---
description: pandas 3 only; copy-on-write and the str dtype, no APIs removed in 3.0
globs: "**/*.py,**/*.ipynb"
alwaysApply: false
---

This project pins pandas 3. Copy-on-write is always on and strings are dtype `str`.

| Never (pandas 1 and 2) | Always (pandas 3) |
|---|---|
| `df["a"][0] = 1` | `df.loc[0, "a"] = 1` |
| `df["a"].fillna(0, inplace=True)` | `df["a"] = df["a"].fillna(0)` |
| `df.applymap(f)` | `df.map(f)` |
| `df.append(other)` | `pd.concat([df, other], ignore_index=True)` |
| `fillna(method="ffill")` | `.ffill()` |
| `read_csv(delim_whitespace=True)` | `read_csv(sep=r"\s+")` |
| `read_csv(date_parser=...)` | `read_csv(date_format="%Y-%m-%d")` |
| `freq="H"`, `"T"`, `"M"`, `"Y"` | `freq="h"`, `"min"`, `"ME"`, `"YE"` |
| `to_numeric(s, errors="ignore")` | `to_numeric(s, errors="coerce")` |
| `pd.read_json('[{...}]')` | `pd.read_json(io.StringIO(text))` |
| `s.dtype == object` for strings | `pd.api.types.is_string_dtype(s)` |
| `agg(np.sum)` | `agg("sum")` |

- No defensive `.copy()` to silence SettingWithCopyWarning; that warning no longer exists.
- After editing, run the `pandas-3-guard` skill's scanner and the tests.