add lazy error

serket/nn/utils.py

@@ -364,6 +364,23 @@ def inner(instance, *a, **k):
     return inner
 
 
+LAZY_CALL_ERROR = """ 
+Cannot call ``{func_name}`` directly on a lazy layer.
+use ``layer.at['{func_name}'](...)`` instead to return a tuple of:
+    - The layer output. 
+    - Materialized layer.
+
+Example:
+    >>> layer = {class_name}(...)
+    >>> layer(x)  # this will raise an error
+    Traceback (most recent call last):
+        ...
+    >>> _, materialized_layer = layer.at['{func_name}'](x)
+    >>> materialized_layer(x)
+    ...
+""".lstrip()
+
+
 def maybe_lazy_call(
     func: Callable[P, T],
     is_lazy: Callable[..., bool],
@@ -395,9 +412,16 @@ def inner(instance, *a, **k):
             kwargs[key] = update(instance, *a, **k)
 
         for key in kwargs:
-            # clear the instance information (i.e. the initial input arguments)
-            # use ``delattr`` to raise an error if the instance is immutable
-            delattr(instance, key)
+            try:
+                # clear the instance information (i.e. the initial input arguments)
+                # use ``delattr`` to raise an error if the instance is immutable
+                delattr(instance, key)
+            except AttributeError:
+                # the instance is lazy and immutable
+                func_name = func.__name__
+                class_name = type(instance).__name__
+                kwargs = dict(func_name=func_name, class_name=class_name)
+                raise RuntimeError(LAZY_CALL_ERROR.format(**kwargs))
 
         # re-initialize the instance with the resolved arguments
         getattr(type(instance), "__init__")(instance, **kwargs)