diff --git a/metaflow/plugins/pypi/conda_decorator.py b/metaflow/plugins/pypi/conda_decorator.py
index fc0e4d83a1b..b1b7ee833d9 100644
--- a/metaflow/plugins/pypi/conda_decorator.py
+++ b/metaflow/plugins/pypi/conda_decorator.py
@@ -49,6 +49,13 @@ class CondaStepDecorator(StepDecorator):
     # CONDA_CHANNELS in their environment. For pinning specific packages to specific
     # conda channels, users can specify channel::package as the package name.
 
+    def __init__(self, attributes=None, statically_defined=False):
+        self._attributes_with_user_values = (
+            set(attributes.keys()) if attributes is not None else set()
+        )
+
+        super(CondaStepDecorator, self).__init__(attributes, statically_defined)
+
     def init(self):
         super(CondaStepDecorator, self).init()
 
@@ -58,9 +65,11 @@ def init(self):
             **self.attributes["packages"],
         }
         del self.attributes["libraries"]
+        if self.attributes["packages"]:
+            self._attributes_with_user_values.add("packages")
 
     def is_attribute_user_defined(self, name):
-        return name in self._user_defined_attributes
+        return name in self._attributes_with_user_values
 
     def step_init(self, flow, graph, step, decos, environment, flow_datastore, logger):
         # The init_environment hook for Environment creates the relevant virtual
@@ -80,9 +89,10 @@ def step_init(self, flow, graph, step, decos, environment, flow_datastore, logge
                 **super_attributes["packages"],
                 **self.attributes["packages"],
             }
-            self._user_defined_attributes = self._user_defined_attributes.union(
-                conda_base._user_defined_attributes
+            self._attributes_with_user_values.update(
+                conda_base._attributes_with_user_values
             )
+
             self.attributes["python"] = (
                 self.attributes["python"] or super_attributes["python"]
             )
@@ -329,22 +339,15 @@ class CondaFlowDecorator(FlowDecorator):
     }
 
     def __init__(self, attributes=None, statically_defined=False):
-        self._user_defined_attributes = (
-            attributes.copy() if attributes is not None else {}
+        self._attributes_with_user_values = (
+            set(attributes.keys()) if attributes is not None else set()
         )
+
         super(CondaFlowDecorator, self).__init__(attributes, statically_defined)
 
     def init(self):
         super(CondaFlowDecorator, self).init()
 
-        # We have to go back and fixup _user_defined_attributes for potential
-        # config resolution
-        self._user_defined_attributes = {
-            k: v
-            for k, v in self.attributes.items()
-            if k in self._user_defined_attributes
-        }
-
         # Support legacy 'libraries=' attribute for the decorator.
         self.attributes["packages"] = {
             **self.attributes["libraries"],
@@ -355,7 +358,7 @@ def init(self):
             self.attributes["python"] = str(self.attributes["python"])
 
     def is_attribute_user_defined(self, name):
-        return name in self._user_defined_attributes
+        return name in self._attributes_with_user_values
 
     def flow_init(
         self, flow, graph, environment, flow_datastore, metadata, logger, echo, options
diff --git a/metaflow/plugins/pypi/pypi_decorator.py b/metaflow/plugins/pypi/pypi_decorator.py
index 575531eb458..bcc7dda891a 100644
--- a/metaflow/plugins/pypi/pypi_decorator.py
+++ b/metaflow/plugins/pypi/pypi_decorator.py
@@ -24,6 +24,13 @@ class PyPIStepDecorator(StepDecorator):
     name = "pypi"
     defaults = {"packages": {}, "python": None, "disabled": None}  # wheels
 
+    def __init__(self, attributes=None, statically_defined=False):
+        self._attributes_with_user_values = (
+            set(attributes.keys()) if attributes is not None else set()
+        )
+
+        super().__init__(attributes, statically_defined)
+
     def step_init(self, flow, graph, step, decos, environment, flow_datastore, logger):
         # The init_environment hook for Environment creates the relevant virtual
         # environments. The step_init hook sets up the relevant state for that hook to
@@ -36,8 +43,8 @@ def step_init(self, flow, graph, step, decos, environment, flow_datastore, logge
         if "pypi_base" in self.flow._flow_decorators:
             pypi_base = self.flow._flow_decorators["pypi_base"][0]
             super_attributes = pypi_base.attributes
-            self._user_defined_attributes = self._user_defined_attributes.union(
-                pypi_base._user_defined_attributes
+            self._attributes_with_user_values.update(
+                pypi_base._attributes_with_user_values
             )
             self.attributes["packages"] = {
                 **super_attributes["packages"],
@@ -99,7 +106,7 @@ def step_init(self, flow, graph, step, decos, environment, flow_datastore, logge
         environment.set_local_root(LocalStorage.get_datastore_root_from_config(logger))
 
     def is_attribute_user_defined(self, name):
-        return name in self._user_defined_attributes
+        return name in self._attributes_with_user_values
 
 
 class PyPIFlowDecorator(FlowDecorator):
@@ -122,9 +129,10 @@ class PyPIFlowDecorator(FlowDecorator):
     defaults = {"packages": {}, "python": None, "disabled": None}
 
     def __init__(self, attributes=None, statically_defined=False):
-        self._user_defined_attributes = (
-            attributes.copy() if attributes is not None else {}
+        self._attributes_with_user_values = (
+            set(attributes.keys()) if attributes is not None else set()
         )
+
         super().__init__(attributes, statically_defined)
 
     def flow_init(
diff --git a/test/unit/test_conda_decorator.py b/test/unit/test_conda_decorator.py
new file mode 100644
index 00000000000..1f8e2d75649
--- /dev/null
+++ b/test/unit/test_conda_decorator.py
@@ -0,0 +1,29 @@
+from metaflow.plugins.pypi.conda_decorator import CondaStepDecorator
+
+
+def test_decorator_custom_attributes():
+    deco = CondaStepDecorator(attributes={"python": "3.9"})
+    deco.init()
+    assert deco.is_attribute_user_defined(
+        "python"
+    ), "python is supposed to be an user-defined attribute"
+    assert not deco.is_attribute_user_defined(
+        "packages"
+    ), "packages is supposed to be default"
+    assert not deco.is_attribute_user_defined(
+        "libraries"
+    ), "libraries is supposed to be default"
+
+
+def test_decorator_custom_attributes_with_backward_compatibility():
+    deco = CondaStepDecorator(attributes={"libraries": {"a": "test"}})
+    deco.init()
+    assert not deco.is_attribute_user_defined(
+        "python"
+    ), "python is supposed to be default"
+    assert deco.is_attribute_user_defined(
+        "packages"
+    ), "packages is supposed to be user-defined"
+    assert deco.is_attribute_user_defined(
+        "libraries"
+    ), "libraries is supposed to be user-defined"
diff --git a/test/unit/test_pypi_decorator.py b/test/unit/test_pypi_decorator.py
new file mode 100644
index 00000000000..32289080524
--- /dev/null
+++ b/test/unit/test_pypi_decorator.py
@@ -0,0 +1,12 @@
+from metaflow.plugins.pypi.pypi_decorator import PyPIStepDecorator
+
+
+def test_decorator_custom_attributes():
+    deco = PyPIStepDecorator(attributes={"python": "3.9"})
+    deco.init()
+    assert deco.is_attribute_user_defined(
+        "python"
+    ), "python is supposed to be an user-defined attribute"
+    assert not deco.is_attribute_user_defined(
+        "packages"
+    ), "packages is supposed to be default"