Pandas

pandas DataFrame AI Skill Guide

Overview & Engine Architecture

pandas is the default Python tabular toolkit: Series and DataFrame over NumPy (and Arrow-backed dtypes in newer builds). IO loaders materialize tables; vectorized ops and groupby/merge reshape them. Agents prefer explicit dtypes, avoid row-wise apply on large frames, and profile memory before chaining copies.

CSV/Parquet/SQL
      -> DataFrame (columns = Series)
          -> filter / assign / groupby / merge
          -> to_parquet / to_sql / plot

When to use this skill

• Cleaning and joining tabular files
• Feature tables for ML (@scikit-learn, @pytorch)
• Quick exploratory aggregates before moving to Polars/DuckDB/Spark

Operational directives

1. Set dtypes at read time (dtype=, parse_dates=) instead of fixing later.
2. Prefer boolean masks and vectorized ops over Python loops / apply.
3. Use merge with explicit how and validate key uniqueness when required.
4. Chain with .copy() only when you must break SettingWithCopy hazards.
5. For multi-GB data, switch to @polars, @duckdb, or chunked reads.

Read + aggregate example

import pandas as pd

df = pd.read_csv(
    "orders.csv",
    usecols=["order_id", "customer_id", "amount", "created_at"],
    parse_dates=["created_at"],
    dtype={"order_id": "string", "customer_id": "string"},
)

monthly = (
    df.assign(month=df["created_at"].dt.to_period("M").astype(str))
    .groupby(["customer_id", "month"], as_index=False)
    .agg(revenue=("amount", "sum"), orders=("order_id", "nunique"))
)

Merge hygiene

out = orders.merge(
    customers[["customer_id", "segment"]],
    on="customer_id",
    how="left",
    validate="m:1",
)

Common failures

Best practices

• Prefer Parquet over CSV for repeated pipelines.
• df.memory_usage(deep=True) before widening joins.
• Categoricals for low-cardinality string columns used as keys/filters.
• Keep idempotent transforms; avoid mutating global frames in notebooks without copies.

Limitations

• Single-machine memory bound; not a Spark replacement.
• Time-zone and nullable dtypes differ across pandas majors - pin versions in prod.
• Arrow string dtype behavior depends on pandas/pyarrow versions.

Related skills

• @polars - faster single-node DataFrames
• @duckdb - SQL over files without full materialization
• @jupyter - exploratory notebooks