[jira] [Commented] (SPARK-29212) Add common classes without using JVM backend

["zhengruifeng (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/SPARK-29212?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16935678#comment-16935678 ] 

zhengruifeng commented on SPARK-29212:
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It seems useful to impl some algs in pure python (like wrap scikit-learn as a pyspark.ml alg)

I personally think [~zero323]'s proposal is reasonable, although Pyspark.ML is now
mostly there to wrap the Scala side.

I had a discussion with [~huaxingao] and [~srowen] , I guess they are fairly neutral on it.

 

How do you think of this? [~holden.karau@gmail.com] [~bryanc]

> Add common classes without using JVM backend
> --------------------------------------------
>
>                 Key: SPARK-29212
>                 URL: https://issues.apache.org/jira/browse/SPARK-29212
>             Project: Spark
>          Issue Type: Sub-task
>          Components: ML, PySpark
>    Affects Versions: 3.0.0
>            Reporter: zhengruifeng
>            Priority: Major
>
> copyed from [https://github.com/apache/spark/pull/25776.]
>  
> Maciej's *Concern*:
> *Use cases for public ML type hierarchy*
>  * Add Python-only Transformer implementations:
>  ** I am Python user and want to implement pure Python ML classifier without providing JVM backend.
>  ** I want this classifier to be meaningfully positioned in the existing type hierarchy.
>  ** However I have access only to high level classes ({{Estimator}}, {{Model}}, {{MLReader}} / {{MLReadable}}).
>  * Run time parameter validation for both user defined (see above) and existing class hierarchy,
>  ** I am a library developer who provides functions that are meaningful only for specific categories of {{Estimators}} - here classifiers.
>  ** I want to validate that user passed argument is indeed a classifier:
>  *** For built-in objects using "private" type hierarchy is not really satisfying (actually, what is the rationale behind making it "private"? If the goal is Scala API parity, and Scala counterparts are public, shouldn't these be too?).
>  ** For user defined objects I can:
>  *** Use duck typing (on {{setRawPredictionCol}} for classifier, on {{numClasses}} for classification model) but it hardly satisfying.
>  *** Provide parallel non-abstract type hierarchy ({{Classifier}} or {{PythonClassifier}} and so on) and require users to implement such interfaces. That however would require separate logic for checking for built-in and and user-provided classes.
>  *** Provide parallel abstract type hierarchy, register all existing built-in classes and require users to do the same.
> Clearly these are not satisfying solutions as they require either defensive programming or reinventing the same functionality for different 3rd party APIs.
>  * Static type checking
>  ** I am either end user or library developer and want to use PEP-484 annotations to indicate components that require classifier or classification model.
>  ** Currently I can provide only imprecise annotations, [such as|https://github.com/zero323/pyspark-stubs/blob/dd5cfc9ef1737fc3ccc85c247c5116eaa4b9df18/third_party/3/pyspark/ml/classification.pyi#L241]
> def setClassifier(self, value: Estimator[M]) -> OneVsRest: ...
> or try to narrow things down using structural subtyping:
> class Classifier(Protocol, Estimator[M]): def setRawPredictionCol(self, value: str) -> Classifier: ... class Classifier(Protocol, Model): def setRawPredictionCol(self, value: str) -> Model: ... def numClasses(self) -> int: ...
>  
> Maciej's *Proposal*:
> {code:java}
> Python ML hierarchy should reflect Scala hierarchy first (@srowen), i.e.
> class ClassifierParams: ...
> class Predictor(Estimator,PredictorParams):
>     def setLabelCol(self, value): ...
>     def setFeaturesCol(self, value): ...
>     def setPredictionCol(self, value): ...
> class Classifier(Predictor, ClassifierParams):
>     def setRawPredictionCol(self, value): ...
> class PredictionModel(Model,PredictorParams):
>     def setFeaturesCol(self, value): ...
>     def setPredictionCol(self, value): ...
>     def numFeatures(self): ...
>     def predict(self, value): ...
> and JVM interop should extend from this hierarchy, i.e.
> class JavaPredictionModel(PredictionModel): ...
> In other words it should be consistent with existing approach, where we have ABCs reflecting Scala API (Transformer, Estimator, Model) and so on, and Java* variants are their subclasses.
>  {code}
>  
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