How to do opposite of explode in PySpark?

Question

Let\'s say I have a DataFrame with a column for users and another column for words they\'ve written:

Answer 1

As of the spark 2.3 release we now have Pandas UDF(aka Vectorized UDF). The function below will accomplish the OP's task... A benefit of using this function is the order is guaranteed to be preserved. Order is essential in many cases such as time series analysis.

import pandas as pd
import findspark

findspark.init()
import pyspark
from pyspark.sql import SparkSession, Row
from pyspark.sql.functions import pandas_udf, PandasUDFType
from pyspark.sql.types import StructType, StructField, ArrayType

spark = SparkSession.builder.appName('test_collect_array_grouped').getOrCreate()

def collect_array_grouped(df, groupbyCols, aggregateCol, outputCol):
    """
    Aggregate function: returns a new :class:`DataFrame` such that for a given column, aggregateCol,
    in a DataFrame, df, collect into an array the elements for each grouping defined by the groupbyCols list.
    The new DataFrame will have, for each row, the grouping columns and an array of the grouped
    values from aggregateCol in the outputCol.

    :param groupbyCols: list of columns to group by.
            Each element should be a column name (string) or an expression (:class:`Column`).
    :param aggregateCol: the column name of the column of values to aggregate into an array
            for each grouping.
    :param outputCol: the column name of the column to output the aggregeted array to.
    """
    groupbyCols = [] if groupbyCols is None else groupbyCols
    df = df.select(groupbyCols + [aggregateCol])
    schema = df.select(groupbyCols).schema
    aggSchema = df.select(aggregateCol).schema
    arrayField = StructField(name=outputCol, dataType=ArrayType(aggSchema[0].dataType, False))
    schema = schema.add(arrayField)
    @pandas_udf(schema, PandasUDFType.GROUPED_MAP)
    def _get_array(pd_df):
        vals = pd_df[groupbyCols].iloc[0].tolist()
        vals.append(pd_df[aggregateCol].values)
        return pd.DataFrame([vals])
    return df.groupby(groupbyCols).apply(_get_array)

rdd = spark.sparkContext.parallelize([Row(user='Bob', word='hello'),
                                      Row(user='Bob', word='world'),
                                      Row(user='Mary', word='Have'),
                                      Row(user='Mary', word='a'),
                                      Row(user='Mary', word='nice'),
                                      Row(user='Mary', word='day')])
df = spark.createDataFrame(rdd)

collect_array_grouped(df, ['user'], 'word', 'users_words').show()

+----+--------------------+
|user|         users_words|
+----+--------------------+
|Mary|[Have, a, nice, day]|
| Bob|      [hello, world]|
+----+--------------------+