Allow providing DecimalField with a custom context

[yasondinalt]

DecimalField has weird behavior in case of float values passed to field.
If decimal_places = 1
float 2.15 will be saved as 2.1 (!)
float 2.25 will be saved as 2.2 (!)
But if float value first converted to str:
float 2.15 will be saved as 0.2 (ok)
float 2.25 will be saved as 0.2 (!)
It's because of default decimal rounding ROUND_HALF_EVEN.
As I understand build-in round() use rounding similar to ROUND_HALF_UP.

I tried first cast float to str, then use ROUND_HALF_UP, so now:
float 2.15 will be saved as 0.2 (ok)
float 2.25 will be saved as 0.3 (ok)

[yasondinalt]

Tests

[Tim Graham]

Could you give a snippet of the application code where you run into this issue?

[yasondinalt]

Replying to timgraham:

Could you give a snippet of the application code where you run into this issue?

class Invoice(models.Model):
    # same as django-paypal use
    mc_gross = models.DecimalField(max_digits=64, decimal_places=2, default=0)
    
invoice = 2.215 # float
invoice.save()

In DB will be value 2.21 (or if 2.225 -> 2.22)

[yasondinalt]

Changes

[Tim Graham]

I'm not sure whether or not to accept the ticket so I asked for advice on the ​django-developers mailing list.

p.s. You don't need to attach patches on the ticket when you also send a ​pull request.

[Tim Graham]

Accepting per Aymeric's proposal on the mailing list:

Currently DecimalField accepts max_digits and decimal_places options. I think it should accept a decimal context and delegate all operations to that context.

I suggest the following behavior:

• start with the decimal context provided in a kwarg to DecimalField or, if there is None, the current context returned by getcontext().
• modify that context to take into account max_digits and decimal_places
• ask the context to perform whatever operations we need

[Mariusz Felisiak]

#31841 was marked as a duplicate.

[Mariusz Felisiak]

#32555 was marked as a duplicate for ArrayField(DecimalField()).