Change the way the requirements are installed into the internal venv. They...

Change the way the requirements are installed into the internal venv. They will now be installed during the configure stage. The actual dune package is installed only if requirements were installed successfully using --no-deps and --no-build-isolation. This way there is no failure if there is no internet connection.

slight change to previous approach: during configure install external packages (not dune modules), during installation of the dune package, locally available packages (wheelhouse) will be installed as well but with --no-index to avoid requiring internet access

cmake/modules/DunePythonInstallPackage.cmake

@@ -65,7 +65,6 @@ function(dune_python_install_package)
   endif()
 
   # Configure setup.py.in if present
-
   set(RequiredPythonModules "${ProjectPythonRequires}")
   foreach(mod ${ALL_DEPENDENCIES})
     if(${${mod}_HASPYTHON}) # module found and has python bindings
@@ -107,13 +106,6 @@ function(dune_python_install_package)
     message(FATAL_ERROR "dune_python_install_package: Requested installations, but pip was not found!")
   endif()
 
-  #
-  # Define build rules that install the Python package into the Dune virtualenv at the build stage
-  #
-
-  # Install the Python Package into the Dune virtual environment in the build stage
-  string(REPLACE "/" "_" envtargetname "env_install_python_${CMAKE_CURRENT_SOURCE_DIR}_${PYINST_PATH}")
-
   # A hack which shouldn't be necesary once the ci is updated
   # https://gitlab.dune-project.org/docker/ci/-/merge_requests/101
   option(DUNE_RUNNING_IN_CI "This is turned on if running in dune gitlab ci" OFF)
@@ -122,21 +114,48 @@ function(dune_python_install_package)
   else()
       set(PACKAGE_INDEX "")
   endif()
+
+  # install external requirements (i.e. not dune packages) once at configure stage - install of package is
+  # only carried out if this succeeded and with --no-index, i.e., without using any package indices but only local wheels
+  string(REPLACE " " "\n" RequiredPypiModules "${ProjectPythonRequires}")
+  file(WRITE "${PYINST_FULLPATH}/requirements.txt" "${RequiredPypiModules}")
+  dune_execute_process(COMMAND ${DUNE_PYTHON_VIRTUALENV_EXECUTABLE} -m pip install
+                                "${WHEEL_OPTION}"
+                                # we can't use the same additional parameters for both internal
+                                # install and normal install so not including these flags at the moment
+                                "${PACKAGE_INDEX}"          # stopgap solution until ci repo fixed
+                                -r "${PYINST_FULLPATH}/requirements.txt"
+                       RESULT_VARIABLE DUNE_PYTHON_DEPENDENCIES_FAILED
+                       WARNING_MESSAGE "python package requirements could not be installed - possibly connection to the python package index failed"
+                      )
+  if(NOT DUNE_PYTHON_DEPENDENCIES_FAILED)
+  #
+  # Define build rules that install the Python package into the Dune virtualenv at the build stage
+  #
+
+  # Install the Python Package into the Dune virtual environment in the build stage
+  string(REPLACE "/" "_" envtargetname "env_install_python_${CMAKE_CURRENT_SOURCE_DIR}_${PYINST_PATH}")
+
+  # installation target for dune package into local env - external requirements are already sorted and we want this step to not require
+  # internet access. Dune packages need to be installed at this stage and should not be optained from pypi (those packages include the C++ part
+  # of the module which we don't want to install. So only use available wheels.
   add_custom_target(
     ${envtargetname}
     ALL
     COMMAND ${DUNE_PYTHON_VIRTUALENV_EXECUTABLE} -m pip install
+      --no-build-isolation      # avoid looking for packages during 'make' if they in the internal venv from previous 'make'
       --no-warn-script-location # supress warnings that dune-env/bin not in path
+      --no-index
       "${WHEEL_OPTION}"
       # we can't use the same additional parameters for both internal
       # install and normal install so not including these flags at the moment
       # ${PYINST_ADDITIONAL_PIP_PARAMS} ${DUNE_PYTHON_ADDITIONAL_PIP_PARAMS}
-      "${PACKAGE_INDEX}"          # stopgap solution until ci repo fixed
       --editable                  # Installations into the internal env are always editable
       "${PYINST_FULLPATH}"
     COMMENT "Installing Python package at ${PYINST_FULLPATH} into Dune virtual environment (${PACKAGE_INDEX})."
     DEPENDS ${PYINST_DEPENDS}
   )
+  endif()
 
   #
   # Now define rules for `make install_python`.
@@ -247,6 +266,7 @@ function(dune_python_install_package)
     #
 
     # Make sure to generate the metadata for the build stage
+    if(NOT DUNE_PYTHON_DEPENDENCIES_FAILED)
     if(SKBUILD)
       # this is the only version of the metadata we need for the package insallation
       add_custom_target(
@@ -311,6 +331,9 @@ function(dune_python_install_package)
     add_custom_command(TARGET ${envtargetname} POST_BUILD
                        COMMAND ${DUNE_PYTHON_VIRTUALENV_EXECUTABLE} -m dune configure
                       )
+    endif() # NOT DUNE_PYTHON_DEPENDENCIES_FAILED)
+
+
     # Add a custom command that triggers the configuration of dune-py when installing package
     if(NOT "${DUNE_PYTHON_INSTALL_LOCATION}" STREQUAL "none")
       add_custom_command(TARGET ${targetname} POST_BUILD