Consumer polling and partitions


I have a question regarding polling from different partitions by a single consumer.

Let’s assume a consumer is assigned to partitions A and B. Partitions A is full of data. For example:

B: [Y][Y]

Let’s assume the consume will start polling from partition A. When will it switch to partition B?. In other words: will the consumer drain partition A first and only then switch to partition B once partition A is empty?

Thanks in advance for you reply