UDF can not be found due to 2 instances of SparkSession is created

mvn clean package -Pspark-2.0 -Phadoop-2.6 -Pyarn -Ppyspark -DskipTests

%pyspark
from pyspark.sql import SQLContext, Row
from pyspark.sql.types import StringType, IntegerType, StructType, StructField, MapType, FloatType, ArrayType
from pyspark.sql.functions import udf

sqlContext.registerFunction("stringLengthString", lambda x: len(x))

Traceback (most recent call last):
  File "/tmp/zeppelin_pyspark-3201872850141735060.py", line 266, in <module>
    raise Exception(traceback.format_exc())
Exception: Traceback (most recent call last):
  File "/tmp/zeppelin_pyspark-3201872850141735060.py", line 264, in <module>
    exec(code)
  File "<stdin>", line 7, in <module>
  File "/home/egor/spark/python/pyspark/sql/context.py", line 203, in registerFunction
    self.sparkSession.catalog.registerFunction(name, f, returnType)
AttributeError: 'JavaMember' object has no attribute 'registerFunction'