[SPARK-14555] First cut of Python API for Structured Streaming

## What changes were proposed in this pull request?

This patch provides a first cut of python APIs for structured streaming. This PR provides the new classes:
 - ContinuousQuery
 - Trigger
 - ProcessingTime
in pyspark under `pyspark.sql.streaming`.

In addition, it contains the new methods added under:
 -  `DataFrameWriter`
     a) `startStream`
     b) `trigger`
     c) `queryName`

 -  `DataFrameReader`
     a) `stream`

 - `DataFrame`
    a) `isStreaming`

This PR doesn't contain all methods exposed for `ContinuousQuery`, for example:
 - `exception`
 - `sourceStatuses`
 - `sinkStatus`

They may be added in a follow up.

This PR also contains some very minor doc fixes in the Scala side.

## How was this patch tested?

Python doc tests

TODO:
 - [ ] verify Python docs look good

Author: Burak Yavuz <brkyvz@gmail.com>
Author: Burak Yavuz <burak@databricks.com>

Closes #12320 from brkyvz/stream-python.

1 files changed, 12 insertions, 0 deletions

diff --git a/python/pyspark/sql/dataframe.py b/python/pyspark/sql/dataframe.py
index 328bda6601..bbe15f5f90 100644
--- a/python/pyspark/sql/dataframe.py
+++ b/python/pyspark/sql/dataframe.py
@@ -197,6 +197,18 @@ class DataFrame(object):
         """
         return self._jdf.isLocal()
 
+    @property
+    @since(2.0)
+    def isStreaming(self):
+        """Returns true if this :class:`Dataset` contains one or more sources that continuously
+        return data as it arrives. A :class:`Dataset` that reads data from a streaming source
+        must be executed as a :class:`ContinuousQuery` using the :func:`startStream` method in
+        :class:`DataFrameWriter`.  Methods that return a single answer, (e.g., :func:`count` or
+        :func:`collect`) will throw an :class:`AnalysisException` when there is a streaming
+        source present.
+        """
+        return self._jdf.isStreaming()
+
     @since(1.3)
     def show(self, n=20, truncate=True):
         """Prints the first ``n`` rows to the console.