zero323 commented on a change in pull request #27406:
[SPARK-30681][PYSPARK][SQL] Add higher order functions API to PySpark
URL: https://github.com/apache/spark/pull/27406#discussion_r373485315
##########
File path: python/pyspark/sql/functions.py
##########
@@ -2840,6 +2840,367 @@ def from_csv(col, schema, options={}):
return Column(jc)
+def _invoke_higher_order_function(name, cols, funs):
+ """
+ Invokes expression identified by name,
+ (relative to ```org.apache.spark.sql.catalyst.expressions``)
+ and wraps the result with Column (first Scala one, then Python).
+
+ :param name: Name of the expression
+ :param cols: a list of columns
+ :param funs: a list of tuples ((*Column) -> Column, Iterable[int])
+ where the second element represent allowed arities
+
+ :return: a Column
+ """
+ sc = SparkContext._active_spark_context
+ expressions = sc._jvm.org.apache.spark.sql.catalyst.expressions
+ expr = getattr(expressions, name)
+
+ jcols = [_to_java_column(col).expr() for col in cols]
+ jfuns = [_create_lambda(f, a) for f, a in funs]
+
+ return Column(sc._jvm.Column(expr(*jcols + jfuns)))
+
+
+@since(3.0)
+def transform(col, f):
+ """
+ Returns an array of elements after applying a transformation to each
element in the input array.
+
+ :param col: name of column or expression
+ :param f: a function that is applied to each element of the input array.
+ Can take one of the following forms:
+
+ - Unary ``(x: Column) -> Column: ...``
+ - Binary ``(x: Column, i: Column) -> Column...``, where the second
argument is
+ a 0-based index of the element.
+
+ and can use methods of :class:`pyspark.sql.Column`, functions defined
in
+ :py:mod:`pyspark.sql.functions` and Scala ``UserDefinedFunctions``.
+ Python ``UserDefinedFunctions`` are not supported
+ (`SPARK-27052 <https://issues.apache.org/jira/browse/SPARK-27052>`__).
+
+ :return: a :class:`pyspark.sql.Column`
+
+ >>> df = spark.createDataFrame([(1, [1, 2, 3, 4])], ("key", "values"))
+ >>> df.select(transform("values", lambda x: x * 2).alias("doubled")).show()
+ +------------+
+ | doubled|
+ +------------+
+ |[2, 4, 6, 8]|
+ +------------+
+
+ >>> def alternate(x, i):
+ ... return when(i % 2 == 0, x).otherwise(-x)
+ >>> df.select(transform("values", alternate).alias("alternated")).show()
+ +--------------+
+ | alternated|
+ +--------------+
+ |[1, -2, 3, -4]|
+ +--------------+
+ """
+ return _invoke_higher_order_function("ArrayTransform", [col], [(f, {1,
2})])
+
+
+@since(3.0)
+def exists(col, f):
+ """
+ Returns whether a predicate holds for one or more elements in the array.
+
+ :param col: name of column or expression
+ :param f: an function ``(x: Column) -> Column: ...`` returning the
Boolean expression.
+ Can use methods of :class:`pyspark.sql.Column`, functions defined in
+ :py:mod:`pyspark.sql.functions` and Scala ``UserDefinedFunctions``.
+ Python ``UserDefinedFunctions`` are not supported
+ (`SPARK-27052 <https://issues.apache.org/jira/browse/SPARK-27052>`__).
+ :return: a :class:`pyspark.sql.Column`
+
+ >>> df = spark.createDataFrame([(1, [1, 2, 3, 4]), (2, [3, -1,
0])],("key", "values"))
+ >>> df.select(exists("values", lambda x: x <
0).alias("any_negative")).show()
+ +------------+
+ |any_negative|
+ +------------+
+ | false|
+ | true|
+ +------------+
+ """
+ return _invoke_higher_order_function("ArrayExists", [col], [(f, {1})])
Review comment:
That would work, but
- we have `aggregate` case which takes two functions, and I wanted to avoid
unnecessary type checks or proliferation of utility functions (and we don't
seem to use dispatching). It means we need at least
`_invoke_higher_order_function("ArrayExists", [col], [f])`
- Checking arguments requires some fun with `inspect`, which is version
specific (2.7 sigh...), and it would have to be repeated in both.
- DRY ‒ that's the same line for each variant.
- It spreads logic over two points which, in my opinion, is harder to
follow.
I guess we could build lambdas before invoking, roughly like this
```python
jf = _create_lambda(func=f, expected_nargs={1})
return _invoke_higher_order_function("ArrayExists", [col], [jf])
```
but I am still not fond of repeating this all over the place.
> looks potentially somewhat confusing
If intention is not clear I'd prefer
```python
from collections import namedtuple
_LambdaSpec = namedtuple("_LambdaSpec", ["func", "expected_nargs"])
```
and then
```python
return _invoke_higher_order_function(
"ArrayExists",
[col],
[_LambdaSpec(func=f, expected_nargs={1})])
```
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