F2py fortran example

.github/workflows/ci.yml

@@ -26,6 +26,11 @@ jobs:
 
     steps:
     - uses: actions/checkout@v3
+    - uses: awvwgk/setup-fortran@main
+      id: setup-fortran
+      with:
+        compiler: gcc
+        version: 11
     - uses: wntrblm/nox@2022.11.21
     - run: nox
 

noxfile.py

@@ -4,7 +4,7 @@
 
 hello_list = ["hello-pure", "hello-cpp", "hello-pybind11", "hello-cython"]
 if not sys.platform.startswith("win"):
-    hello_list.append("hello-cmake-package")
+    hello_list.extend(["hello-cmake-package", "pi-fortran"])
 long_hello_list = [*hello_list, "pen2-cython", "core-c-hello", "core-pybind11-hello"]
 
 

projects/pi-fortran/CMakeLists.txt

@@ -0,0 +1,44 @@
+cmake_minimum_required(VERSION 3.17.2...3.26)
+
+project(pi
+  VERSION 1.0.1
+  DESCRIPTION "pi estimator"
+  LANGUAGES C Fortran
+)
+
+find_package(Python REQUIRED COMPONENTS Interpreter Development.Module NumPy)
+
+# F2PY headers
+execute_process(
+  COMMAND "${Python_EXECUTABLE}"
+  -c "import numpy.f2py; print(numpy.f2py.get_include())"
+  OUTPUT_VARIABLE F2PY_INCLUDE_DIR
+  OUTPUT_STRIP_TRAILING_WHITESPACE
+)
+
+set(f2py_module_name "pi")
+set(fortran_src_file "${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
+                           ${F2PY_INCLUDE_DIR}
+                          )
+
+target_link_libraries(${CMAKE_PROJECT_NAME} PUBLIC Python::NumPy)
+
+install(TARGETS ${CMAKE_PROJECT_NAME} DESTINATION pi)

projects/pi-fortran/README.md

@@ -0,0 +1,25 @@
+# Pi
+
+This is an example project demonstrating the use of scikit-build for distributing a standalone FORTRAN library, *pi*;
+a CMake package for that library; and a Python wrapper implemented in f2py.
+
+The example assume some familiarity with CMake and f2py, only really going into detail on the scikit-build parts.
+
+To install the package run in the project directory
+
+```bash
+pip install .
+```
+
+To run the Python tests, first install the package, then in the project directory run
+
+```bash
+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 $\pi$.
+
+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).
+2. The module will require numpy to work.
+3. Between failed builds, it is best to clear out the `_skbuild` folder.

projects/pi-fortran/pi/_pi.f

@@ -0,0 +1,17 @@
+      subroutine estimate_pi(pi_estimated, num_trials)
+C     Estimate pi using Monte Carlo
+      integer num_trials, num_in_circle
+      real*8 pi_estimated, x, y
+Cf2py intent(in) num_trials
+Cf2py intent(out) pi_estimated
+
+      num_in_circle = 0d0
+      do i = 1, num_trials
+          call random_number(x)
+          call random_number(y)
+          if (x**2 + y**2 < 1.0d0) then
+              num_in_circle = num_in_circle + 1
+          endif
+      end do
+      pi_estimated = (4d0 * num_in_circle) / num_trials
+      end subroutine estimate_pi

projects/pi-fortran/pi/pi.pyf

@@ -0,0 +1,10 @@
+!    -*- f90 -*-
+! Note: the context of this file is case sensitive.
+pymodule pi
+interface
+subroutine estimate_pi(pi_estimated,num_trials) ! in pi/_pi.f
+    real*8 intent(out) :: pi_estimated
+    integer intent(in) :: num_trials
+end subroutine estimate_pi
+end interface
+end pymodule pi

projects/pi-fortran/pyproject.toml

@@ -0,0 +1,16 @@
+[build-system]
+requires = [
+    "scikit-build-core",
+    "numpy>=1.21",
+]
+build-backend = "scikit_build_core.build"
+
+[project]
+name = "pi-fortran"
+version = "1.0.1"
+requires-python = ">=3.7"
+dependencies = ["numpy>=1.21"]
+
+[tool.scikit-build]
+ninja.minimum-version = "1.10"
+cmake.minimum-version = "3.17.2"

projects/pi-fortran/tests/test_pi.py

@@ -0,0 +1,9 @@
+import math
+
+import pytest
+from pi import pi
+
+
+def test_estimate_pi():
+    pi_est = pi.estimate_pi(1e8)
+    assert pi_est == pytest.approx(math.pi, rel=0.001)