Dockerizing a Python App

The Node Dockerfile you already know

If you have containerized a Node/TypeScript app before, you know the pattern: copy package.json, run npm ci, copy source, build, and run. Python’s Dockerfile follows the same multi-stage logic — but with a few Python-specific best practices you need to internalize.

Side-by-side: Node vs Python Dockerfile

TypeScript

# ---- Node / TypeScript multi-stage Dockerfile ----
FROM node:20-alpine AS deps
WORKDIR /app
COPY package*.json ./
RUN npm ci --only=production

FROM node:20-alpine AS builder
WORKDIR /app
COPY package*.json ./
RUN npm ci
COPY . .
RUN npm run build

FROM node:20-alpine AS runner
WORKDIR /app
ENV NODE_ENV=production
COPY --from=deps /app/node_modules ./node_modules
COPY --from=builder /app/dist ./dist
RUN addgroup -S appgroup && adduser -S appuser -G appgroup
USER appuser
EXPOSE 3000
CMD ["node", "dist/main.js"]

Python

# ---- Python multi-stage Dockerfile ----
FROM python:3.12-slim AS builder
WORKDIR /app

# Install build tools, then create venv inside the image
RUN python -m venv /app/.venv
ENV PATH="/app/.venv/bin:$PATH"

COPY requirements.txt .
RUN pip install --no-cache-dir -r requirements.txt

FROM python:3.12-slim AS runner
WORKDIR /app

# Copy the pre-built venv from builder
COPY --from=builder /app/.venv /app/.venv
ENV PATH="/app/.venv/bin:$PATH"

# Copy application source
COPY src/ ./src/

# Run as non-root user
RUN addgroup --system appgroup && adduser --system --ingroup appgroup appuser
USER appuser

EXPOSE 8000
CMD ["uvicorn", "src.main:app", "--host", "0.0.0.0", "--port", "8000"]

The full Python Dockerfile explained

Here is the production-grade Dockerfile with comments:

# syntax=docker/dockerfile:1

# ---- Stage 1: Build / dependency installation ----
FROM python:3.12-slim AS builder

WORKDIR /app

# PYTHONDONTWRITEBYTECODE: don't write .pyc files (saves space)
# PYTHONUNBUFFERED: log output immediately (12-Factor: log to stdout)
ENV PYTHONDONTWRITEBYTECODE=1 \
    PYTHONUNBUFFERED=1

# Create the virtual environment INSIDE the image
# This lets us copy the entire /app/.venv to the final stage
RUN python -m venv /app/.venv
ENV PATH="/app/.venv/bin:$PATH"

# Copy dependency manifest first (Docker layer caching)
COPY pyproject.toml uv.lock* requirements.txt* ./

# Install dependencies — no cache to keep image small
RUN pip install --no-cache-dir --upgrade pip && \
    pip install --no-cache-dir -r requirements.txt

# ---- Stage 2: Runtime image ----
FROM python:3.12-slim AS runner

WORKDIR /app

# Copy the fully-populated venv from builder
COPY --from=builder /app/.venv /app/.venv
ENV PATH="/app/.venv/bin:$PATH" \
    PYTHONDONTWRITEBYTECODE=1 \
    PYTHONUNBUFFERED=1

# Copy application source (after deps — better cache hit on rebuilds)
COPY src/ ./src/

# Create and switch to a non-root user (security best practice)
RUN addgroup --system appgroup && \
    adduser --system --ingroup appgroup --no-create-home appuser
USER appuser

# Expose the port the app listens on
EXPOSE 8000

# CMD uses exec form (no shell wrapper — signals go directly to process)
CMD ["uvicorn", "src.main:app", "--host", "0.0.0.0", "--port", "8000"]

Why venv inside the Docker image?

Run this in your terminal to build and run:

docker build -t my-python-app .
docker run -p 8000:8000 my-python-app

# Or with docker compose:
# docker compose up --build

🟦 PYTHONUNBUFFERED and PYTHONDONTWRITEBYTECODE explained

These two env vars are standard in every production Python Dockerfile:

PYTHONUNBUFFERED=1 — Python buffers stdout/stderr by default. In a container, this means your logs may never appear (they are stuck in a buffer). Setting this to 1 forces immediate flushing, which is the 12-Factor “log to stdout” requirement.

PYTHONDONTWRITEBYTECODE=1 — Python normally writes compiled .pyc files alongside your .py source files to speed up startup. In containers this is noise: it wastes layer space and can cause stale-cache bugs. Disabling it keeps the image clean.

Node developers do not need these flags because Node’s runtime model works differently — this is a Python-specific footgun.

.dockerignore

Like .gitignore for Docker. Always create one to avoid copying your venv, cache, and secrets into the image:

.venv/
__pycache__/
*.pyc
*.pyo
.pytest_cache/
.mypy_cache/
.ruff_cache/
.env
.env.*
*.log

Why should you create a virtual environment INSIDE the Docker image instead of relying on the system Python?

Docker requires it by specification A venv inside the image lets you copy the entire isolated dependency set cleanly between build and runtime stages System Python is not available in slim images pip refuses to install without a venv

What does PYTHONUNBUFFERED=1 ensure in a containerized Python app?

Python runs faster by skipping the GIL Python logs are flushed to stdout immediately instead of being buffered Python skips writing .pyc bytecode files uvicorn listens on all network interfaces

In a multi-stage Dockerfile, how do you bring the installed packages from the builder stage into the runner stage?

RUN pip install again in the runner stage COPY --from=builder /app/.venv /app/.venv ADD venv /app/.venv Docker automatically merges layers between stages

Submit

Mark as complete