grounded-python-skill

A Claude Code skill that forces Claude to verify every Python library API before writing code — no more hallucinated function signatures.

The problem

Claude writes plausible-looking Python code from memory. The method names look right. The arguments look right. But DataFrame.merge(how='left', on='id') has subtly different behavior than what Claude remembered, and the bug only surfaces at runtime.

What this skill does

Before writing any code that calls a third-party library or a custom project function, Claude must:

1. List every external symbol it's about to use (pd.read_csv, DataFrame.groupby, etc.)
2. Look up the actual signature — via Context7 MCP for libraries, via grep for local code
3. Write code grounded in what it found, briefly noting what it verified

Builtins and stdlib (os, sys, pathlib, json, etc.) are skipped — only third-party packages and project-local symbols require verification.

Requirements

• Claude Code
• Context7 MCP configured in Claude Code (used for library doc lookups)

Install

npx skills add com55/grounded-python-skill

Or manually:

git clone https://github.com/com55/grounded-python-skill ~/.claude/skills/grounded-python

Then start a new Claude Code session — the skill is active automatically.

Usage

Invoke it with /grounded-python before asking Claude to write Python code:

/grounded-python write a pandas pipeline that reads sales.csv, groups by region, and computes mean revenue

Or just use it — Claude Code will trigger the skill automatically when you ask it to write Python code using any third-party library.

What gets verified

Symbol type	Verification method
Third-party library (pandas, httpx, fastapi, …)	Context7 MCP docs lookup
Custom/local class or function	grep + read source
Python builtins + stdlib	Skipped (no lookup needed)

Hard rules Claude follows

• Never write a function call with argument names that weren't verified
• Never use **kwargs to avoid looking up the real signature
• Never infer arguments from what "makes sense" — look them up
• If a local symbol isn't found in the codebase: stop and ask, never guess

License

MIT