Missing build-system in pyproject.toml prevents package installation via `uv sync`

[bwentl]

Description

The PopulationSim repository's pyproject.toml is missing a [build-system] section, which prevents the package from being installed when using modern Python package managers like uv sync. This causes ModuleNotFoundError: No module named 'populationsim.run' when attempting to import the package after environment setup.

Root Cause

The pyproject.toml lacks a [build-system] section that tells packaging tools (pip, uv, build, etc.) which build backend to use. Without this section:

• uv sync only installs dependencies but does not install the package itself
• The package remains unimportable despite having a valid .venv with all dependencies

Current pyproject.toml structure

[project]
name = "populationsim"
version = "0.10.0"
# ... dependencies ...

[tool.setuptools.packages.find]
where = ["."]

[tool.uv.sources]
populationsim = { workspace = true }  # ← Also problematic - not a monorepo

[project.scripts]
populationsim = "populationsim.__main__:main"

Comparison with ActivitySim (works correctly)

[build-system]
requires = ["setuptools>=69", "wheel", "setuptools_scm>=8.0"]
build-backend = "setuptools.build_meta"

[project]
name = "activitysim"
# ... rest of config ...

Proposed Fix

Add a [build-system] section at the top of pyproject.toml:

[build-system]
requires = ["setuptools>=45", "wheel"]
build-backend = "setuptools.build_meta"

[project]
name = "populationsim"
# ... rest remains the same ...

Additional Issues

1. The line [tool.uv.sources] populationsim = { workspace = true } should be removed - it's only for monorepo setups and causes confusion
2. Consider adding setuptools_scm if you want version management from git tags

Impact

• Affected workflows: Any installation using uv sync, pip install -e . with modern pip versions, or PEP 517-compliant build tools
• Workaround: Users must run uv pip install -e . explicitly after uv sync to trigger legacy setuptools detection
• Similar projects affected: None - ActivitySim and other similar projects have proper [build-system] configurations

Steps to Reproduce

1. Clone the PopulationSim repository
2. Run uv sync --python 3.10
3. Attempt to import: python -c "from populationsim.run import add_run_args"
4. Observe: ModuleNotFoundError: No module named 'populationsim.run'
5. Run uv pip install -e . as workaround
6. Import now succeeds

References

• PEP 517 - A build-system independent format for source trees
• PEP 518 - Specifying Minimum Build System Requirements