- PySpark is an interface for Apache Spark in Python.
- It not only allows you to write Spark applications using Python APIs but also provides the PySpark shell for interactively analyzing your data in a distributed environment.
- PySpark supports most of Spark’s features such as Spark SQL, DataFrame, Streaming, MLlib (Machine Learning), and Spark Core.
- In this case study, we will look into creating a basic spark cluster, importing some data, checking the various dataframe functionalities, and performing EDA on the data using PySpark.
You can find the notebook here with the codes and plots.
- So let’s get started!
Installing & Importing Libraries
Before we begin, we need to install two important libraries for this case study:
- Pyspark: Using PySpark, you can work with RDDs in the Python programming language. A library called Py4j allows you to achieve this and offers PySpark Shell which links the Python API to the spark core and initializes the Spark context.
- HandySpark: It is a library that allows you to plot visualizations with a pyspark dataframe.
To install these libraries, simply write the following commands:
!pip install -q pyspark
!pip install -q handyspark
After installing the libraries, we need to import the pyspark library along with some important classes and modules:
- SparkContext or SparkSession are used as the entry point to any spark functionality. When we run any Spark application, a driver program starts, which has the main function and your SparkContext/SparkSession gets initiated here. The driver program then runs the operations inside the executors on worker nodes. SparkContext uses Py4J to launch a JVM and creates a JavaSparkContext.
- SQL module functions like col, when, isnan, Window, Row, and various other sub-functions.
- We are also importing some standard libraries like matplotlib and seaborn that will assist during the data visualization part.
Initializing a Spark Session
You can create a spark session using the SparkSession.builder.appName() module using the following code:
# Building a spark app/session
spark = SparkSession.builder.appName(“carsSpark”).getOrCreate()# single cluster information
- Here, we have named our app “carSpark” and used the getOrCreate() function to retrieve (if exists already) or create the app.
- You get the following output after writing the above code:
About the Dataset
- We will use the CarDekho Indian Car Price dataset that can be found here. It is a simple dataset that can be used for this exercise.
- The dataset has the following features:
- PySpark has its own dataframe and functionalities.
- Here we are going to read a single CSV into the dataframe using
spark.read.csv()and then use that dataframe to perform the analysis:
# Reading the data
df = spark.read.csv(‘/content/car data.csv’, header=True, inferSchema=”true”)# Shape of the dataset
print(‘Shape of the dataset: ‘, (df.count(), len(df.columns)))# Displaying top n=10 rows
- We set the inferSchema attribute as True, this will go through the CSV file and automatically adapt its schema into PySpark Dataframe.
- We get the following output for the above command:
df.describe().show()allows us to see a general description of the dataset, similar to what pandas
- We get the following output:
- We can further use
df.printSchema()to get the schema information of the dataset:
- Pyspark dataframes don’t have readily available, easy to run functions to check for data inconsistencies and null values.
- We are using a driver function, with the help of Pyspark’s inbuilt SQL functions to check for the null values in our dataframe:
- The dataset doesn't need any preprocessing so we can simply proceed with the EDA of this dataset.
Exploratory Data Analysis
- PySpark dataframes do not support visualizations like pandas does with its plot() method.
- A lot of users simply convert their dataframe to pandas which do not translate well for real-world big data as pandas require the whole data to be in the memory for processing and visualization.
- But there is another library that bridges this gap by giving pandas-like functionality to your PySpark dataframe, without compromising with the drawbacks of pandas — HandySpark.
- We can simply convert our pyspark dataframe to handyspark dataframe using the following code:
hdf = df.toHandy()
- HandySpark provides a
hist()function that can be used to plot histograms and bar plots.
- Some of the univariate analysis that uses this
hist()function method is as follows:
Question 1: What is the spread of the
- We see a right-skewed distribution.
- Max price can go till 35 lacs causing the skew.
- The majority of the cars have price listings under 15 lacs.
Question 2: What is the distribution of classes for the
- More than 175 sellers are dealers and the rest are simply individuals looking for selling their cars.
Question 3: What is the distribution of classes for the
- Most of the listed cars (n(cars) > 250) are of Manual transmission type.
Question 4: What is the distribution of classes for
- We have three owner types where 0 indicates brand new cars (0 previous owners), 1 indicates that the car is second-hand, and 3 indicates that there were 3 owners before the car was listed.
- There is just one row that has Owner=3.
Let’s see some Multivariate Analysis. HandySpark also allows you to plot a scatterplot with its
scatterplot() function. Let’s see some examples:
Question 5: Determine the relation between
- We don’t observe any linear relation between Kms_Driven and Selling_Price.
Question 6: Determine the relation between
- We observe a slight linear relation between the present price and the selling price.
Question 7: Determine the relation between
Transmission Type features?
- HandySpark offers a unique take on groupby like commands with the help of
stratify()method which works like the split-apply-combine approach.
- It will first split your HandyFrame according to the specified (discrete) columns, then it will apply some function to each stratum of data and finally combine the results back together.
- This is better illustrated with an example — let’s try the stratified version of
Name: Selling_Price, dtype: float64
- The mean value for Manual Transmission cars is 3.9 lacs and it is 9.4 lacs for Automatic. Let’s see the distribution of the same:
- Most of the cars are of Manual transmission and are available under 20 lacs.
- We can see that an equal number of Automatic transmission cars are present for all price ranges.
Question 8: Determine the relation between
- Individual sellers are selling their cars relatively cheaply than the proper dealers.
Question 9: Determine the relation between
- CNG fuel-type cars are being sold the cheapest with their selling prices less than 5 lacs.
- We can then see that petrol cars are listed in the most with max selling price around 20 lacs.
- Diesel car prices can go as high as 35 lacs.
Question 10: Plot a heatmap to check for correlations between features?
- Selling_Price is highly correlated with Presnet_Price and is mildly influenced by the Year of making as well.
- We can see Present_Price is slightly related to Kms_Driven but is not influenced as much.
- Kms_Driven and Year are inversely correlated, indicating that some listed cars are being driven since their make year.
- We have finally performed EDA on the cars data, extracted some important insights that can be useful for model building.
- You can find the notebook here.
- In the next article, we will use a VectorAssembler for preparing our data for the machine learning model.
- This will be proceeded by a linear regression training and evaluation where we will observe whether the model is able to fit the data properly or not.
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