[SPARK-9014] [SQL] Allow Python spark API to use built-in exponential operator

This PR addresses (SPARK-9014)[https://issues.apache.org/jira/browse/SPARK-9014]
Added functionality: `Column` object in Python now supports exponential operator `**`
Example:
```
from pyspark.sql import *
df = sqlContext.createDataFrame([Row(a=2)])
df.select(3**df.a,df.a**3,df.a**df.a).collect()
```
Outputs:
```
[Row(POWER(3.0, a)=9.0, POWER(a, 3.0)=8.0, POWER(a, a)=4.0)]
```

Author: 0x0FFF <programmerag@gmail.com>

Closes #8658 from 0x0FFF/SPARK-9014.

python/pyspark/sql/column.py

@@ -91,6 +91,17 @@ def _func_op(name, doc=''):
     return _
 
 
+def _bin_func_op(name, reverse=False, doc="binary function"):
+    def _(self, other):
+        sc = SparkContext._active_spark_context
+        fn = getattr(sc._jvm.functions, name)
+        jc = other._jc if isinstance(other, Column) else _create_column_from_literal(other)
+        njc = fn(self._jc, jc) if not reverse else fn(jc, self._jc)
+        return Column(njc)
+    _.__doc__ = doc
+    return _
+
+
 def _bin_op(name, doc="binary operator"):
     """ Create a method for given binary operator
     """
@@ -151,6 +162,8 @@ class Column(object):
     __rdiv__ = _reverse_op("divide")
     __rtruediv__ = _reverse_op("divide")
     __rmod__ = _reverse_op("mod")
+    __pow__ = _bin_func_op("pow")
+    __rpow__ = _bin_func_op("pow", reverse=True)
 
     # logistic operators
     __eq__ = _bin_op("equalTo")

python/pyspark/sql/tests.py

@@ -568,7 +568,7 @@ class SQLTests(ReusedPySparkTestCase):
         cs = self.df.value
         c = ci == cs
         self.assertTrue(isinstance((- ci - 1 - 2) % 3 * 2.5 / 3.5, Column))
-        rcc = (1 + ci), (1 - ci), (1 * ci), (1 / ci), (1 % ci)
+        rcc = (1 + ci), (1 - ci), (1 * ci), (1 / ci), (1 % ci), (1 ** ci), (ci ** 1)
         self.assertTrue(all(isinstance(c, Column) for c in rcc))
         cb = [ci == 5, ci != 0, ci > 3, ci < 4, ci >= 0, ci <= 7]
         self.assertTrue(all(isinstance(c, Column) for c in cb))