[SPARK-6117] [SQL] Improvements to DataFrame.describe()

1. Slightly modifications to the code to make it more readable.
2. Added Python implementation.
3. Updated the documentation to state that we don't guarantee the output schema for this function and it should only be used for exploratory data analysis.

Author: Reynold Xin <rxin@databricks.com>

Closes #5201 from rxin/df-describe and squashes the following commits:

25a7834 [Reynold Xin] Reset run-tests.
6abdfee [Reynold Xin] [SPARK-6117] [SQL] Improvements to DataFrame.describe()

1 files changed, 19 insertions, 0 deletions

diff --git a/python/pyspark/sql/dataframe.py b/python/pyspark/sql/dataframe.py
index bf7c47b726..d51309f7ef 100644
--- a/python/pyspark/sql/dataframe.py
+++ b/python/pyspark/sql/dataframe.py
@@ -520,6 +520,25 @@ class DataFrame(object):
 
     orderBy = sort
 
+    def describe(self, *cols):
+        """Computes statistics for numeric columns.
+
+        This include count, mean, stddev, min, and max. If no columns are
+        given, this function computes statistics for all numerical columns.
+
+        >>> df.describe().show()
+        summary age
+        count   2
+        mean    3.5
+        stddev  1.5
+        min     2
+        max     5
+        """
+        cols = ListConverter().convert(cols,
+                                       self.sql_ctx._sc._gateway._gateway_client)
+        jdf = self._jdf.describe(self.sql_ctx._sc._jvm.PythonUtils.toSeq(cols))
+        return DataFrame(jdf, self.sql_ctx)
+
     def head(self, n=None):
         """ Return the first `n` rows or the first row if n is None.