Update documentation

Author: Peter554

docs/usage.rst

@@ -304,7 +304,7 @@ Higher level analysis
         passing ``independent=False`` when instantiating the :class:`.Layer`. For convenience, if a layer consists
         only of one module name then a string may be passed in place of the :class:`.Layer` object. Additionally, if
         the layer consists of multiple *independent* modules, that can be passed as a set of strings instead of a
-        :class:`.Layer` object.
+        :class:`.Layer` object. A closed layer may be created by passing ``open=False`` (see below).
         *Any modules specified that don't exist in the graph will be silently ignored.*
     :param set[str] containers: The parent modules of the layers, as absolute names that you could
         import, such as ``mypackage.foo``. (Optional.)
@@ -409,6 +409,14 @@ Higher level analysis
             ),
         )
 
+    Closed layers
+    ^^^^^^^^^^^^^
+
+    A closed layer may be created by passing ``open=False``. Imports from higher to lower
+    layers cannot bypass closed layers - the closed layer must be included in the import
+    chain. For example, given the layers high -> mid (closed) -> low then all import chains
+    from high -> low must go via mid.
+
     Return value
     ^^^^^^^^^^^^
 
@@ -575,4 +583,4 @@ Module expressions
   - ``mypackage.foo*``: is not a valid expression. (The wildcard must replace a whole module name.)
 
 .. _namespace packages: https://docs.python.org/3/glossary.html#term-namespace-package
-.. _namespace portion: https://docs.python.org/3/glossary.html#term-portion
+.. _namespace portion: https://docs.python.org/3/glossary.html#term-portion

src/grimp/application/ports/graph.py

@@ -373,6 +373,11 @@ def find_illegal_dependencies_for_layers(
         compatibility it is also possible to pass a simple `set[str]` to describe a layer. In this
         case the sibling modules within the layer will be considered independent.
 
+        By default layers are open. `Layer.open` can be set to false to create a closed layer.
+        Imports from higher to lower layers cannot bypass closed layers - the closed layer must be
+        included in the import chain. For example, given the layers high -> mid (closed) -> low then
+        all import chains from high -> low must go via mid.
+
         Arguments:
 
         - layers:     A sequence, each element of which consists either of a `Layer`, the name