# error: subprocess-exited-with-error

× python setup.py bdist_wheel did not run successfully.
│ exit code: 1
╰─> [12 lines of output]
    ModuleNotFoundError: No module named 'Cython'

- **ID:** `python/pip-subprocess-exited-with-error-build-backend`
- **Domain:** python
- **Category:** install_error
- **Verification:** ai_generated
- **Fix Rate:** 80%

## Root Cause

The package's build backend requires build-time dependencies (Cython, numpy, etc.) that are not declared in pyproject.toml's [build-system].requires, so the isolated build environment lacks them.

## Version Compatibility

| Version | Status | Introduced | Deprecated |
|---------|--------|------------|------------|
| 3.x | active | — | — |

## Workarounds

1. **** (85% success)
   ```
   python -m pip install Cython numpy
python -m pip install --no-build-isolation mypkg
   ```
2. **** (90% success)
   ```
   # If the project ships pyproject.toml with [build-system].requires
python -m pip install build
python -m build  # installs build-system.requires automatically
   ```

## Dead Ends

- **** — PEP 517 build isolation uses a fresh env; packages in your venv are invisible to the build. (80% fail)
- **** — The missing dependency is deterministic; retrying produces identical failure. (90% fail)