You can use HiveQL's cast() type conversion function to cast an element of a nested map in Python as follows:

from pyspark.sql import Row 
df = sqlContext.createDataFrame([Row(a={'b': 1})])
str = df.selectExpr("cast(a['b'] AS STRING)")

or in Scala as follows:

val df = Seq((Map("a" -> 1))).toDF("a") 
df.selectExpr("cast(a['a'] AS STRING)")