build: Use Python venv by default

build-support/custom-modules/ns3-check-dependencies.cmake

@@ -40,6 +40,27 @@ function(check_deps missing_deps)
     )
     if(NOT (${return_code} EQUAL 0))
       list(APPEND local_missing_deps ${package})
+    else()
+      # To make sure CMake import files can be found from venv site packages, we
+      # manually add them to CMAKE_PREFIX_PATH
+      execute_process(
+        COMMAND
+          ${Python3_EXECUTABLE} -c
+          "import os; import ${package}; print(os.path.abspath(os.path.dirname(${package}.__file__)))"
+        OUTPUT_VARIABLE venv_site_packages_path
+      )
+      # Remove newlines (\n, \r, \r\n)
+      string(REGEX REPLACE "[\r\n]+$" "" venv_site_packages_path
+                           "${venv_site_packages_path}"
+      )
+      if(EXISTS ${venv_site_packages_path})
+        if(NOT (DEFINED CMAKE_PREFIX_PATH))
+          set(CMAKE_PREFIX_PATH "" PARENT_SCOPE)
+        endif()
+        set(CMAKE_PREFIX_PATH "${CMAKE_PREFIX_PATH};${venv_site_packages_path}"
+            PARENT_SCOPE
+        )
+      endif()
     endif()
   endforeach()
 

build-support/macros-and-definitions.cmake

@@ -628,16 +628,11 @@ macro(process_options)
   set(Python3_FOUND FALSE)
   set(Python3_INCLUDE_DIRS)
   set(Python3_Interpreter_FOUND FALSE)
+  set(Python3_FIND_VIRTUALENV FIRST)
   if(${NS3_PYTHON_BINDINGS})
     find_package(Python3 COMPONENTS Interpreter Development)
   else()
-    # If Python was not set yet, use the version found by check_deps
-    check_deps(python3_deps EXECUTABLES python3)
-    if(python3_deps)
-      message(FATAL_ERROR "Python3 was not found")
-    else()
-      set(Python3_EXECUTABLE ${PYTHON3})
-    endif()
+    find_package(Python3 COMPONENTS Interpreter)
   endif()
 
   # Check if both Python interpreter and development libraries were found