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pythonintermediate

Polars Expressions API Patterns

Use Polars expression API for complex column-level transformations without apply or loops.

python
import polars as pl

df = pl.DataFrame({'name':['alice','bob','charlie'],'score':[85,92,78],'dept':['eng','hr','eng']})

result = df.with_columns([
    pl.col('name').str.to_titlecase().alias('Name'),
    pl.col('score').rank(descending=True).over('dept').alias('dept_rank'),
    pl.when(pl.col('score') >= 90).then(pl.lit('A')).otherwise(pl.lit('B')).alias('grade'),
    (pl.col('score') - pl.col('score').mean().over('dept')).alias('score_delta'),
])
print(result)

Use Cases

  • column transformations
  • ranking within groups
  • conditional expressions

Tags

#polars#expressions#etl#performance

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