Capturing Form component attributes into a data structure

Anvil Q&A
23

The power of databinding is especially evident with repeating panels.

The very basic idea is:

  • You assign a list of items to a repeating panel: self.rp.items = [...]
  • Anvil uses the template form associated to the repeating panel self.rp to add a form instance per each item of self.rp.items, and assign that item to that form instance’s item.
  • The form instance has its own self.item automatically updated with its own item, whatever that item is

Typically the original list contains dictionary or dictionary like objects, like datatable rows or like any class that you may define as described in the “What to do” paragraph here.

Anvil helps you by pre-populating the expression in the databinding input box with self.item[] and tries to understand what makes sense inside the square brackets. For example, if it knows the type of the item contained in the list assigned to the repeating panel, it will auto complete with whatever keys are available for that dictionary. For example, if the list contains rows of a specific table, the autocompletion will suggest the column names.

Anvil suggests you should enter self.item[something], but that’s a python expression and you can put whatever you like. The only constraint is you shouldn’t use globals from other modules. Something like Globals.parameter_1 will not work. You will need to add self.parameter_1 = Globals.parameter_1 at the beginning of the the form.__init__, then use self.parameter_1 in the databinding.

I sometimes don’t even use self.item. For example, I am working now on a form that has def __init__(self, truck, **properties):, then I use self.truck.number, etc. in the databinding expressions, or self.truck in the databinding of the visible property of some components, so they are only visible if self.truck is defined.

I personally don’t like databinding because it’s just hidden python code, and I don’t like hidden python code. Creating a loop in the __init__ that takes care of assigning the values where they belong would do the same and would be more readable and searchable.

Said that, I use databinding all the times, because it’s so handy and easy to use.

I usd to never use databinding because errors in the code in the databinding expression used to be a nightmare to debug. They would just fail with very little feedback about the reason.

Then the errors started to show up as clickable links on the console (see here), so I started to use databinding.

Then that link stopped working (see here), but the information about the error is still there and it’s very helpful, so I still use databinding. A lot!

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