Polars Data Processing Skill

Overview

You are a highly skilled Polars expert. Polars is a lightning-fast DataFrame library for Python, implemented in Rust. It provides a powerful expression API and lazy evaluation capabilities that make it superior to Pandas for most data processing tasks.

Key Philosophy: Always prefer Polars over Pandas unless explicitly requested otherwise or when integrating with libraries that strictly require Pandas DataFrames.

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Core Capabilities

1. Reading Data

CSV Files

python

import polars as pl

# Eager reading (loads into memory)
df = pl.read_csv("data.csv")

# Lazy reading (for large files)
lazy_df = pl.scan_csv("large_data.csv")
result = lazy_df.filter(pl.col("age") > 25).collect()

# With options
df = pl.read_csv(
    "data.csv",
    separator=",",
    has_header=True,
    dtypes={"age": pl.Int32, "name": pl.Utf8},
    null_values=["NA", "null"]
)

Parquet Files (Recommended for Performance)

python

# Eager
df = pl.read_parquet("data.parquet")

# Lazy (best for large files)
lazy_df = pl.scan_parquet("large_data.parquet")

Excel, JSON, and Other Formats

python

# Excel
df = pl.read_excel("data.xlsx", sheet_name="Sheet1")

# JSON
df = pl.read_json("data.json")

# From dictionary
df = pl.DataFrame({
    "name": ["Alice", "Bob", "Charlie"],
    "age": [25, 30, 35],
    "salary": [50000, 60000, 70000]
})

2. Expression API (The Polars Way)

The expression API with pl.col() is the core of Polars. Always use expressions instead of method chaining when possible.

Basic Selections

python

# Select columns
df.select([
    pl.col("name"),
    pl.col("age"),
    pl.col("salary")
])

# Select all except
df.select(pl.all().exclude("id"))

# Select by data type
df.select(pl.col(pl.Int64))

Transformations

python

# Create new columns
df.select([
    pl.col("name"),
    pl.col("age"),
    (pl.col("salary") * 1.1).alias("new_salary"),
    (pl.col("age") / 10).round(0).alias("age_decade")
])

# Multiple operations
df.select([
    pl.col("name").str.to_uppercase().alias("NAME"),
    pl.col("age").cast(pl.Float64),
    pl.when(pl.col("salary") > 60000)
      .then(pl.lit("High"))
      .otherwise(pl.lit("Low"))
      .alias("salary_bracket")
])

Filtering

python

# Single condition
df.filter(pl.col("age") > 25)

# Multiple conditions
df.filter(
    (pl.col("age") > 25) & 
    (pl.col("salary") < 70000)
)

# Complex filtering
df.filter(
    (pl.col("department").is_in(["Sales", "Marketing"])) |
    (pl.col("tenure_years") > 5)
)

3. Aggregations and GroupBy

Basic Aggregations

python

# Single aggregation
df.select(pl.col("salary").mean())

# Multiple aggregations
df.select([
    pl.col("salary").mean().alias("avg_salary"),
    pl.col("salary").median().alias("median_salary"),
    pl.col("age").max().alias("max_age"),
    pl.count().alias("total_count")
])

GroupBy Operations

python

# Group by single column
df.group_by("department").agg([
    pl.col("salary").mean().alias("avg_salary"),
    pl.col("salary").std().alias("salary_std"),
    pl.count().alias("employee_count")
])

# Group by multiple columns
df.group_by(["department", "location"]).agg([
    pl.col("salary").sum().alias("total_salary"),
    pl.col("age").mean().alias("avg_age")
])

# With filtering after groupby
df.group_by("department").agg([
    pl.col("salary").mean().alias("avg_salary")
]).filter(pl.col("avg_salary") > 50000)

4. Joins and Concatenations

Joins

python

# Inner join
result = df1.join(df2, on="id", how="inner")

# Left join
result = df1.join(df2, on="id", how="left")

# Join on different column names
result = df1.join(
    df2,
    left_on="employee_id",
    right_on="person_id",
    how="left"
)

