fixes related to imports

automata/base/utils.py

@@ -17,6 +17,7 @@
     Literal,
     Set,
     Tuple,
+    TypeAlias,
     TypeVar,
     Union,
 )
@@ -30,6 +31,10 @@
     _visual_imports = False
 else:
     _visual_imports = True
+finally:
+    # create a type for type checker
+    # irrespective of whether the imports were successful
+    GraphT: TypeAlias = "pgv.AGraph"
 
 
 LayoutMethod = Literal["neato", "dot", "twopi", "circo", "fdp", "nop"]
@@ -81,7 +86,7 @@ def create_graph(
     reverse_orientation: bool = False,
     fig_size: Union[Tuple[float, float], Tuple[float], None] = None,
     state_separation: float = 0.5,
-) -> pgv.AGraph:
+) -> GraphT:
     """Creates and returns a graph object
     Args:
         - horizontal (bool, optional): Direction of node layout. Defaults
@@ -118,7 +123,7 @@ def create_graph(
 
 
 def save_graph(
-    graph: pgv.AGraph,
+    graph: GraphT,
     path: Union[str, os.PathLike],
 ) -> None:
     """Write `graph` to file given by `path`. PNG, SVG, etc.

automata/fa/fa.py

@@ -20,7 +20,9 @@
     import coloraide
     import pygraphviz as pgv
 except ImportError:
-    pass
+    _visual_imports = False
+else:
+    _visual_imports = True
 
 
 FAStateT = AutomatonStateT
@@ -73,6 +75,12 @@ def show_diagram(
             AGraph corresponding to the given automaton.
         """
 
+        if not _visual_imports:
+            raise ImportError(
+                "Missing visualization packages; "
+                "please install coloraide and pygraphviz."
+            )
+
         # Defining the graph.
         graph = create_graph(
             horizontal, reverse_orientation, fig_size, state_separation

automata/pda/pda.py

@@ -33,7 +33,9 @@
     import coloraide
     import pygraphviz as pgv
 except ImportError:
-    pass
+    _visual_imports = False
+else:
+    _visual_imports = True
 
 PDAStateT = AutomatonStateT
 PDATransitionsT = AutomatonTransitionsT
@@ -134,6 +136,12 @@ def show_diagram(
             AGraph corresponding to the given automaton.
         """
 
+        if not _visual_imports:
+            raise ImportError(
+                "Missing visualization packages; "
+                "please install coloraide and pygraphviz."
+            )
+
         # Defining the graph.
         graph = create_graph(
             horizontal, reverse_orientation, fig_size, state_separation