Scala vs python

why we cant use python from Bigdata prospective ; in real time application of machine learning what is the role of scala ?

You can use Python for Big-data. In lab the codes are mostly written in Python only like word-counts. n-grams etc.
You can use Pyspark for your Big-data kennedy space project :

https://spark.apache.org/docs/latest/api/python/index.html

Scala language was originally built for the Java Virtual Machine (JVM) and interact with java code.
If you want to implement ML task in Java then you can use Scala. ( Which is not recommend due to not used in ML community )

But you can use Spark for ML using the MLlib machine learning library.
Check here. All the ML Algorithms are Implemented using Spark which is good to learn.

https://spark.apache.org/docs/latest/ml-guide.html