Document customSchema (#199)

* document customSchema

* Update samples/src/main/python/pyKusto.py

Co-authored-by: Yochai Gilad <yogilad@microsoft.com>

* Update pyKusto.py

* Update pyKusto.py

Co-authored-by: Yochai Gilad <yogilad@microsoft.com>

samples/src/main/python/pyKusto.py

@@ -46,11 +46,16 @@ kustoDf  = pyKusto.read. \
 # Read the data from the kusto table in forced 'distributed' mode and with advanced options
 # Please refer to https://github.com/Azure/azure-kusto-spark/blob/master/connector/src/main/scala/com/microsoft/kusto/spark/datasource/KustoSourceOptions.scala
 # in order to get the string representation of the options - as pyspark does not support calling properties of scala objects.
+# ClientRequestProperties are used in every command executed on the service (schema inference, export command or query. in older versions it is used just the for the export command)
 
 crp = sc._jvm.com.microsoft.azure.kusto.data.ClientRequestProperties()
 crp.setOption("norequesttimeout",True)
 crp.toString()
 
+# Use customSchema to enforce the schema and remove the initial command to the service for schema inference. 
+# The provided Schema must be a subset of the query result schema.
+ddl = spark.sparkContext._jvm.org.apache.spark.sql.types.DataType.fromJson(customSchema.json()).toDDL()
+
 kustoDf  = pyKusto.read. \
             format("com.microsoft.kusto.spark.datasource"). \
             option("kustoCluster", kustoOptions["kustoCluster"]). \
@@ -61,17 +66,7 @@ kustoDf  = pyKusto.read. \
             option("kustoAadAuthorityID", kustoOptions["kustoAadAuthorityID"]). \
             option("clientRequestPropertiesJson", crp.toString()). \
             option("readMode", 'ForceDistributedMode'). \
-            load()
-
-kustoDf  = pyKusto.read. \
-            format("com.microsoft.kusto.spark.datasource"). \
-            option("kustoCluster", kustoOptions["kustoCluster"]). \
-            option("kustoDatabase", kustoOptions["kustoDatabase"]). \
-            option("kustoQuery", kustoOptions["kustoTable"]). \
-            option("kustoAadAppId", kustoOptions["kustoAadAppId"]). \
-            option("kustoAadAppSecret", kustoOptions["kustoAadAppSecret"]). \
-            option("kustoAadAuthorityID", kustoOptions["kustoAadAuthorityID"]). \
-            option("clientRequestPropertiesJson", crp.toString()). \
+            option("customSchema", ddl). \
             load()
 
 
@@ -154,7 +149,7 @@ kustoQ.start().awaitTermination(60*8)
 
 # Device authentication for databricks (Scala users can just discard any authentication parameters and get the same result)
 # Acquire a token with device authentication and pass the token to the connector, this token will expire in one hour but
-# it should be enough for reading as the call to the service is done at the start of the flow but writing should be done
+# it should be enough for reading as the call to the service is done at the start of the flow. Write commands should be done
 # in an hour.
 # Prints done inside the JVM are not shown in the notebooks, therefore the user has to print himself the device code.