import pyspark.sql.functions as f

join_keys = ["D_ACCOUNTS_ID", "D_APPS_ID", "D_CONTENT_PAGE_ID"]

df = (
    df
    .join(df_forecast, join_keys, 'left')
    .select(
        *join_keys,
        # selecting columns from left side of the join that are not in the join keys.
        *[df[element].alias('df_'+element) for element in df.columns if element not in join_keys],
        # selecting columns from right side of the join that are not in the join keys.
        *[df_forecast[element].alias('df_forecast_'+element) for element in df_forecast.columns if element not in join_keys]
    )
)