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Python Data Wrangling

Modern patterns for pandas and polars data manipulation.

Decision Matrix: Pandas vs Polars

General guidance:

• Use pandas when: <1GB data, heavy ML/viz integration, team familiarity critical
• Use polars when: >1GB data, performance critical, greenfield projects

Modern Pandas Patterns

Method Chaining

Chain operations for readability

result = (
    df
    .assign(
        total=lambda x: x["price"] * x["quantity"],
        date=lambda x: pd.to_datetime(x["date"])
    )
    .query("total > 100")
    .sort_values("total", ascending=False)
    .groupby("category")
    .agg({"total": ["sum", "mean"]})
    .reset_index()
)

See pandas-method-chaining.md for:

• Lambda vs direct assignment
• Pipe with custom functions
• Handling complex transformations

Idiomatic Operations

# Use .loc for explicit indexing
df.loc[df["score"] > 80, "grade"] = "A"

# Use .pipe() for custom transformations
result = df.pipe(normalize_columns).pipe(remove_duplicates)

# Use .assign() for new columns
df = df.assign(
    log_value=lambda x: np.log(x["value"]),
    is_high=lambda x: x["value"] > x["value"].median()
)

GroupBy Patterns

# Named aggregations (pandas 0.25+)
summary = df.groupby("category").agg(
    total_sales=("sales", "sum"),
    avg_sales=("sales", "mean"),
    num_transactions=("sales", "count")
)

See pandas-groupby-patterns.md for:

• Window functions
• Multiple grouping levels
• Custom aggregations

Polars Patterns

Lazy Evaluation

Use lazy API for query optimization

import polars as pl

result = (
    pl.scan_csv("data.csv")  # Lazy
    .filter(pl.col("value") > 100)
    .group_by("category")
    .agg([
        pl.col("sales").sum().alias("total_sales"),
        pl.col("sales").mean().alias("avg_sales")
    ])
    .collect()  # Execute
)

See polars-lazy-evaluation.md for:

• When lazy helps vs hurts
• Streaming for huge data
• Query plan inspection

Polars Expressions

Use expressions for vectorized operations

result = df.select([
    pl.col("name"),
    (pl.col("salary") * 1.1).alias("new_salary"),
    pl.when(pl.col("age") > 30)
      .then(pl.lit("senior"))
      .otherwise(pl.lit("junior"))
      .alias("level")
])

See polars-expressions.md for:

• Expression composition
• when().then().otherwise() patterns
• List and struct operations

Migration Guide

Pandas → Polars

See migration-pandas-polars.md for complete patterns.

Performance Tips

Pandas Performance

# Use vectorized operations
df["result"] = df["a"] + df["b"]  # Good

# Use categorical for low-cardinality columns
df["category"] = df["category"].astype("category")

# Use eval for complex expressions
df.eval("total = price * quantity", inplace=True)

Polars Performance

# Use scan instead of read for lazy
df = pl.scan_csv("data.csv")

# Use streaming for data larger than memory
result = df.collect(streaming=True)

Anti-Patterns to Avoid

source: pandas user guide, polars documentation

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