What does % mean/does in this context?

[Marcelo de Moraes Serpa]

Take the following piece of code:

  for item in cart.values():
            v = _button_cart % {"idx": idx,
                                "itemname": item.name,
                                "amount": item.cost,
                                "quantity": item.quantity,}
            cartitems.append(v)


What does the % operator is doing there?

Thanks.
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