Usage of custom Python object in Pyspark UDF

问题:

When running following piece of PySpark code:

nlp = NLPFunctions()

def parse_ingredients(ingredient_lines):
    parsed_ingredients = nlp.getingredients_bulk(ingredient_lines)[0]
    return list(chain.from_iterable(parsed_ingredients))


udf_parse_ingredients = UserDefinedFunction(parse_ingredients, ArrayType(StringType()))

I get the following error: _pickle.PicklingError: Could not serialize object: TypeError: can't pickle _thread.lock objects

I imagine this is because PySpark can not serialize this custom class. But how can I avoid the overhead of instantiating this expensive object on every run of the parse_ingredients_line function?

回答1:

Edit: this answer is wrong. The object is still serialized and then de-serialized when it is broadcast, and so serialization is not avoided. (Tips for properly using large broadcast variables?)

---

Try using a broadcast variable.

sc = SparkContext()
nlp_broadcast = sc.broadcast(nlp) # Stores nlp in de-serialized format.

def parse_ingredients(ingredient_lines):
    parsed_ingredients = nlp_broadcast.value.getingredients_bulk(ingredient_lines)[0]
    return list(chain.from_iterable(parsed_ingredients))

回答2:

I solved it based on (https://github.com/scikit-learn/scikit-learn/issues/6975) by making all dependencies of the NLPFunctions class serializable.

回答3:

Let's say you want to use Identity class defined like this (identity.py):

class Identity(object):                   
    def __getstate__(self):
        raise NotImplementedError("Not serializable")

    def identity(self, x):
        return x

you can for example use a callable object (f.py) and store an Identity instance as a class member:

from identity import Identity

class F(object):                          
    identity = None

    def __call__(self, x):
        if not F.identity:
            F.identity = Identity()
        return F.identity.identity(x)

and use these as shown below:

from pyspark.sql.functions import udf
import f

sc.addPyFile("identity.py")
sc.addPyFile("f.py")

f_ = udf(f.F())

spark.range(3).select(f_("id")).show()

+-----+
|F(id)|
+-----+
|    0|
|    1|
|    2|
+-----+

or standalone function and closure:

from pyspark.sql.functions import udf
import identity

sc.addPyFile("identity.py")

def f(): 
    dict_ = {}                 
    @udf()              
    def f_(x):                 
        if "identity" not in dict_:
            dict_["identity"] = identity.Identity()
        return dict_["identity"].identity(x)
    return f_


spark.range(3).select(f()("id")).show()

+------+
|f_(id)|
+------+
|     0|
|     1|
|     2|
+------+