Class Variables vs Methods in Django Class Based Views

Question

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?

I'm not really sure what you're asking, since you know what the problem is and what the solution is. And you correctly identify class variables, so you obviously know the difference between class and instance variables. So what are you missing? — Daniel Roseman
– Daniel Roseman, Commented Jul 27, 2017 at 21:36
I was just wondering why I can load a page, the CBV can deliver data, but apparently it could be old data that is no longer true. Does the original Class Variable persist as long as the server is running, after it is defined on the first page load? — Adam Starrh
– Adam Starrh, Commented Jul 27, 2017 at 21:55

Alasdair · Accepted Answer · 2017-07-27 21:58:26Z

1

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.

answered Jul 27, 2017 at 21:58

Alasdair

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1 Comment

Adam Starrh Over a year ago

That clears it up, and I now understand the difference. Thank you!

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