Pyspark UDF column on Dataframe

问题

I'm trying to create a new column on a dataframe based on the values of some columns. It's returning null in all cases. Anyone know what's going wrong with this simple example?

df = pd.DataFrame([[0,1,0],[1,0,0],[1,1,1]],columns = ['Foo','Bar','Baz'])

spark_df = spark.createDataFrame(df)

def get_profile():
    if 'Foo'==1:
        return 'Foo'
    elif 'Bar' == 1:
        return 'Bar'
    elif 'Baz' ==1 :
        return 'Baz'

spark_df = spark_df.withColumn('get_profile', lit(get_profile()))
spark_df.show()

   Foo  Bar  Baz get_profile
    0    1    0        None
    1    0    0        None
    1    1    1        None

I would expect that the get_profile column would be filled out for all rows.

I also tried this:

spark_udf = udf(get_profile,StringType())

spark_df = spark_df.withColumn('get_profile', spark_udf())
print(spark_df.toPandas())

to the same effect.

回答1:

The udf has no knowledge of what the column names are. So it checks each of your conditions in your if/elif block and all of them evaluate to False. Thus the function will return None.

You'd have to rewrite your udf to take in the columns you want to check:

from pyspark.sql.functions import udf
from pyspark.sql.types import StringType

def get_profile(foo, bar, baz):
    if foo == 1:
        return 'Foo'
    elif bar == 1:
        return 'Bar'
    elif baz == 1 :
        return 'Baz'

spark_udf = udf(get_profile, StringType())
spark_df = spark_df.withColumn('get_profile',spark_udf('Foo', 'Bar', 'Baz'))
spark_df.show()
#+---+---+---+-----------+
#|Foo|Bar|Baz|get_profile|
#+---+---+---+-----------+
#|  0|  1|  0|        Bar|
#|  1|  0|  0|        Foo|
#|  1|  1|  1|        Foo|
#+---+---+---+-----------+

If you have a lot of columns and want to pass them all (in order):

spark_df = spark_df.withColumn('get_profile', spark_udf(*spark_df.columns))

More generally, you can unpack any ordered list of columns:

cols_to_pass_to_udf = ['Foo', 'Bar', 'Baz']
spark_df = spark_df.withColumn('get_profile', spark_udf(*cols_to_pass_to_udf ))

But this particular operation does not require a udf. I would do it this way:

from pyspark.sql.functions import coalesce, when, col, lit

spark_df.withColumn(
    "get_profile",
    coalesce(*[when(col(c)==1, lit(c)) for c in spark_df.columns])
).show()
#+---+---+---+-----------+
#|Foo|Bar|Baz|get_profile|
#+---+---+---+-----------+
#|  0|  1|  0|        Bar|
#|  1|  0|  0|        Foo|
#|  1|  1|  1|        Foo|
#+---+---+---+-----------+

This works because pyspark.sql.functions.when() will return null by default if the condition evaluates to False and no otherwise is specified. Then the list comprehension of pyspark.sql.functions.coalesce will return the first non-null column.

Note this is equivalent to the udf ONLY if the order of the columns is the same as the sequence that's evaluated in the get_profile function. To be more explicit, you should do:

spark_df.withColumn(
    "get_profile",
    coalesce(*[when(col(c)==1, lit(c)) for c in ['Foo', 'Bar', 'Baz'])
).show()

来源：https://stackoverflow.com/questions/52522057/pyspark-udf-column-on-dataframe

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