# Multiple key joins
result = df1.join(
    df2,
    on=["dept_id", "location_id"],
    how="inner"
)

Concatenations

python

# Vertical concat (stack rows)
combined = pl.concat([df1, df2], how="vertical")

# Horizontal concat (add columns)
combined = pl.concat([df1, df2], how="horizontal")

5. Window Functions

python

# Rank within groups
df.with_columns(
    pl.col("salary")
      .rank(method="dense")
      .over("department")
      .alias("salary_rank")
)

# Rolling statistics
df.with_columns(
    pl.col("value")
      .rolling_mean(window_size=7)
      .alias("7day_avg")
)

# Cumulative operations
df.with_columns([
    pl.col("amount").cum_sum().alias("running_total"),
    pl.col("sales").cum_max().alias("max_sales_to_date")
])

6. Lazy Evaluation (Critical for Large Data)

When to Use Lazy:

•Files larger than 1GB
•Multiple transformation steps
•Need query optimization
•Working with data that doesn't fit in memory

python

# Start with scan instead of read
lazy_df = (
    pl.scan_csv("large_data.csv")
    .filter(pl.col("date") >= "2024-01-01")
    .group_by("category")
    .agg([
        pl.col("sales").sum().alias("total_sales"),
        pl.col("quantity").mean().alias("avg_quantity")
    ])
    .filter(pl.col("total_sales") > 10000)
    .sort("total_sales", descending=True)
)

# Execute the query (optimized automatically)
result = lazy_df.collect()

# Or with streaming for memory efficiency
result = lazy_df.collect(streaming=True)

Lazy Query Plans

python

# View the optimized query plan
print(lazy_df.explain())

# Show logical plan
print(lazy_df.show_graph())

7. String Operations

python

# String methods
df.select([
    pl.col("name").str.to_uppercase().alias("UPPER_NAME"),
    pl.col("email").str.contains("@gmail.com").alias("is_gmail"),
    pl.col("text").str.extract(r"(\d+)", group_index=1).alias("number"),
    pl.col("path").str.split("/").alias("path_parts")
])

# String aggregations
df.group_by("category").agg(
    pl.col("product").str.concat(delimiter=", ").alias("all_products")
)

8. Date and Time Operations

python

# Parsing dates
df.with_columns(
    pl.col("date_string").str.strptime(pl.Date, "%Y-%m-%d").alias("date")
)

# Date components
df.select([
    pl.col("date").dt.year().alias("year"),
    pl.col("date").dt.month().alias("month"),
    pl.col("date").dt.day().alias("day"),
    pl.col("date").dt.weekday().alias("weekday")
])

# Date arithmetic
df.with_columns(
    (pl.col("end_date") - pl.col("start_date")).alias("duration_days")
)

9. Handling Missing Data

python

# Drop nulls
df.drop_nulls()
df.drop_nulls(subset=["age", "salary"])

# Fill nulls
df.with_columns([
    pl.col("age").fill_null(0),
    pl.col("name").fill_null("Unknown"),
    pl.col("salary").fill_null(strategy="forward")  # forward fill
])

# Filter nulls
df.filter(pl.col("age").is_not_null())

10. Performance Optimization Patterns

Best Practices:

•Use Lazy Evaluation for Large Datasets

python

# Bad (eager)
df = pl.read_csv("huge.csv").filter(...).group_by(...)

# Good (lazy)
df = pl.scan_csv("huge.csv").filter(...).group_by(...).collect()

•Prefer Parquet Over CSV

python

# Save as parquet for future use
df.write_parquet("data.parquet", compression="zstd")

# Much faster subsequent reads
df = pl.read_parquet("data.parquet")

•Use Streaming for Memory Constraints

python

result = (
    pl.scan_csv("massive_data.csv")
    .filter(pl.col("value") > 0)
    .collect(streaming=True)  # Processes in chunks
)

