DecimalRenderer ignores its currency when formatting for an Amount

Original comment by Justus Pendleton (Bitbucket: hoostus, GitHub: hoostus).

I've spent a while going through the code in query_render.py gradually (mostly, sorta, kinda) figuring out how the PositionRenderer -> AmountRenderer -> DecimalRenderer stuff all hangs together. To be honest, I don't (yet, at least) totally understand what problem there is with ignoring the key in DecimalRenderer.format(). But during the course of looking over the code I think I've come across a bigger issue, DecimalRender is mixing up global scope (i.e. all transactions in the bean file) and local scope (i.e. just the transactions in the query result set).

As I began to understand the rendering code I began to wonder...why is DecimalRenderer doing all this work anyway? It seems like the whole thing can be ripped out and replaced with a DisplayFormatter using Align.DOT (and turning off commas).

#!python

    def format(self, number, key=None):
        display_formatter = self.dcontext.build(alignment=display_context.Align.DOT, commas=False)
        return display_formatter.format(number)

Doing this doesn't result in exactly the same output, however. And investigating why that is led me to realise: in DecimalRenderer.update() there's a call to the global display context

#!python

    def update(self, number, key=None):
        if number is None:
            return
        # Quantize the number based on the display context.
        qnumber = self.dcontext.quantize(number, key)

However, the rest of DecimalRenderer is building up what is effectively a DisplayFormatter but with a scope of just the query result set. What exactly does that mean and what are the consequences?

Imagine:

The full beanfile has 10,000 transactions with 15 currencies, whose precisions vary from 0 to 15.
The query result set only has 10 transactions with 2 currencies, whose precisions are both 2.

The results of Precision.MOST_COMMON or Precision.MAXIMUM will now vary.

#!python

self.dcontext.quantize(number, key) == 4
self.dists[key].mode() == 2

So it seems clear (to me at least!) that using the global DisplayContext but constructing a local ad-hoc formatter in DecimalFormatter is wrong. The question is which of the two needs to be fixed:

Do we stop using the global DisplayContext for quantizing?
Or do we stop building a local formatter and just use the DisplayFormatter from the global DisplayContext? (ed: this would be way easier :point_up:)

I can start working on a patch once there's some agreement on which direction is best.

tbm / test

DecimalRenderer ignores its currency when formatting for an Amount #68