The CPython build system is complex, but lacks comprehensive
documentation explaining how the pieces fit together. This expands
the existing "Python Build System" section in Doc/using/configure.rst
with detailed explanations of:
- Build system overview and key phases
- Platform-specific build system components (autotools, MSBuild, etc.)
- The 4-stage bootstrap process explained step-by-step
- Module configuration system (3-layer architecture)
- Detailed build flow with phase-by-phase breakdown
- Visual build flow diagram
This helps new contributors understand the build system architecture
and reduces questions about "how does CPython build?"
The documentation focuses on conceptual understanding rather than
step-by-step instructions, which are better suited for the devguide.
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