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https://issues.apache.org/jira/browse/SPARK-21542?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16609495#comment-16609495
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Peter Knight commented on SPARK-21542:
--------------------------------------

It would be really helpful to have some example code on how to use these.

I have tried: 
{code}
from pyspark.ml import Transformer
from pyspark.ml.util import DefaultParamsReadable, DefaultParamsWritable

class MedianTrend(Transformer, DefaultParamsReadable, DefaultParamsWritable):
# code here to define Params and transform

# instantiate it
mt1 = MedianTrend(inputColList = ["v1"], outputColList = ["v1_trend_no_reset"], 
sortCol = "date")

# then save andit
path1 = "test_MedianTrend" 
mt1.write().overwrite().save(path1)

# then load it
mt1_loaded = mt1.load(path1)
df2 = mt1_loaded.transform(df)
df2.show()
{code}
This gives the following error:
{noformat}
'module' object has no attribute 'MedianTrend'{noformat}
 

> Helper functions for custom Python Persistence
> ----------------------------------------------
>
>                 Key: SPARK-21542
>                 URL: https://issues.apache.org/jira/browse/SPARK-21542
>             Project: Spark
>          Issue Type: New Feature
>          Components: ML, PySpark
>    Affects Versions: 2.2.0
>            Reporter: Ajay Saini
>            Assignee: Ajay Saini
>            Priority: Major
>             Fix For: 2.3.0
>
>
> Currently, there is no way to easily persist Json-serializable parameters in 
> Python only. All parameters in Python are persisted by converting them to 
> Java objects and using the Java persistence implementation. In order to 
> facilitate the creation of custom Python-only pipeline stages, it would be 
> good to have a Python-only persistence framework so that these stages do not 
> need to be implemented in Scala for persistence. 
> This task involves:
> - Adding implementations for DefaultParamsReadable, DefaultParamsWriteable, 
> DefaultParamsReader, and DefaultParamsWriter in pyspark.



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