Python: disable environment variables by default

This avoids the problem where Blender doesn't start because
the PYTHONPATH points to an incompatible Python version,
see T72807.

Previously we chose to assume people who set the PYTHONPATH know what
they're doing, however users may have set this for non Blender projects.
So it's not obvious that this is the cause of Blender not to launch
on their system.

To use Python's environment vars, pass the argument:
--python-use-system-env

Note that this only impacts Python run-time environment variables
documented in `python --help`, Access from `os.environ` remains.

1 files changed, 15 insertions, 0 deletions

diff --git a/source/creator/creator_args.c b/source/creator/creator_args.c
index 6e3988a5ea6..c7df4888c0d 100644
--- a/source/creator/creator_args.c
+++ b/source/creator/creator_args.c
@@ -559,6 +559,7 @@ static int arg_handle_print_help(int UNUSED(argc), const char **UNUSED(argv), vo
   BLI_argsPrintArgDoc(ba, "--python-expr");
   BLI_argsPrintArgDoc(ba, "--python-console");
   BLI_argsPrintArgDoc(ba, "--python-exit-code");
+  BLI_argsPrintArgDoc(ba, "--python-use-system-env");
   BLI_argsPrintArgDoc(ba, "--addons");
 
   printf("\n");
@@ -1907,6 +1908,17 @@ static int arg_handle_python_exit_code_set(int argc, const char **argv, void *UN
   }
 }
 
+static const char arg_handle_python_use_system_env_set_doc[] =
+    "\n\t"
+    "Allow Python to use system environment variables such as 'PYTHONPATH'.";
+static int arg_handle_python_use_system_env_set(int UNUSED(argc),
+                                                const char **UNUSED(argv),
+                                                void *UNUSED(data))
+{
+  BPY_python_use_system_env();
+  return 0;
+}
+
 static const char arg_handle_addons_set_doc[] =
     "<addon(s)>\n"
     "\tComma separated list of add-ons (no spaces).";
@@ -2188,6 +2200,9 @@ void main_args_setup(bContext *C, bArgs *ba)
       ba, 1, NULL, "--env-system-scripts", CB_EX(arg_handle_env_system_set, scripts), NULL);
   BLI_argsAdd(ba, 1, NULL, "--env-system-python", CB_EX(arg_handle_env_system_set, python), NULL);
 
+  BLI_argsAdd(
+      ba, 1, NULL, "--python-use-system-env", CB(arg_handle_python_use_system_env_set), NULL);
+
   /* second pass: custom window stuff */
   BLI_argsAdd(ba, 2, "-p", "--window-geometry", CB(arg_handle_window_geometry), NULL);
   BLI_argsAdd(ba, 2, "-w", "--window-border", CB(arg_handle_with_borders), NULL);