CODEWITHSUNDEEP
pythondjangodjango-class-based-views
Adam Starrh
Adam Starrh

Reputation: 6968

Class Variables vs Methods in Django Class Based Views

I am a self-taught amateur ever trying to understand fundamentals of Python, Django and programming in general. I'm looking to understand an issue I was having.

So I have this class

class ContractsView(InventoryView):
    template_name = "contracts.html"
    page = "Contracts"
    primary_table, secondary_table = build_contracts_tables(**{"commodity": None})

and it uses the following function:

def build_contracts_tables(**kwargs):
    print('fire')
    primary_table_query = Purchase.inventory.current_contracts_totals(**kwargs)
    primary_table_fields = ("total_meats", "total_value", "meat_cost")
    primary_table_html = build_table([primary_table_query,], *primary_table_fields) if primary_table_query else err_str

    secondary_table_query = Purchase.inventory.current_contracts(**kwargs)
    secondary_table_fields = ("invoice", "supplier", "variety", "meats", "value", "meat_cost", "ship_date")
    secondary_table_html = build_table(secondary_table_query, *secondary_table_fields) if secondary_table_query else err_str

    return primary_table_html, secondary_table_html

Somehow, the view is sending something to the template, as it does render some data. However, the data doesn't update right away (it eventually does), meaning I will refresh it after changes to the database, but old data will persist. Additionally, I don't ever see my print appear in the console.

However, when I convert the class variables into functions, it works just fine:

class ContractsView(InventoryView):
    template_name = "contracts.html"
    page = "Contracts"

    def primary_table(self):
        x,y = build_contracts_tables(**{"commodity": None})
        return x

    def secondary_table(self):
        x, y = build_contracts_tables(**{"commodity": None})
        return y

Could someone help me understand the rule I am breaking in my original attempt?

Upvotes: 1

Views: 107

Answers (1)

Alasdair
Alasdair

Reputation: 309099

You shouldn't set primary_table and secondary_table as class variables, because they will be calculated once when the module is loaded.

As you have already worked out, the correct approach is to use methods. This way, the method runs when the view runs, so you get the up to date value.

Upvotes: 1

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