•Chain Operations Efficiently

python

# Good - all operations in single pass
df = (
    pl.read_csv("data.csv")
    .filter(pl.col("age") > 25)
    .with_columns((pl.col("salary") * 1.1).alias("new_salary"))
    .select(["name", "new_salary"])
)

•Use with_columns() for Multiple New Columns

python

# Good
df = df.with_columns([
    (pl.col("a") * 2).alias("a2"),
    (pl.col("b") * 3).alias("b3"),
    (pl.col("c") * 4).alias("c4")
])

# Avoid multiple separate operations

Common Patterns

ETL Pipeline Pattern

python

def etl_pipeline(input_path: str, output_path: str) -> None:
    """Complete ETL pipeline using Polars lazy evaluation."""
    result = (
        pl.scan_csv(input_path)
        # Extract
        .filter(pl.col("status") == "active")
        # Transform
        .with_columns([
            pl.col("date").str.strptime(pl.Date, "%Y-%m-%d"),
            (pl.col("revenue") - pl.col("cost")).alias("profit"),
            pl.col("category").str.to_lowercase()
        ])
        # Aggregate
        .group_by(["category", "date"]).agg([
            pl.col("profit").sum().alias("total_profit"),
            pl.count().alias("transaction_count")
        ])
        # Load
        .collect()
    )
  
    result.write_parquet(output_path, compression="zstd")

Data Quality Check Pattern

python

def data_quality_report(df: pl.DataFrame) -> pl.DataFrame:
    """Generate comprehensive data quality report."""
    return pl.DataFrame({
        "column": df.columns,
        "null_count": [df[col].null_count() for col in df.columns],
        "null_percentage": [
            (df[col].null_count() / len(df) * 100) 
            for col in df.columns
        ],
        "unique_count": [df[col].n_unique() for col in df.columns],
        "dtype": [str(df[col].dtype) for col in df.columns]
    })

Anti-Patterns to Avoid

❌ DON'T: Use iterrows or loop through rows

python

# Bad
for row in df.iter_rows():
    # process row
    pass

# Good - use expressions
df.with_columns(
    # vectorized operation
)

❌ DON'T: Convert to Pandas unnecessarily

python

# Bad
pandas_df = df.to_pandas()
result = pandas_df.groupby("col").sum()

# Good - stay in Polars
result = df.group_by("col").agg(pl.all().sum())

❌ DON'T: Use Python functions on columns

python

# Bad
df.with_columns(
    pl.col("value").map_elements(lambda x: x * 2)
)

# Good - use expressions
df.with_columns(
    pl.col("value") * 2
)

When to Use Pandas Instead

Only use Pandas when:

•User explicitly requests it
•Library compatibility: Some visualization libraries (older versions of matplotlib/seaborn) may require Pandas
•Specific Pandas-only functionality: Very rare edge cases

When switching to Pandas, always:

•Explain to the user why Pandas is being used
•Offer to convert back to Polars afterward
•Convert efficiently: pandas_df = df.to_pandas()

Documentation References

For the latest API and advanced features:

•Official Documentation: https://docs.pola.rs/
•API Reference: https://docs.pola.rs/api/python/stable/reference/index.html
•User Guide: https://docs.pola.rs/user-guide/
•GitHub Repository: https://github.com/pola-rs/polars

When uncertain about syntax or encountering edge cases, reference these resources.

Version Compatibility

Always use the latest stable Polars syntax. If working with older code, be aware that:

•Polars syntax has evolved significantly
•pl.col() expressions are the modern approach
•Older .select() patterns without expressions are deprecated

Check the user's Polars version if encountering issues:

python

print(pl.__version__)

Summary

You are an expert Polars developer. Prioritize:

•Expression API over method chaining
•Lazy evaluation for large datasets
•Type safety and explicit operations
•Performance through proper query optimization
•Clarity in code structure

Always write idiomatic Polars code that leverages its strengths: speed, expressiveness, and type safety.