feat(pi-fortran): use f2py-cmake instead of hand-rolled f2py wiring (#73)

Replace the manual numpy.f2py include probe, add_custom_command wrapper
generation, and explicit fortranobject.c plumbing with f2py-cmake's
UseF2Py module and a single f2py_add_module call. Default --lower and
F77 detection preserve the previous behavior; the checked-in .pyf
signature (the underscore-problem fix) is passed straight through.

Assisted-by: ClaudeCode:claude-opus-4.8

Author: henryiii

projects/pi-fortran/CMakeLists.txt

@@ -1,52 +1,27 @@
-cmake_minimum_required(VERSION 3.17.2...3.26)
+cmake_minimum_required(VERSION 3.17.2...3.29)
 
 project(pi
   VERSION 1.0.1
   DESCRIPTION "pi estimator"
   LANGUAGES C Fortran
 )
 
-find_package(Python REQUIRED COMPONENTS Interpreter Development.Module NumPy)
+find_package(
+  Python
+  COMPONENTS Interpreter Development.Module NumPy
+  REQUIRED)
 
-# F2PY headers
-execute_process(
-  COMMAND "${Python_EXECUTABLE}"
-  -c "import numpy.f2py; print(numpy.f2py.get_include())"
-  OUTPUT_VARIABLE F2PY_INCLUDE_DIR
-  OUTPUT_STRIP_TRAILING_WHITESPACE
-)
-# On Windows the path comes back with backslashes, which CMake reads as escapes.
-file(TO_CMAKE_PATH "${F2PY_INCLUDE_DIR}" F2PY_INCLUDE_DIR)
-
-set(f2py_module_name "pi")
-set(fortran_src_file "${CMAKE_SOURCE_DIR}/pi/_pi.f")
-set(f2py_module_c "${f2py_module_name}module.c")
-
-add_custom_command(
-  OUTPUT "${CMAKE_CURRENT_BINARY_DIR}/${f2py_module_c}"
-  COMMAND ${PYTHON_EXECUTABLE}  -m "numpy.f2py"
-                   "${fortran_src_file}"
-                   "${CMAKE_SOURCE_DIR}/pi/pi.pyf"  # Must include custom .pyf file
-                   -m "pi"
-                   --lower # Important
-  DEPENDS pi/_pi.f # Fortran source
-)
-
-python_add_library(${CMAKE_PROJECT_NAME} MODULE
-            "${f2py_module_name}module.c"
-            "${F2PY_INCLUDE_DIR}/fortranobject.c"
-            "${fortran_src_file}" WITH_SOABI)
-
-target_include_directories(${CMAKE_PROJECT_NAME} PUBLIC
-                           ${F2PY_INCLUDE_DIR}
-                          )
+include(UseF2Py)
 
-target_link_libraries(${CMAKE_PROJECT_NAME} PUBLIC Python::NumPy)
+# Build the extension in one call: f2py generates the wrappers from the
+# checked-in signature file, then it and _pi.f are compiled and linked together
+# with fortranobject. The module name (pi) is taken from the .pyf stem.
+f2py_add_module(pi/pi.pyf pi/_pi.f)
 
 # With the MinGW toolchain (Windows), statically link the GNU runtime so the
 # module has no external libgfortran/libgcc/libwinpthread DLL dependencies.
 if(WIN32 AND CMAKE_Fortran_COMPILER_ID STREQUAL "GNU")
-  target_link_options(${CMAKE_PROJECT_NAME} PRIVATE -static)
+  target_link_options(pi PRIVATE -static)
 endif()
 
-install(TARGETS ${CMAKE_PROJECT_NAME} DESTINATION pi)
+install(TARGETS pi DESTINATION pi)

projects/pi-fortran/README.md

@@ -19,7 +19,9 @@ pytest
 
 This is slightly modified from the example in the [numpy docs](https://numpy.org/devdocs/f2py/buildtools/skbuild.html), but we are using Monte Carlo to estimate the value of $\pi$.
 
+The f2py glue is handled by [f2py-cmake](https://github.com/scikit-build/f2py-cmake): `include(UseF2Py)` and a single `f2py_add_module(pi/pi.pyf pi/_pi.f)` call generate the wrappers and build the extension, instead of wiring up `add_custom_command` and `fortranobject.c` by hand.
+
 A few surprises:
-1. The dreaded underscore problem has a way of cropping up. One solution is explicitly writing out the interface in a [signature (`.pyf`) file](https://numpy.org/devdocs/f2py/signature-file.html).
+1. The dreaded underscore problem has a way of cropping up. One solution is explicitly writing out the interface in a [signature (`.pyf`) file](https://numpy.org/devdocs/f2py/signature-file.html), which we pass to `f2py_add_module`.
 2. The module will require numpy to work.
-3. Between failed builds, it is best to clear out the `_skbuild` folder.
+3. Between failed builds, it is best to clear out the `build` folder.

projects/pi-fortran/pyproject.toml

@@ -2,6 +2,7 @@
 requires = [
     "scikit-build-core",
     "numpy>=1.21",
+    "f2py-cmake>=0.2",
 ]
 build-backend = "scikit_build_core.build"