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Description:

djangocalculation 1.0.0

django-calculation

Make simple calculations in your django forms using django-calculation. This app provide a Django Widget that derives its value from a expression defined in the widget instance.
The field is updated when any of the source fields change.

Installation
pip install django-calculation

Add calculation to your INSTALLED_APPS
INSTALLED_APPS = [
...
'calculation',
]

Usage
Import calculation and complete the definition.
Example
Using FormulaInput widget
from django import forms

import calculation

class TestForm(forms.Form):
quantity = forms.DecimalField()
price = forms.DecimalField()
amount = forms.DecimalField(
widget=calculation.FormulaInput('quantity*price') # <- using single math expression
)
apply_taxes = forms.BooleanField(initial=True)
tax = forms.DecimalField(
# using math expression and javascript functions.
widget=calculation.FormulaInput('apply_taxes ? parseFloat(amount/11).toFixed(2) : 0.0')
)

django-calculation works with static files and therefore it is necessary to include the media of the form in the template file.
<form method="post">
{% csrf_token %}
{{ form }}
<input type="submit" value="Submit">
</form>

{{ form.media }}

Modes
Currently the app support two modes of calculation FORMULA and SUMMARY.
FORMULA
The field value derive from a formula expression. In the expression you can refer to the form field using its name.
amount = forms.DecimalField(
widget=calculation.FormulaInput('quantity*price')
)

SUMMARY
The field value derive from a summary definition, it is useful when you need to get the sum of a django formset field.
total = forms.DecimalField(
widget=calculation.SummaryInput(
function=calculation.SUM,
field='amount'
)

Summary example
Summary definition in OrderForm
class OrderForm(forms.ModelForm):
total = forms.DecimalField(
# using SumInput a SummaryInput abstraction
widget=calculation.SumInput('subtotal')
)
class Meta:
model = Order
fields = ['date', 'customer']

OrderDetForm also contain a calculated field subtotal.
class OrderDetForm(forms.ModelForm):
subtotal = forms.DecimalField(
widget=calculation.FormulaInput('quantity*price')
)
class Meta:
model = OrderDet
fields = ['product', 'price', 'quantity', 'subtotal']

# formset definition
OrderDetFormSet = forms.inlineformset_factory(Order, OrderDet, OrderDetForm)

Roadmap

Create demo project.
Create documentation.
Add changelog.