Add an empty column to Spark DataFrame

问题:

As mentioned in many other locations on the web, adding a new column to an existing DataFrame is not straightforward. Unfortunately it is important to have this functionality (even though it is inefficient in a distributed environment) especially when trying to concatenate two DataFrames using unionAll.

What is the most elegant workaround for adding a null column to a DataFrame to facilitate a unionAll?

My version goes like this:

from pyspark.sql.types import StringType
from pyspark.sql.functions import UserDefinedFunction
to_none = UserDefinedFunction(lambda x: None, StringType())
new_df = old_df.withColumn('new_column', to_none(df_old['any_col_from_old']))

回答1:

All you need here is a literal and cast:

from pyspark.sql.functions import lit

new_df = old_df.withColumn('new_column', lit(None).cast(StringType()))

A full example:

df = sc.parallelize([row(1, "2"), row(2, "3")]).toDF()
df.printSchema()

## root
##  |-- foo: long (nullable = true)
##  |-- bar: string (nullable = true)

new_df = df.withColumn('new_column', lit(None).cast(StringType()))
new_df.printSchema()

## root
##  |-- foo: long (nullable = true)
##  |-- bar: string (nullable = true)
##  |-- new_column: string (nullable = true)

new_df.show()

## +---+---+----------+
## |foo|bar|new_column|
## +---+---+----------+
## |  1|  2|      null|
## |  2|  3|      null|
## +---+---+----------+

A Scala equivalent can be found here: Create new Dataframe with empty/null field values

回答2:

I would cast lit(None) to NullType instead of StringType. So that if we ever have to filter out not null rows on that column...it can be easily done as follows

df = sc.parallelize([Row(1, "2"), Row(2, "3")]).toDF()

new_df = df.withColumn('new_column', lit(None).cast(NullType()))

new_df.printSchema() 

df_null = new_df.filter(col("new_column").isNull()).show()
df_non_null = new_df.filter(col("new_column").isNotNull()).show()

Also be careful about not using lit("None")(with quotes) if you are casting to StringType since it would fail for searching for records with filter condition .isNull() on col("new_column").