use an external venv if active

cmake/modules/DunePythonVirtualenv.cmake

@@ -82,6 +82,16 @@ set(DUNE_PYTHON_EXTERNAL_VIRTUALENV_FOR_ABSOLUTE_BUILDDIR ON CACHE BOOL
   "Place Python virtualenv in top-level directory \"dune-python-env\" when using an absolute build directory"
   )
 
+# check if we are in a venv already
+if(DUNE_PYTHON_VIRTUALENV_PATH STREQUAL "")
+  dune_execute_process(COMMAND ${Python3_EXECUTABLE}
+                                ${scriptdir}/venvpath.py
+                       OUTPUT_VARIABLE DUNE_PYTHON_VIRTUALENV_PATH
+                       OUTPUT_STRIP_TRAILING_WHITESPACE
+                      )
+endif()
+
+# if no virtual env path set so far check dune modules
 if(DUNE_PYTHON_VIRTUALENV_PATH STREQUAL "")
   foreach(mod ${ALL_DEPENDENCIES})
     if(IS_DIRECTORY ${${mod}_DIR}/dune-env)

cmake/scripts/CMakeLists.txt

@@ -3,6 +3,7 @@ install(FILES
   conf.py.in
   CreateDoxyFile.cmake
   envdetect.py
+  venvpath.py
   FinalizeHeadercheck.cmake
   index.rst.in
   InstallFile.cmake

cmake/scripts/venvpath.py

+import sys,os
+
+def inVEnv():
+    # check whether we are in a anaconda environment
+    # were the checks based on prefix and base_prefix
+    # seem to fail
+    if "CONDA_DEFAULT_ENV" in os.environ:
+        return 1
+
+    # If sys.real_prefix exists, this is a virtualenv set up with the virtualenv package
+    real_prefix = hasattr(sys, 'real_prefix')
+    if real_prefix:
+        return 1
+    # If a virtualenv is set up with pyvenv, we check for equality of base_prefix and prefix
+    if hasattr(sys, 'base_prefix'):
+        return (sys.prefix != sys.base_prefix)
+    # If none of the above conditions triggered, this is probably no virtualenv interpreter
+    return 0
+
+if __name__ == "__main__":
+    if not inVEnv():
+        print("")
+    else:
+        print(sys.prefix)