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A PySpark SQL DataFrame is a distributed collection of data with a specific row and column structure. Under the hood, DataFrames are built on top of RDDs. Like pandas, PySpark SQL DataFrames allow a developer to analyze data more easily than by writing functions directly on underlying data.

DataFrames can be created manually from RDDs using rdd.toDF(["names", "of", "columns"]). In the example below, we create a DataFrame from a manually constructed RDD and name its columns article_title and view_count.

# Create an RDD from a list
hrly_views_rdd  = spark.sparkContext.parallelize([
    ["Betty_White" , 288886],
    ["Main_Page", 139564],
    ["New_Year's_Day", 7892],
    ["ABBA", 8154]
])

# Convert RDD to DataFrame
hrly_views_df = hrly_views_rdd\
    .toDF(["article_title", "view_count"])

Let’s take a look at our new DataFrame. We can use the DataFrame.show(n_rows) method to print the first n_rows of a Spark DataFrame. It can also be helpful to pass truncate=False to ensure all columns are visible.

hrly_views_df.show(4, truncate=False)

+--------------+-----------+
| article_title| view_count|
+--------------+-----------+
|   Betty_White|     288886|
|     Main_Page|     139564|
|New_Year's_Day|       7892|
|          ABBA|       8154|
+--------------+-----------+

Great! Now that this data is loaded in as a DataFrame, we can access the underlying RDD with DataFrame.rdd. You likely won’t need the underlying data often, but it can be helpful to keep in mind that a DataFrame is a structure built on top of an RDD. When we check the type of hrly_views_df_rdd, we can see that it’s an RDD!

# Access DataFrame's underlying RDD
hrly_views_df_rdd = hrly_views_df.rdd

# Check object type
print(type(hrly_views_df_rdd)) 
# <class 'pyspark.rdd.RDD'>

Instructions

Because we learned about SparkSession in the first exercise, all remaining exercises in this lesson will include the code to create a SparkSession named spark for you to use. Be sure to run these cells!

Using the RDD sample_page_views, create a DataFrame named sample_page_views_df with columns named language_code, title, date, and count.

In the same code cell, add code to show the first five rows of the DataFrame. Set truncate=False to ensure all columns are visible.

Access the RDD underlying sample_page_views_df and save it as sample_page_views_rdd_restored. In the same code cell, run sample_page_views_rdd_restored.collect() to view the restored RDD.

Note: You may notice that the restored RDD is not identical to the original RDD! Although the data is the same, when we converted the data to a DataFrame, PySpark automatically wrapped the original content into a Row. Behind the scenes, rows allow for more efficient calculations over large distributed data.

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