如何在 Django 中将浮点数和小数相乘？

Question

I am using django 2.0 and Python 3.6.4

I have two fields in my model called total and tax_total both of them are decimal fields.

total       = models.DecimalField(default=0.00, max_digits=100, decimal_places=2)
tax_total   = models.DecimalField(default=0.00, max_digits=100, decimal_places=2)

I want to calculate the value after multiplying it with 1.15 in a function.

def pre_save_cart_receiver(sender, instance, *args, **kwargs):
    new_tax_total      = instance.total * Decimal(1.15)
    instance.tax_total = format(new_tax_total, '.2f') 

pre_save.connect(pre_save_cart_receiver, sender=Cart)

It's giving me errors: unsupported operand type(s) for *: 'float' and 'Decimal'

When I want to add two decimals, I do:

    new_total           = math.fsum([cart_total,  shipping_total])
    new_total_formatted = format(new_total, '.2f')

Is there anything like that from math ?

Answer 1

try this

new_tax_total      = float(instance.total) * 1.15

if you need to round the result, use round()
new_tax_total = round(float(instance.total) * 1.15,2) where 2 is the rounding interger

Answer 2

For starters and unrelated unless you want uneeded precision on something called "tax" I would put your tax in a string.

from decimal import Decimal
Decimal(3.16)
=> Decimal('3.160000000000000142108547152020037174224853515625')
Decimal('3.16')
=> Decimal('3.16')
Decimal('3.16') * Decimal(2)
=> Decimal('6.32')
Decimal(3.16) * Decimal(2)
=> Decimal('6.320000000000000284217094304')
Decimal(3.16) * Decimal(2.5434)
=> Decimal('8.037144000000000693745505487')
Decimal('3.16') * Decimal('2.5434')
=> Decimal('8.037144')

As for why it thinks you have a float from a DecimalField , that is unknown to me. So this is a partial answer at best, sorry.

Answer 3

A bit late, but in case someone is looking for a solution, i used this one, adding a method in the custom_tags.py

def multiply_unversal(item1, item2):
    return float(item1) * float(item2)

Then in the view I call {{ data.est_number_of_shares | intcomma }}. Note the intcomma here is add comma separator for thousands.