--- a/python/mozbuild/mozbuild/backend/tup.py
+++ b/python/mozbuild/mozbuild/backend/tup.py
@@ -485,17 +485,17 @@ class TupOnly(CommonBackend, PartialBack
             acdefines_flags += ' -DBUILD_FASTER=1'
 
             fh.write('MOZ_OBJ_ROOT = $(TUP_CWD)\n')
             fh.write('DIST = $(MOZ_OBJ_ROOT)/dist\n')
             fh.write('ACDEFINES = %s\n' % acdefines_flags)
             fh.write('topsrcdir = $(MOZ_OBJ_ROOT)/%s\n' % (
                 os.path.relpath(self.environment.topsrcdir, self.environment.topobjdir)
             ))
-            fh.write('PYTHON = $(MOZ_OBJ_ROOT)/_virtualenv/bin/python -B\n')
+            fh.write('PYTHON = PYTHONDONTWRITEBYTECODE=1 $(MOZ_OBJ_ROOT)/_virtualenv/bin/python\n')
             fh.write('PYTHON_PATH = $(PYTHON) $(topsrcdir)/config/pythonpath.py\n')
             fh.write('PLY_INCLUDE = -I$(topsrcdir)/other-licenses/ply\n')
             fh.write('IDL_PARSER_DIR = $(topsrcdir)/xpcom/idl-parser\n')
             fh.write('IDL_PARSER_CACHE_DIR = $(MOZ_OBJ_ROOT)/xpcom/idl-parser/xpidl\n')
 
         # Run 'tup init' if necessary.
         if not os.path.exists(mozpath.join(self.environment.topsrcdir, ".tup")):
             tup = self.environment.substs.get('TUP', 'tup')