Lernen Handling Missing Values and Duplicates

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Real-world datasets are rarely clean. The flights dataset contains nulls in delay columns for cancelled flights, and potentially duplicate rows from data pipeline issues. Handling these correctly before analysis prevents skewed results.

Detecting Nulls


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import urllib.request
from pyspark.sql import SparkSession
from pyspark.sql.functions import col, count, when

urllib.request.urlretrieve(
    "https://staging-content-media-cdn.codefinity.com/courses/aa80ac56-0d50-49e8-9231-2c2374cd3e9d/flights.csv",
    "flights.csv"
)

spark = SparkSession.builder \
    .appName("MissingDuplicates") \
    .master("local[*]") \
    .getOrCreate()

flights_df = spark.read.csv("flights.csv", header=True, inferSchema=True)

# Counting nulls per column
flights_df.select([
    count(when(col(c).isNull(), c)).alias(c)
    for c in ["DEPARTURE_DELAY", "ARRIVAL_DELAY", "CANCELLATION_REASON", "AIR_TIME"]
]).show()

Dropping Nulls


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# Dropping rows where ARRIVAL_DELAY is null
clean_df = flights_df.dropna(subset=["ARRIVAL_DELAY"])

print(f"Before: {flights_df.count()}, After: {clean_df.count()}")

subset limits dropping to specific columns – without it, any row with a null in any column is removed, which is often too aggressive.

Filling Nulls

For delay columns, nulls typically mean the flight was cancelled – filling with 0 would be misleading. A better approach is to fill with a sentinel value or keep them as nulls and filter before aggregation:


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# Filling nulls in delay columns with 0 (use with care)
filled_df = flights_df.fillna(0, subset=["DEPARTURE_DELAY", "ARRIVAL_DELAY"])

# Filling CANCELLATION_REASON nulls with a placeholder string
filled_df = filled_df.fillna("N/A", subset=["CANCELLATION_REASON"])

Removing Duplicates


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# Removing fully duplicate rows
deduped_df = flights_df.dropDuplicates()

# Removing duplicates based on key columns only
deduped_df = flights_df.dropDuplicates(["AIRLINE", "FLIGHT_NUMBER", "YEAR", "MONTH", "DAY"])

print(f"Before: {flights_df.count()}, After: {deduped_df.count()}")

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