Small corrections: About 1) — actually decimal.Decimal(<float>) was an error Python 3.0 and 3.1.
About 2) — I mixed up two options for the factory to build decimals from floats: - the decimal.Decimal.from_float class method — which I assume uses the global context - the decimal.Context..create_decimal_from_float method — which relies on a specific context https://docs.python.org/3.5/library/decimal.html#decimal.Decimal.from_float I assume the latter will be more convenient for low level control. -- Aymeric. > On 05 Apr 2016, at 16:17, Aymeric Augustin > <aymeric.augus...@polytechnique.org> wrote: > > 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 > > <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 > <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 <timogra...@gmail.com >> <mailto:timogra...@gmail.com>> 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> >> >> -- >> You received this message because you are subscribed to the Google Groups >> "Django developers (Contributions to Django itself)" group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to django-developers+unsubscr...@googlegroups.com >> <mailto:django-developers+unsubscr...@googlegroups.com>. >> To post to this group, send email to django-developers@googlegroups.com >> <mailto:django-developers@googlegroups.com>. >> Visit this group at https://groups.google.com/group/django-developers >> <https://groups.google.com/group/django-developers>. >> To view this discussion on the web visit >> https://groups.google.com/d/msgid/django-developers/3749bbaf-ded7-4de3-912b-3a9e654b0207%40googlegroups.com >> >> <https://groups.google.com/d/msgid/django-developers/3749bbaf-ded7-4de3-912b-3a9e654b0207%40googlegroups.com?utm_medium=email&utm_source=footer>. >> For more options, visit https://groups.google.com/d/optout >> <https://groups.google.com/d/optout>. > -- You received this message because you are subscribed to the Google Groups "Django developers (Contributions to Django itself)" group. 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