diff --git a/repo2docker/buildpacks/base.py b/repo2docker/buildpacks/base.py
index 796710b9..8a78a6be 100644
--- a/repo2docker/buildpacks/base.py
+++ b/repo2docker/buildpacks/base.py
@@ -222,9 +222,8 @@ class BuildPack(LoggingConfigurable):
         """
         return []
 
-    assemble_scripts = List(
-        [],
-        help="""
+    def get_assemble_scripts(self):
+        """
         Ordered list of shell script snippets to build the repo into the image.
 
         A list of tuples, where the first item is a username & the
@@ -247,7 +246,7 @@ class BuildPack(LoggingConfigurable):
         You can use environment variable substitutions in both the
         username and the execution script.
         """
-    )
+        return []
 
     post_build_scripts = List(
         [],
@@ -295,9 +294,8 @@ class BuildPack(LoggingConfigurable):
         result.get_labels = lambda: _merge_dicts(self.get_labels(), other.get_labels())
         result.get_build_script_files = lambda: _merge_dicts(self.get_build_script_files(), other.get_build_script_files())
         result.get_build_scripts = lambda: self.get_build_scripts() + other.get_build_scripts()
+        result.get_assemble_scripts = lambda: self.get_assemble_scripts() + other.get_assemble_scripts()
 
-        result.assemble_scripts = (self.assemble_scripts +
-                                   other.assemble_scripts)
         result.post_build_scripts = (self.post_build_scripts +
                                      other.post_build_scripts)
 
@@ -335,7 +333,7 @@ class BuildPack(LoggingConfigurable):
 
         assemble_script_directives = []
         last_user = 'root'
-        for user, script in self.assemble_scripts:
+        for user, script in self.get_assemble_scripts():
             if last_user != user:
                 assemble_script_directives.append("USER {}".format(user))
                 last_user = user
diff --git a/repo2docker/buildpacks/conda/__init__.py b/repo2docker/buildpacks/conda/__init__.py
index 59703d01..c46d0b12 100644
--- a/repo2docker/buildpacks/conda/__init__.py
+++ b/repo2docker/buildpacks/conda/__init__.py
@@ -108,8 +108,7 @@ class CondaBuildPack(BuildPack):
         """Am I building a Python 2 kernel environment?"""
         return self.python_version and self.python_version.split('.')[0] == '2'
 
-    #@default('assemble_scripts')
-    def setup_assembly(self):
+    def get_assemble_scripts(self):
         assembly_scripts = []
         environment_yml = self.binder_path('environment.yml')
         env_name = 'kernel' if self.py2 else 'root'
diff --git a/repo2docker/buildpacks/julia.py b/repo2docker/buildpacks/julia.py
index 021398ef..51226196 100644
--- a/repo2docker/buildpacks/julia.py
+++ b/repo2docker/buildpacks/julia.py
@@ -49,8 +49,7 @@ class JuliaBuildPack(BuildPack):
             )
         ]
 
-    @default('assemble_scripts')
-    def setup_assembly(self):
+    def get_assemble_scripts(self):
         require = self.binder_path('REQUIRE')
         return [(
             "${NB_USER}",
diff --git a/repo2docker/buildpacks/python/__init__.py b/repo2docker/buildpacks/python/__init__.py
index 561593ad..1fdffc92 100644
--- a/repo2docker/buildpacks/python/__init__.py
+++ b/repo2docker/buildpacks/python/__init__.py
@@ -60,8 +60,7 @@ class PythonBuildPack(BuildPack):
             )
         ]
 
-    @default('assemble_scripts')
-    def setup_assembly(self):
+    def get_assemble_scripts(self):
         # If we have a runtime.txt & that's set to python-2.7,
         # we will *not* install requirements.txt but will find &
         # install a requirements3.txt file if it exists.
@@ -140,8 +139,7 @@ class Python2BuildPack(BuildPack):
             )
         ]
 
-    @default('assemble_scripts')
-    def setup_assembly(self):
+    def get_assemble_scripts(self):
         return [
             (
                 '${NB_USER}',