developing-with-python

developing-with-python is a skill for Claude Code, Codex from FortiumPartners/ensemble. It costs 40 tokens per session (3,388 once invoked), scanned A, original, MIT.

A guide to building software with Python, a programming language, using type hints, asynchronous code, web APIs, command-line tools, and automated tests.

In plain words
What is it for?
It helps create backend services, FastAPI applications, command-line tools, data-processing programs, and pytest tests.
Why use it?
It gives Python projects consistent patterns for organizing code, handling errors, building APIs, and testing behavior.

Skill for Claude CodeCodex

Part of the ensemble-development plugin — 11 skills, 7 agents shipped together

Install

Getting it into your agent

One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.

agentmods
npx agentmods add skills/fortiumpartners/ensemble/developing-with-python
Any agent
npx skills add FortiumPartners/ensemble --skill developing-with-python
Clone the repo
git clone --depth 1 https://github.com/FortiumPartners/ensemble

Made for: Claude Code, Codex.

Or install ensemble-development, the plugin that ships this one along with the rest of its 11 skills, 7 agents.

Wrote this? Show the measurements

A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.

agentmods badge for developing-with-python

README.md
[![agentmods](https://agentmods.dev/badge/skills/fortiumpartners/ensemble/developing-with-python.svg)](https://agentmods.dev/skills/fortiumpartners/ensemble/developing-with-python)
Your own site
<a href="https://agentmods.dev/skills/fortiumpartners/ensemble/developing-with-python"><img src="https://agentmods.dev/badge/skills/fortiumpartners/ensemble/developing-with-python.svg" alt="Measured on agentmods" height="20"></a>
Per session 40 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.
When invoked 3,388 The whole file, excluding the scripts and references it only reads on demand.
Security scan A 0 findings. Scan, not verified.
Origin original No closer match found in the catalogue.
Token cost

What it costs to keep this loaded

Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.

ModelPer sessionOnce invoked
Fable 5 $0.00040 $0.03388
Opus 5 $0.00020 $0.01694
Sonnet 5 $0.00008 $0.00678
Haiku 4.5 $0.00004 $0.00339

Measured yesterday against content hash ea3950b1aeaa, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

developing-with-python scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured yesterday.

The scan reads SKILL.md. This mod also ships 7 executable files (examples/fastapi_app.example.py, examples/patterns.example.py, templates/cli.template.py, …), listed below but not scanned — reading those needs a real analyzer, not pattern matching.

A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.

Nothing flagged

None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.

packages/development/skills/developing-with-python/SKILL.md · 517 lines

How it starts

The opening of the file, as written. The whole thing — 517 lines — stays where its author put it; the contents beside it link to each section on GitHub.

Python Development Skill

Python 3.11+ development with modern patterns including type hints, async/await, FastAPI, and pytest.

Progressive Disclosure: This file provides quick reference patterns. For comprehensive guides, see REFERENCE.md.

Table of Contents

  1. When to Use
  2. Quick Start
  3. Project Structure
  4. Type Hints
  5. Dataclasses & Pydantic
  6. FastAPI Patterns
  7. Async Patterns
  8. Testing with pytest
  9. Error Handling
  10. Anti-Patterns
  11. CLI Commands
  12. Configuration
  13. See Also

When to Use

Loaded by backend-developer when:

  • pyproject.toml or setup.py present
  • requirements.txt with Python dependencies
  • .py files in project root or src/
  • FastAPI, Django, Flask detected

Quick Start

Basic Module

from __future__ import annotations
from dataclasses import dataclass
from typing import TypeVar, Generic

T = TypeVar("T")

@dataclass
class Result(Generic[T]):
    value: T
    success: bool = True
    error: str | None = None

    @classmethod
    def ok(cls, value: T) -> Result[T]:
        return cls(value=value, success=True)

    @classmethod
    def fail(cls, error: str) -> Result[T]:
        return cls(value=None, success=False, error=error)  # type: ignore

FastAPI Endpoint

from fastapi import FastAPI, HTTPException
from pydantic import BaseModel, Field

app = FastAPI(title="My API", version="1.0.0")

class UserCreate(BaseModel):
    email: str = Field(..., pattern=r"^[\w\.-]+@[\w\.-]+\.\w+$")
    name: str = Field(..., min_length=1, max_length=100)

class UserResponse(BaseModel):
    id: int
    email: str
    name: str

@app.post("/users", response_model=UserResponse, status_code=201)
async def create_user(user: UserCreate) -> UserResponse:
    return UserResponse(id=1, email=user.email, name=user.name)

Read the full file on GitHub · 517 lines

Changes

What this file has done since we first saw it

Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.

  1. yesterday First seen · 517 lines · 40 tokens per session scan A ea3950b1aeaa

Subscribe to this mod's changes

developing-with-python is a skill published in the GitHub repository FortiumPartners/ensemble (11 stars, last pushed 1mo ago), licensed MIT. It adds 40 tokens to every session and 3,388 once invoked, about $0.0002 per session on Opus 5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-09-03.

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