For-loop implementation of a key for an object HTML

Question

I have a queryset full of weather objects and I need to loop through a custom array to extract specific weather metrics in each of the weather objects in an HTML template for a Django application.

detail.html

<table>
<tr>
   {% for weather_object in weather_objects %}
      {% for variable in variable_list %}  ## variable_list = ['temp', 'humidity']
          <td>{{weather_object.variable}}</td>
      {% endfor  %}

   {% endfor %}
</tr>
</table>

Views.py

context = {'weather_objects': weather_objects,
           'variable_list': variable_list}

return render(
    request,
    'webapp/detail.html',
    context=context
)

There could be better ways to do this, but honestly, I'm really stumped here. Why can't I loop through the variable list and extract data from weather_object? It's specifically 'weather_object.variable' that just acts nonexistent.

I can do it manually, I can write specifically

<td>{{weather_objects.temp}}</td>

or

<td>{{weather_objects.humidity}}</td>

but I can't automate it in a for a loop. Why?? I have verified that the variables in the variable list are correct and should work. Could it be because the variable is only a string substitute?

because {{ weather_object.variable }} will look for weather_object.variable (so an attributed named variable), and weather_object['variable'], not for weather_object.temp or weather_object['temp'] if 'temp' is assigned to variable. — willeM_ Van Onsem
– willeM_ Van Onsem, Commented Dec 3, 2021 at 18:18

willeM_ Van Onsem · Accepted Answer · 2021-12-03 18:33:52Z

If you write:

{{weather_object.variable }}

it aims to obtain the attribute named variable from the weather_object, and if that fails, it will try to use weather_object['variable']. The fact that there is a variable named variable does not matter. That would also be very unstable: imagine that you assign a variable named humidity = 'temp'. It would easily break the application since now all item.humiditys all of a sudden should use item.temp instead. That would be bad code design.

However a template should not implement business logic: it should only implement rendering logic. It is the view that needs to pass data in an accessible way to the view.

You for example can construct a list of lists for your weather_objects with:

variable_list = ['temp', 'humidity']
weather_data = [
    [getattr(datum, v) for v in variable_list]
    for datum in wheather_objects
  ]
context = {'weather_data': weather_data }

return render(
    request,
    'webapp/detail.html',
    context=context
)

and then render this as:

<table>
  {% for weather_object in weather_data %}
    <tr>
      {% for item in weather_object %}
          <td>{{ item }}</td>
      {% endfor %}
    </tr>
  {% endfor %}
</table>

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