Generate fallback Dockerfile for Python apps without UV

[Copilot]

Previously, Dockerfiles were only generated for Python apps using UV. Non-UV Python apps had no automatic containerization support.

Changes

• Split Dockerfile generation logic into GenerateUvDockerfile and GenerateFallbackDockerfile methods
• Fallback Dockerfiles for non-UV apps:
  • Use same base image as UV workflow: python:{version}-slim-bookworm
  • Detect requirements.txt and run pip install --no-cache-dir -r requirements.txt when present
  • Single-stage build with non-root user
  • Support all entrypoint types (Script, Module, Executable)
• Updated PythonVersionDetector to accept nullable VirtualEnvironment for version detection without venv

Example

// Python app without UV now generates Dockerfile automatically
var python = builder.AddPythonScript("api", "../python-api", "main.py");
// Publishes with generated Dockerfile that installs requirements.txt dependencies

Generated Dockerfile (with requirements.txt):

FROM python:3.12-slim-bookworm

RUN groupadd --system --gid 999 appuser && useradd --system --gid 999 --uid 999 --create-home appuser
WORKDIR /app

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

COPY --chown=appuser:appuser . /app
ENV PYTHONDONTWRITEBYTECODE=1
ENV PYTHONUNBUFFERED=1

USER appuser
ENTRYPOINT ["python","main.py"]

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Generate fallback Dockerfile for Python apps without UV support</issue_title>
<issue_description>Currently, the dotnet/aspire tooling only generates a Dockerfile for Python apps when UV is being used. There is no automatic Dockerfile generation as a fallback when UV is not present.

This issue tracks adding support for generating a default Dockerfile for all Python apps when UV is not being used and no Dockerfile exists. The generated Dockerfile should:

• Detect if a requirements.txt is present and run pip install for dependencies if so.
• Use the same runtime image as the UV workflow for consistency.
• Apply this fallback logic for all Python apps, not just certain scenarios.

Reference: The current implementation at src/Aspire.Hosting.Python/PythonAppResourceBuilderExtensions.cs only generates a Dockerfile when UV is used. This enhancement will ensure Python apps are supported even without UV and that dependencies are installed correctly via pip when requirements.txt is present.</issue_description>

Comments on the Issue (you are @copilot in this section)

@davidfowl For 13, we will provide a good error message.

• Fixes Generate fallback Dockerfile for Python apps without UV support #12049

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.

[github-actions]

🚀 Dogfood this PR with:

⚠️ WARNING: Do not do this without first carefully reviewing the code of this PR to satisfy yourself it is safe.

curl -fsSL https://raw.githubusercontent.com/dotnet/aspire/main/eng/scripts/get-aspire-cli-pr.sh | bash -s -- 12627

Or

• Run remotely in PowerShell:

iex "& { $(irm https://raw.githubusercontent.com/dotnet/aspire/main/eng/scripts/get-aspire-cli-pr.ps1) } 12627"

[eerhardt]

I just had the one comment. Beyond that, this looks good to me.

[Copilot]

Pull Request Overview

This PR adds support for generating fallback Dockerfiles for Python applications that don't use UV (the modern Python package installer). Previously, Dockerfiles were only generated for UV-based projects; now projects using traditional requirements.txt or no dependencies at all will also get appropriate Dockerfiles generated.

Key changes:

• Refactored Dockerfile generation logic to support both UV and non-UV Python projects
• Added a new GenerateFallbackDockerfile method for traditional Python projects using pip/requirements.txt
• Updated version detection to handle cases where no virtual environment exists

Reviewed Changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

File	Description
src/Aspire.Hosting.Python/PythonAppResourceBuilderExtensions.cs	Refactored Dockerfile generation into separate methods (GenerateUvDockerfile and GenerateFallbackDockerfile) to support both UV and traditional Python projects
src/Aspire.Hosting.Python/PythonVersionDetector.cs	Changed virtualEnvironment parameter to nullable and added null check to support version detection without a virtual environment
tests/Aspire.Hosting.Python.Tests/AddPythonAppTests.cs	Added three new test methods to verify fallback Dockerfile generation for various scenarios (with/without requirements.txt, different entrypoint types)
tests/Aspire.Hosting.Python.Tests/Snapshots/*.verified.txt	Added snapshot files for the new test cases showing expected Dockerfile content

[eerhardt]

I just had the one comment. Beyond that, this looks good to me.

[eerhardt]

/backport to release/13.0

[github-actions]

Started backporting to release/13.0: https://github.com/dotnet/aspire/actions/runs/19051517713

[eerhardt]

/backport to release/13.0

[github-actions]

Started backporting to release/13.0: https://github.com/dotnet/aspire/actions/runs/19051857972