Data analysis with pandas

pandas, numpy, and scipy are already installed in the sandbox — no install_packages call needed for any workflow below.

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• Pin dtypes on load rather than letting pandas infer them, when you know the schema — pd.read_csv(path, dtype={"customer_id": str, "amount": float}). Inference is usually right but silently wrong in a specific, dangerous way for ID-like columns: a numeric-looking ID (e.g. "00123") inferred as int64 loses its leading zeros.
• Parse date columns explicitly (parse_dates=["order_date"] or pd.to_datetime(df["col"]) post-load) rather than leaving them as strings — string dates sort and filter lexicographically, not chronologically, which silently produces wrong "most recent N" or date-range results.
• For Excel sources with multiple sheets, always pass sheet_name= explicitly (or inspect pd.ExcelFile(path).sheet_names first) — read_excel's default of the first sheet is a common source of "the numbers don't match" bugs when the data you actually need is on a different sheet.

Cleaning

• Check for nulls (df.isnull().sum()) and duplicates (df.duplicated().sum()) before any aggregation — decide explicitly whether to drop, fill, or flag them, rather than letting them silently skew a sum/mean (pandas aggregations skip NaN by default, which is not always the right call — a "total" that silently excludes rows with a missing value is a different number than "total of all rows", and the user should know which one they're getting).
• Coerce types explicitly after cleaning (pd.to_numeric(df["col"], errors="coerce")) rather than assuming a column that "looks numeric" already is — mixed-type columns from a manually-maintained spreadsheet are common, and a silent string comparison instead of a numeric one produces wrong sort/filter results without raising any error.
• After any cleaning step that drops or modifies rows, print the before/after row count — a cleaning step that unexpectedly drops 80% of the data is a bug, not a result, and should be caught immediately rather than discovered after the whole analysis is built on the reduced dataset.

Aggregation, joins, pivots

• groupby(...).agg(...) for aggregates; always specify the aggregation function explicitly per column (.agg({"amount": "sum", "order_id": "count"})) rather than a bare .sum() across the whole frame, which silently includes every numeric column whether or not it makes sense to sum it.
• For joins (pd.merge), always specify how= explicitly (don't rely on the "inner" default when the user's intent might be "left"/"outer") and check the resulting row count against expectations — an unexpected row-count change after a merge (either fewer rows than the left frame, or way more) almost always means a many-to-many relationship you didn't account for, or a key-matching problem (mismatched types, whitespace, or casing in the join column).
• pivot_table for cross-tabulations; pass fill_value=0 explicitly when a missing combination should read as zero rather than NaN, since which one is correct depends on the question being asked.

Verification discipline (do this before reporting any number)

1. Always run df.info() and df.describe() (or .value_counts() for categorical columns) before analyzing — this catches wrong dtypes, unexpected nulls, and outliers early, before they propagate into a wrong conclusion.
2. Cross-check any headline number by computing it a second, independent way — e.g. if you report a total via groupby().sum(), also sanity-check it against df["amount"].sum() on the ungrouped frame (should match), or against a manual filter-and-sum on a known subset. Two different code paths agreeing is meaningfully more trustworthy than one path you didn't double-check.
3. Never report a number your code didn't actually print. If you're describing a trend or a comparison, make sure the specific figures you state in your response are ones that appeared in your program's printed output — do not round, estimate, or extrapolate a number that wasn't literally computed and displayed.