Zapier magically collapses lists with one item

I’m building a zap which is triggered by invoices. The trigger outputs a record with numbered line items (like a nested dictionary). The test record that I have contains only one line item but there could be many. The attribute that I'm interested in is called "Invoice Line Items".
example:

invoice_number
invoice_date
line_items
  1
    description
    quantity
    price

The next step is "Run Python in Code by Zapier", which I pass the line_items attribute to.

When python reads line_items I was expecting it to see:

1
  description
  quantity
  price

Instead it sees:

description
quantity
price

This works for invoices with one line item but how does it handle invoices with more than one line item? I only have test data for invoices with one line item so I can’t find out by experimentation.

My best guess is that for situations with 2 or more items it will structure it as I expect but would appreciate clarification. I have read lots of docs and tried the zapier ai support bot which got confused.

Pointers much appreciated!