diff --git a/tools/python/defs.bzl b/tools/python/defs.bzl
index abaac17..4e750bd 100644
--- a/tools/python/defs.bzl
+++ b/tools/python/defs.bzl
@@ -160,6 +160,7 @@ def py_project(name=None,
                main_deps=None,
                main_data=None,
                shebang=None,
+               zip_safe=True,
                lib_srcs=None,
                lib_deps=None,
                lib_data=None,
@@ -177,26 +178,33 @@ def py_project(name=None,
 
     """
 
-    lib_srcs = lib_srcs or native.glob(["src/python/**/*.py"],
-                                       exclude=[
-                                           "**/*.pyc",
-                                       ])
-    lib_data = lib_data or native.glob(["src/resources/**/*",
-                                        "src/python/**/*"],
-                                       exclude=[
-                                           "**/*.py",
-                                           "**/*.pyc",
-                                       ])
-    test_srcs = test_srcs or native.glob(["test/python/**/*.py"],
-                                         exclude=[
-                                             "**/*.pyc",
-                                         ])
-    test_data = test_data or native.glob(["test/resources/**/*",
-                                          "test/python/**/*"],
-                                         exclude=[
-                                             "**/*.py",
-                                             "**/*.pyc",
-                                         ])
+    lib_srcs = (lib_srcs or []) + \
+        native.glob(["src/python/**/*.py"],
+                    exclude=[
+                        "**/*.pyc",
+                    ])
+
+    lib_data = (lib_data or []) + \
+        native.glob(["src/resources/**/*",
+                     "src/python/**/*"],
+                    exclude=[
+                        "**/*.py",
+                        "**/*.pyc",
+                    ])
+
+    test_srcs = (test_srcs or []) + \
+        native.glob(["test/python/**/*.py"],
+                    exclude=[
+                        "**/*.pyc",
+                    ])
+
+    test_data = (test_data or []) + \
+        native.glob(["test/resources/**/*",
+                     "test/python/**/*"],
+                    exclude=[
+                        "**/*.py",
+                        "**/*.pyc",
+                    ])
 
     lib_name = name if not main else "lib"
 
@@ -242,6 +250,7 @@ def py_project(name=None,
                 "//visibility:public",
             ],
             shebang = shebang,
+            zip_safe=zip_safe,
         )
 
     for src in test_srcs: