Databricks Applications — Python backends

First, confirm this skill is the right one. The default for new Databricks Apps is databricks-apps (AppKit — Node.js + TypeScript + React SDK). Load that skill first unless the user explicitly asks for a Python backend, is extending an existing Python app, or the team is Python-only. Everything below is the Python-backend alternative.

Critical Rules for Python apps (always follow)

• MUST confirm framework choice or use Python Framework Selection below
• MUST use SDK Config() for authentication (never hardcode tokens)
• MUST use app.yaml valueFrom for resources (never hardcode resource IDs)
• MUST use dash-bootstrap-components for Dash app layout and styling
• MUST use @st.cache_resource for Streamlit database connections
• MUST deploy Flask with Gunicorn, FastAPI with uvicorn (not dev servers)

Required Steps for Python apps

Copy this checklist and verify each item:

- [ ] Framework selected
- [ ] Auth strategy decided: app auth, user auth, or both
- [ ] App resources identified (SQL warehouse, Lakebase, serving endpoint, etc.)
- [ ] Backend data strategy decided (SQL warehouse, Lakebase, or SDK)
- [ ] Deployment method: CLI or DABs

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Python Framework Selection

Framework	Best For	app.yaml Command
FastAPI (default)	Any Python backend by default — async APIs, auto-generated OpenAPI docs, JSON-serving apps	["uvicorn", "app:app", "--host", "0.0.0.0", "--port", "8000"]
Flask	Custom REST APIs, lightweight apps, webhooks	["gunicorn", "app:app", "-w", "4", "-b", "0.0.0.0:8000"]
Dash	Production dashboards, BI tools, complex interactivity	["python", "app.py"]
Streamlit	Rapid prototyping, data science apps, internal tools where the UI is a series of Python widgets	["streamlit", "run", "app.py"]
Gradio	ML demos, model interfaces, chat UIs	["python", "app.py"]
Reflex	Full-stack Python apps without JavaScript	["reflex", "run", "--env", "prod"]

Default: FastAPI. Reach for FastAPI unless the user explicitly asks for Streamlit-style widget prototyping (Streamlit), a heavy dashboard grid (Dash), or a Gradio-style ML demo. FastAPI pairs naturally with a JS/HTML frontend or a JSON-consuming caller — the same posture databricks-apps uses on the Node side.

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Quick Reference

Concept	Details
Runtime	Python 3.11, Ubuntu 22.04, 2 vCPU, 6 GB RAM
Pre-installed	Dash 2.18.1, Streamlit 1.38.0, Gradio 4.44.0, Flask 3.0.3, FastAPI 0.115.0
Auth (app)	Service principal via Config() — auto-injected DATABRICKS_CLIENT_ID/DATABRICKS_CLIENT_SECRET
Auth (user)	x-forwarded-access-token header — see references/1-authorization.md
Resources	valueFrom in app.yaml — see references/2-app-resources.md
SDK / Foundation Models / Vector Search / Model Serving	Use the databricks-python-sdk skill — same WorkspaceClient and OpenAI-compatible foundation-model patterns work inside a Databricks App
Docs	https://docs.databricks.com/dev-tools/databricks-apps/

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Detailed Guides

Authorization: Use references/1-authorization.md when configuring app or user authorization — covers service principal auth, on-behalf-of user tokens, OAuth scopes, and per-framework code examples. (Keywords: OAuth, service principal, user auth, on-behalf-of, access token, scopes)

App resources: Use references/2-app-resources.md when connecting your app to Databricks resources — covers SQL warehouses, Lakebase, model serving, secrets, volumes, and the valueFrom pattern. (Keywords: resources, valueFrom, SQL warehouse, model serving, secrets, volumes, connections)

Frameworks: See references/3-frameworks.md for Databricks-specific patterns per framework — FastAPI (default), Flask, Dash, Streamlit, Gradio, Reflex — with auth integration and deployment commands. (Keywords: FastAPI, Flask, Dash, Streamlit, Gradio, Reflex, framework selection)

Deployment: Use references/4-deployment.md when deploying your app — covers Databricks CLI, Asset Bundles (DABs), app.yaml configuration, and post-deployment verification. (Keywords: deploy, CLI, DABs, asset bundles, app.yaml, logs)

Lakebase: Use references/5-lakebase.md when using Lakebase (PostgreSQL) as your app's data layer — covers auto-injected env vars, psycopg2/asyncpg patterns, and when to choose Lakebase vs SQL warehouse. (Keywords: Lakebase, PostgreSQL, psycopg2, asyncpg, transactional, PGHOST)

CLI commands: Use references/6-cli-approach.md for managing app lifecycle via CLI — covers creating, deploying, monitoring, and deleting apps. (Keywords: CLI, create app, deploy app, app logs)

Foundation Models / SDK / Vector Search / Model Serving: Use the databricks-python-sdk skill for the OpenAI-compatible foundation-model client, WorkspaceClient calls, Vector Search, and model-serving invocation — the same patterns apply inside a Databricks App. The examples in this skill's examples/ folder (fm-minimal-chat.py, fm-parallel-calls.py, fm-structured-outputs.py, llm_config.py) show the App-side wiring only.

