Adding a custom expression

Your team frequently calculates per-listener metrics across different reports. To make this pattern even more reusable, create a custom expression that can be chained like any built-in Polars method.

polars is loaded as pl. The DataFrame spotify is available with album streaming data.

This exercise is part of the course

Data Transformation with Polars

View Course

Exercise instructions

Define per_listener() to divide its input by the monthly_listeners column.
Attach the function to pl.Expr to make it chainable.
Apply the custom method on the streams column.

Hands-on interactive exercise

Have a go at this exercise by completing this sample code.

# Define the per_listener function
def per_listener(input):
    return ____ / pl.col("____")


# Attach the function to pl.Expr
pl.Expr.____ = ____

# Use the custom method on the streams column
result = spotify.with_columns(
    pl.col("____").____().alias("streams_per_listener")
)

print(result.head())

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