What is developing-with-python?

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

It helps create backend services, FastAPI applications, command-line tools, data-processing programs, and pytest tests.

It gives Python projects consistent patterns for organizing code, handling errors, building APIs, and testing behavior.

developing-with-python — Skill for Claude Code by FortiumPartners

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.

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.

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.

README.md

[![agentmods](https://agentmods.dev/badge/skills/fortiumpartners/ensemble/developing-with-python/github.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/github.svg" alt="Measured on agentmods" height="20"></a>

Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.

Your own site · 80×15

<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="Reviewed on agentmods" width="80" 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. A grade says what 26 rules found in the file — not that it is safe.

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.

Model	Per session	Once invoked
Fable 5.1	$0.00040	$0.03388
Opus 5.5	$0.00016	$0.01355
Sonnet 5.5	$0.00008	$0.00678
Haiku 4.5	$0.00004	$0.00339

Measured 1mo ago against content hash ea3950b1aeaa, method: parsed. Prices are Anthropic first-party input rates as of 2026-10-07, 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 1mo ago.

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.

When to Use
Quick Start
Project Structure
Type Hints
Dataclasses & Pydantic
FastAPI Patterns
Async Patterns
Testing with pytest
Error Handling
Anti-Patterns
CLI Commands
Configuration
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

Files

What ships with it

12 files beside SKILL.md in the same directory: the scripts, references and assets a skill reads on demand. Not counted in the per-session cost; read them before you install if any of them is executable.

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.

1mo ago 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 (12 stars, last pushed 2d ago), licensed MIT. It adds 40 tokens to every session and 3,388 once invoked, about $0.0002 per session on Opus 5.5. 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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python-backend

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fastapi

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django

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developing-with-python