Spark Error:expected zero arguments for construction of ClassDict (for numpy.core.multiarray._reconstruct)

Question

I have a dataframe in Spark in which one of the columns contains an array.Now,I have written a separate UDF which converts the array to another array with distinct values in it only. See example below:

Ex: [24,23,27,23] should get converted to [24, 23, 27] Code:

def uniq_array(col_array):     x = np.unique(col_array)     return x uniq_array_udf = udf(uniq_array,ArrayType(IntegerType()))  Df3 = Df2.withColumn("age_array_unique",uniq_array_udf(Df2.age_array)) 

In the above code, Df2.age_array is the array on which I am applying the UDF to get a different column "age_array_unique" which should contain only unique values in the array.

However, as soon as I run the command Df3.show(), I get the error:

net.razorvine.pickle.PickleException: expected zero arguments for construction of ClassDict (for numpy.core.multiarray._reconstruct)

Can anyone please let me know why this is happening?

Thanks!

Answer 1

The source of the problem is that object returned from the UDF doesn't conform to the declared type. np.unique not only returns numpy.ndarray but also converts numerics to the corresponding NumPy types which are not compatible with DataFrame API. You can try something like this:

udf(lambda x: list(set(x)), ArrayType(IntegerType())) 

or this (to keep order)

udf(lambda xs: list(OrderedDict((x, None) for x in xs)),      ArrayType(IntegerType())) 

instead.

If you really want np.unique you have to convert the output:

udf(lambda x: np.unique(x).tolist(), ArrayType(IntegerType())) 

Answer 2

You need to convert the final value to a python list. You implement the function as follows:

def uniq_array(col_array):     x = np.unique(col_array)     return list(x) 

This is because Spark doesn't understand the numpy array format. In order to feed a python object that Spark DataFrames understand as an ArrayType, you need to convert the output to a python list before returning it.