Polars

-----|--------|--------| | Select column | df["col"] | df.select("col") | | Filter | df[df["col"] > 10] | df.filter(pl.col("col") > 10) | | Add column | df.assign(x=...) | df.with_columns(x=...) | | Group by | df.groupby("col").agg(...) | df.group_by("col").agg(...) | | Window | df.groupby("col").transform(...) | df.with_columns(...).over("col") |

Key Syntax Patterns

Pandas sequential (slow):

df.assign(
    col_a=lambda df_: df_.value * 10,
    col_b=lambda df_: df_.value * 100
)

Polars parallel (fast):

df.with_columns(
    col_a=pl.col("value") * 10,
    col_b=pl.col("value") * 100,
)

For comprehensive migration guide, load references/pandas_migration.md.

Best Practices

Performance Optimization

1. Use lazy evaluation for large datasets:

   lf = pl.scan_csv("large.csv")  # Don't use read_csv
   result = lf.filter(...).select(...).collect()
   

2. Avoid Python functions in hot paths:

   • Stay within expression API for parallelization
   • Use .map_elements() only when necessary
   • Prefer native Polars operations

3. Use streaming for very large data:

   lf.collect(streaming=True)
   

4. Select only needed columns early:

   # Good: Select columns early
   lf.select("col1", "col2").filter(...)
   
   # Bad: Filter on all columns first
   lf.filter(...).select("col1", "col2")
   

5. Use appropriate data types:

   • Categorical for low-cardinality strings
   • Appropriate integer sizes (i32 vs i64)
   • Date types for temporal data

Expression Patterns

Conditional operations:

pl.when(condition).then(value).otherwise(other_value)

Column operations across multiple columns:

df.select(pl.col("^.*_value$") * 2)  # Regex pattern

Null handling:

pl.col("x").fill_null(0)
pl.col("x").is_null()
pl.col("x").drop_nulls()

For additional best practices and patterns, load references/best_practices.md.

Resources

This skill includes comprehensive reference documentation:

references/

• core_concepts.md - Detailed explanations of expressions, lazy evaluation, and type system
• operations.md - Comprehensive guide to all common operations with examples
• pandas_migration.md - Complete migration guide from pandas to Polars
• io_guide.md - Data I/O operations for all supported formats
• transformations.md - Joins, concatenation, pivots, and reshaping operations
• best_practices.md - Performance optimization tips and common patterns

Load these references as needed when users require detailed information about specific topics.