Development mode installation

development-mode-installation

Summary

Install a Python package in editable mode with development dependencies to enable local testing and code contribution workflows. This skill allows developers to modify source code and immediately test changes without reinstalling the package.

When to use

Apply this skill when you need to run a local test suite, contribute code changes to a repository, or iterate rapidly on package modifications. Specifically, use it before executing pytest tests locally or when the project README instructs 'Run the following command from the root of the project to install the project for development'.

When NOT to use

• When installing a package for end-user production deployment (use pip install package_name instead)
• When the package has not declared a [dev] extras group or development dependencies in its build metadata
• When working in an environment where editable installs are forbidden or unsupported (e.g., restricted container or sandboxed runtime)

Inputs

• Python package source repository root directory
• pyproject.toml or setup.py with [dev] extras defined

Outputs

• Editable package installation in site-packages (as symlink)
• Development dependencies (pytest, black, etc.) available in environment
• Test suite executable and passing (pytest tests/ exit code 0)

How to apply

Navigate to the package repository root directory. Execute pip install -e .[dev] to install the package in editable mode with all development dependencies (including test runners and code formatters). The -e flag installs the package as a symbolic link to the source directory, allowing source code edits to take effect immediately without reinstallation. The [dev] extra installs development-specific dependencies such as pytest and black. After installation, verify by running the test suite with pytest tests/ to confirm all tests pass and dependencies are correctly configured.

Related tools

• pip (Package manager used to install the package in editable mode with development extras)
• pytest (Test runner installed as a development dependency and used to verify successful installation by running the test suite)
• black (Code formatter installed as a development dependency to enforce code style before committing changes) — https://github.com/psf/black

Examples

pip install -e .[dev] && pytest tests/

Evaluation signals

• Command pip install -e .[dev] completes without errors or unresolved dependencies
• pytest test suite runs successfully with command pytest tests/ and all tests pass (exit code 0)
• Source code modifications (e.g., in a .py file) take effect immediately in subsequent test runs without reinstalling
• Development tools (pytest, black) are executable and available in the Python environment (verify with which pytest or pytest --version)
• The package is listed in pip freeze or pip list with an editable path indicator (e.g., -e /path/to/repo or showing a file:// URL)

Limitations

• Editable installs work reliably only with packages using modern build backends (setuptools with pyproject.toml); some legacy packages may not support -e mode
• Changes to compiled extensions or package metadata (e.g., version strings in init.py) may require reinstallation even in editable mode
• Development dependency installation depends on the [dev] extras group being properly defined in the package's build configuration; if missing or misconfigured, pytest and other tools will not be installed

Evidence

• [other] Run the following command from the root of the project to install the project for development: "Run the following command from the root of the project to install the project for development"
• [other] pip install -e .[dev]: "pip install -e .[dev]"
• [other] You can also run the tests locally with the following command: pytest tests/: "You can also run the tests locally with the following command: pytest tests/"
• [other] Please use black to format your code before submitting a pull request: "Please use black to format your code before submitting a pull request"
• [other] The biosynfoni package includes a pytest test suite located in the tests/ directory that can be executed locally: "The biosynfoni package includes a pytest test suite located in the tests/ directory that can be executed locally"