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Workflow

1. Determine the task type:

   New app from scratch? → Load databricks-apps first (AppKit / Node). Only stay in this skill if the user explicitly asks for a Python backend. Python-backend confirmed? → Python Framework Selection — default to FastAPI. Setting up authorization? → Read references/1-authorization.md Connecting to data/resources? → Read references/2-app-resources.md Using Lakebase (PostgreSQL)? → Read references/5-lakebase.md Deploying to Databricks? → Read references/4-deployment.md Using CLI for app lifecycle? → Read references/6-cli-approach.md Calling foundation model / LLM APIs, Vector Search, or model-serving endpoints? → Load the databricks-python-sdk skill. This skill's examples/ folder shows only the App-side wiring on top of those SDK patterns.

2. Follow the instructions in the relevant guide.

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Core Architecture

All Python Databricks apps follow this pattern:

app-directory/
├── app.py                 # Main application (or framework-specific name)
├── models.py              # Pydantic data models
├── backend.py             # Data access layer
├── requirements.txt       # Additional Python dependencies
├── app.yaml               # Databricks Apps configuration
└── README.md

Backend Toggle Pattern

import os
from databricks.sdk.core import Config

USE_MOCK = os.getenv("USE_MOCK_BACKEND", "true").lower() == "true"

if USE_MOCK:
    from backend_mock import MockBackend as Backend
else:
    from backend_real import RealBackend as Backend

backend = Backend()

SQL Warehouse Connection (shared across all frameworks)

from databricks.sdk.core import Config
from databricks import sql

cfg = Config()  # Auto-detects credentials from environment
conn = sql.connect(
    server_hostname=cfg.host,
    http_path=f"/sql/1.0/warehouses/{os.getenv('DATABRICKS_WAREHOUSE_ID')}",
    credentials_provider=lambda: cfg.authenticate,
)

Pydantic Models

from pydantic import BaseModel, Field
from datetime import datetime
from enum import Enum

class Status(str, Enum):
    ACTIVE = "active"
    PENDING = "pending"

class EntityOut(BaseModel):
    id: str
    name: str
    status: Status
    created_at: datetime

class EntityIn(BaseModel):
    name: str = Field(..., min_length=1)
    status: Status = Status.PENDING

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Common Issues

Issue	Solution
Connection exhausted	Use @st.cache_resource (Streamlit) or connection pooling
Auth token not found	Check x-forwarded-access-token header — only available when deployed, not locally
App won't start	Check app.yaml command matches framework; check databricks apps logs <name>
Resource not accessible	Add resource via UI, verify SP has permissions, use valueFrom in app.yaml
Import error on deploy	Add missing packages to requirements.txt (pre-installed packages don't need listing)
Lakebase app crashes on start	psycopg2/asyncpg are NOT pre-installed — MUST add to requirements.txt
Port conflict	Apps must bind to DATABRICKS_APP_PORT env var (defaults to 8000). Never use 8080. Streamlit is auto-configured; for others, read the env var in code or use 8000 in app.yaml command
Streamlit: set_page_config error	st.set_page_config() must be the first Streamlit command
Dash: unstyled layout	Add dash-bootstrap-components; use dbc.themes.BOOTSTRAP
Slow queries	Use Lakebase for transactional/low-latency; SQL warehouse for analytical queries

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Platform Constraints

Constraint	Details
Runtime	Python 3.11, Ubuntu 22.04 LTS
Compute	2 vCPUs, 6 GB memory (default)
Pre-installed frameworks	Dash, Streamlit, Gradio, Flask, FastAPI, Shiny
Custom packages	Add to requirements.txt in app root
Network	Apps can reach Databricks APIs; external access depends on workspace config
User auth	Public Preview — workspace admin must enable before adding scopes

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Official Documentation

• Databricks Apps Overview — main docs hub
• Authorization — app auth and user auth
• Resources — SQL warehouse, Lakebase, serving, secrets
• app.yaml Reference — command and env config
• System Environment — pre-installed packages, runtime details

Related Skills

• databricks-apps — the default for new Databricks Apps (AppKit / Node / TypeScript + React); load it first unless a Python backend is explicitly required
• databricks-python-sdk — WorkspaceClient, OpenAI-compatible foundation-model client, Vector Search, model-serving invocation; the same patterns work inside a Databricks App
• databricks-lakebase — persistent PostgreSQL state (autoscaling managed PG with branching)
• databricks-model-serving — endpoint lifecycle for ML models an App calls
• databricks-dabs — deploying apps via DABs