I have three ideas about this.
1) In my opinion, assigning a float to a decimal field is a programming error.
Sadly Python doesn’t raise an exception in that case.
>>> decimal.Decimal(2.15)
Decimal('2.149999999999999911182158029987476766109466552734375')
So Django has to support it as well.
2) Python 3 provides the `create_decimal_from_float(<float>)` class method on
`decimal.Context` for this purpose. I think that’s what Django should use.
>>> decimal.Context(prec=10).create_decimal_from_float(2.15)
Decimal('2.150000000')
https://docs.python.org/3.5/library/decimal.html#decimal.Context.create_decimal_from_float
(I don’t care much about Python 2 at this point. We should just make sure we
don’t introduce a debatable behavior in the last LTS release supporting Python
2.)
3) 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
https://docs.python.org/3.5/library/decimal.html#decimal.getcontext
I’m -1 on emulating that with arbitrary string conversions and rounding rules
and also -1 on deviating from Python’s default behavior, which matches the IEEE
754 specification, without providing a way to customize that behavior properly.
I hope this helps !
--
Aymeric.
> On 05 Apr 2016, at 15:49, Tim Graham <[email protected]> wrote:
>
> If you assign a float with more decimal places than a DecimalField field
> supports to that field, the value may round differently than if you assign
> the same value as a string. For example:
>
> class Invoice(models.Model):
> gross = models.DecimalField(max_digits=10, decimal_places=1)
>
> invoice.gross = 2.15 # saves as 2.1
> invoice.gross = '2.15' # saves as 2.2
>
> The default behavior of decimal rounding is ROUND_HALF_EVEN (to nearest with
> ties going to nearest even integer). There's a proposal to change this to
> cast floats to string and then use ROUND_HALF_UP to match the value of
> strings [0][1]. Do you have any concerns about this? Is it something we
> should even care about?
>
> [0] https://github.com/django/django/pull/6410
> <https://github.com/django/django/pull/6410>
> [1] https://code.djangoproject.com/ticket/26459
> <https://code.djangoproject.com/ticket/26459>
>
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