precision." decimal.getcontext().prec += 2 yield None decimal.getcontext().prec -= 2

Won't this do the wrong thing if something within the block alters the precision?

Depends on what behavior you want. I wrote it this way partly because that's how the example in the manual was written, and partly because I was thinking "if someone increased the precision by 3 within the block, then we probably want to leave it increased by 3 on block exit". On careful re-consideration, I think it'd be better to require that the block NOT make unbalanced changes to the precision... we could verify it using an assert statement. I avoided caching the context and restoring it, because I WANTED to allow code in the block to make OTHER alterations to the context and not clobber them after the block exits (eg: setting or clearing some trap). It's debatable whether that was a good design choice... there's a great deal of elegence to Guido's version used like this: Guido:

do with_decimal_context() as ctx: ctx.prec += 2 # change other settings # algorithm goes here

However, all of these are minor details compared to the bug that Raymond points out: Raymond:

The final "return +s" should be unindented. It should be at the same level as the "do with_extra_precision()". The purpose of the "+s" is to force the result to be rounded back to the *original* precision.

In effect, the "with_extra_precision" wrapper causes the calculations to be done with higher precision, AND causes any variables set during the block will retain their higher precision. (It's because context controls OPERATIONS but changing context never affects individual Decimal OBJECTS.) So I fear that the whole with_extra_precision() idea is just likely to tempt people into introducing subtle bugs, and it should probably be scrapped anyhow. Guido's approach to save-and- restore context works fine. -- Michael Chermside (PS: Another reason that I avoided a basic save-and-restore is that we have plenty of examples already of 'do' statements that save-and-restore, I was trying to do something different. It's interesting that what I tried may have turned out to be a poor idea.)