diff --git a/repo2docker/buildpacks/base.py b/repo2docker/buildpacks/base.py
index b45c6c77..757ec384 100644
--- a/repo2docker/buildpacks/base.py
+++ b/repo2docker/buildpacks/base.py
@@ -162,9 +162,8 @@ class BuildPack(LoggingConfigurable):
             "npm",
         }
 
-    env = List(
-        [],
-        help="""
+    def get_env(self):
+        """
         Ordered list of environment variables to be set for this image.
 
         Ordered so that environment variables can use other environment
@@ -173,17 +172,16 @@ class BuildPack(LoggingConfigurable):
         Expects tuples, with the first item being the environment variable
         name and the second item being the value.
         """
-    )
+        return []
 
-    path = List(
-        [],
-        help="""
+    def get_path(self):
+        """
         Ordered list of file system paths to look for executables in.
 
         Just sets the PATH environment variable. Separated out since
         it is very commonly set by various buildpacks.
         """
-    )
+        return []
 
     labels = Dict(
         {},
@@ -292,10 +290,9 @@ class BuildPack(LoggingConfigurable):
         # FIXME: Temporary hack so we can refactor this piece by piece instead of all at once!
         result.get_packages = lambda: self.get_packages().union(other.get_packages())
         result.get_base_packages = lambda: self.get_base_packages().union(other.get_base_packages())
+        result.get_path = lambda: self.get_path() + other.get_path()
+        result.get_env = lambda: self.get_env() + other.get_env()
 
-        result.path = self.path + other.path
-        # FIXME: Deduplicate Env
-        result.env = self.env + other.env
         result.build_scripts = self.build_scripts + other.build_scripts
         result.assemble_scripts = (self.assemble_scripts +
                                    other.assemble_scripts)
@@ -351,8 +348,8 @@ class BuildPack(LoggingConfigurable):
 
         return t.render(
             packages=sorted(self.get_packages()),
-            path=self.path,
-            env=self.env,
+            path=self.get_path(),
+            env=self.get_env(),
             labels=self.labels,
             build_script_directives=build_script_directives,
             assemble_script_directives=assemble_script_directives,
diff --git a/repo2docker/buildpacks/conda/__init__.py b/repo2docker/buildpacks/conda/__init__.py
index d9720b50..f592dc5b 100644
--- a/repo2docker/buildpacks/conda/__init__.py
+++ b/repo2docker/buildpacks/conda/__init__.py
@@ -19,12 +19,14 @@ HERE = os.path.dirname(os.path.abspath(__file__))
 class CondaBuildPack(BuildPack):
     name = "conda"
     version = "0.1"
-    env = [
-        ('CONDA_DIR', '${APP_BASE}/conda'),
-        ('NB_PYTHON_PREFIX', '${CONDA_DIR}'),
-    ]
+    def get_env(self):
+        return [
+            ('CONDA_DIR', '${APP_BASE}/conda'),
+            ('NB_PYTHON_PREFIX', '${CONDA_DIR}'),
+        ]
 
-    path = ['${CONDA_DIR}/bin']
+    def get_path(self):
+        return ['${CONDA_DIR}/bin']
 
     build_scripts = [
         (
diff --git a/repo2docker/buildpacks/julia.py b/repo2docker/buildpacks/julia.py
index 515ea39f..7a0afa39 100644
--- a/repo2docker/buildpacks/julia.py
+++ b/repo2docker/buildpacks/julia.py
@@ -9,17 +9,18 @@ from .base import BuildPack
 class JuliaBuildPack(BuildPack):
     name = "julia"
     version = "0.1"
-    env = [
-        ('JULIA_PATH', '${APP_BASE}/julia'),
-        ('JULIA_HOME', '${JULIA_PATH}/bin'),
-        ('JULIA_PKGDIR', '${JULIA_PATH}/pkg'),
-        ('JULIA_VERSION', '0.6.0'),
-        ('JUPYTER', '${NB_PYTHON_PREFIX}/bin/jupyter')
-    ]
 
-    path = [
-        '${JULIA_PATH}/bin'
-    ]
+    def get_env(self):
+        return [
+            ('JULIA_PATH', '${APP_BASE}/julia'),
+            ('JULIA_HOME', '${JULIA_PATH}/bin'),
+            ('JULIA_PKGDIR', '${JULIA_PATH}/pkg'),
+            ('JULIA_VERSION', '0.6.0'),
+            ('JUPYTER', '${NB_PYTHON_PREFIX}/bin/jupyter')
+        ]
+
+    def get_path(self):
+        return ['${JULIA_PATH}/bin']
 
     build_scripts = [
         (
diff --git a/repo2docker/buildpacks/python/__init__.py b/repo2docker/buildpacks/python/__init__.py
index 7f444e41..5073df1b 100644
--- a/repo2docker/buildpacks/python/__init__.py
+++ b/repo2docker/buildpacks/python/__init__.py
@@ -17,15 +17,17 @@ class PythonBuildPack(BuildPack):
             'python3-dev',
         }
 
-    env = [
-        ("VENV_PATH", "${APP_BASE}/venv"),
-        # Prefix to use for installing kernels and finding jupyter binary
-        ("NB_PYTHON_PREFIX", "${VENV_PATH}"),
-    ]
+    def get_env(self):
+        return [
+            ("VENV_PATH", "${APP_BASE}/venv"),
+            # Prefix to use for installing kernels and finding jupyter binary
+            ("NB_PYTHON_PREFIX", "${VENV_PATH}"),
+        ]
 
-    path = [
-        "${VENV_PATH}/bin"
-    ]
+    def get_path(self):
+        return [
+            "${VENV_PATH}/bin"
+        ]
 
 
     build_script_files = {
@@ -100,13 +102,15 @@ class Python2BuildPack(BuildPack):
         'python/requirements2.frozen.txt': '/tmp/requirements2.frozen.txt',
     }
 
-    env = [
-        ('VENV2_PATH', '${APP_BASE}/venv2')
-    ]
+    def get_env(self):
+        return [
+            ('VENV2_PATH', '${APP_BASE}/venv2')
+        ]
 
-    path = [
-        "${VENV2_PATH}/bin"
-    ]
+    def get_path(self):
+        return [
+            "${VENV2_PATH}/bin"
+        ]
 
     build_scripts = [
         (