As for now the recommended way is to use DataFrameWriterV2 API:

• https://iceberg.apache.org/docs/latest/spark-writes/#writing-with-dataframes
• https://spark.apache.org/docs/latest/api/python/reference/pyspark.sql/api/pyspark.sql.DataFrameWriterV2.partitionedBy.html

So the modern way to define partitions using spark DataFrame API is:

import pyspark.sql.functions as F

df.writeTo("catalog.db.table") \
    .partitionedBy(F.days(F.col("created_at_field_name"))) \
    .create()