Is it impossible to populate a spinner in my .kv file using a list in my .py file?

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No, it is not impossible. In fact, it is a common practice to populate a spinner in a .kv file using a list in a .py file. Here's an example:

In your .py file, define a list:

fruits = ['apple', 'banana', 'orange']

In your .kv file, create a spinner and set its values to the list:

Spinner:
    values: root.fruits

Make sure to prefix the list name with "root." to access it from the .kv file.