Selecting only numeric/string columns names from a Spark DF in pyspark

user_7997598 0
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I have a Spark DataFrame in Pyspark (2.1.0) and I am looking to get the names of numeric columns only or string columns only.

For example, this is the Schema of my DF:

root
 |-- Gender: string (nullable = true)
 |-- SeniorCitizen: string (nullable = true)
 |-- MonthlyCharges: double (nullable = true)
 |-- TotalCharges: double (nullable = true)
 |-- Churn: string (nullable = true)

This is what I need:

num_cols = [MonthlyCharges, TotalCharges]
str_cols = [Gender, SeniorCitizen, Churn]

How can I make it?

pyspark apache-spark-sql apache-spark python
3 Answers

PySpark provides a rich API related to schema types. As @DanieldePaula mentioned you can access fields' metadata through df.schema.fields.

Here is a different approach based on statically typed checking:

from pyspark.sql.types import StringType, DoubleType

df = spark.createDataFrame([
  [1, 2.3, "t1"],
  [2, 5.3, "t2"],
  [3, 2.1, "t3"],
  [4, 1.5, "t4"]
], ["cola", "colb", "colc"])

# get string
str_cols = [f.name for f in df.schema.fields if isinstance(f.dataType, StringType)]
# ['colc']

# or double
dbl_cols = [f.name for f in df.schema.fields if isinstance(f.dataType, DoubleType)]
# ['colb']
user_7503760

You can do what zlidme suggested to get only string (categorical columns). To extend on the answer given take a look at the example bellow. It will give you all numeric (continuous) columns in a list called continuousCols, all categorical columns in a list called categoricalCols and all columns in a list called allCols. 

data = {'mylongint': [0, 1, 2],
        'shoes': ['blue', 'green', 'yellow'],
        'hous': ['furnitur', 'roof', 'foundation'],
        'C': [1, 0, 0]}

play_df = pd.DataFrame(data)
play_ddf = spark.createDataFrame(play_df)

#store all column names in a list
allCols = [item[0] for item in play_ddf]

#store all column names that are categorical in a list
categoricalCols = [item[0] for item in play_ddf.dtypes if item[1].startswith('string')]

#store all column names that are continous in a list
continuousCols =[item[0] for item in play_ddf.dtypes if item[1].startswith('bigint')]

print(len(allCols), ' - ', len(continuousCols), ' - ', len(categoricalCols))

This will give the result: 4 - 2 - 2

user_98857470
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