showgraph: give more information when system graphviz not found

catch graphviz ExecutableNotFound error and re-raise to explain the
source of the problem and where to download system graphviz

psyneulink/core/compositions/showgraph.py

@@ -295,6 +295,13 @@
 default_showgraph_subdir = 'pnl-show_graph-output'
 
 
+_gv_executable_not_found_error_msg = (
+    "Graphviz executables were not found on your systems' PATH. These are"
+    " required in addition to the graphviz python package and can be downloaded"
+    " at https://www.graphviz.org/download/"
+)
+
+
 def get_default_showgraph_dir():
     pnl_module_dir = pathlib.Path(psyneulink.__file__).parent.absolute()
     try:
@@ -2613,6 +2620,8 @@ def _generate_output(self,
                          ):
 
         from psyneulink.core.compositions.composition import Composition, NodeRole
+        # graphviz is currently only imported within methods
+        from graphviz.backend.execute import ExecutableNotFound
 
         composition = self.composition
         nodes = self._get_nodes(composition, context)
@@ -2684,39 +2693,35 @@ def get_index_of_node_in_G_body(node, node_type: Literal['MECHANISM', 'Projectio
         # GENERATE OUTPUT ---------------------------------------------------------------------
 
         # Show as pdf
-        try:
-            if output_fmt == 'pdf':
-                # G.format = 'svg'
+        if output_fmt == 'pdf':
+            # G.format = 'svg'
+            try:
                 G.view(composition.name.replace(" ", "-"), cleanup=True, directory=get_default_showgraph_dir().joinpath('PDFS'))
+            except ExecutableNotFound as e:
+                raise ShowGraphError(_gv_executable_not_found_error_msg) from e
+            except Exception as e:
+                raise ShowGraphError(f"Problem displaying graph for {composition.name}: {e}") from e
 
-            # Generate images for animation
-            elif output_fmt == 'gif':
-                if composition.active_item_rendered or INITIAL_FRAME in active_items:
-                    self._generate_gifs(G, active_items, context)
+        # Generate images for animation
+        elif output_fmt == 'gif':
+            if composition.active_item_rendered or INITIAL_FRAME in active_items:
+                self._generate_gifs(G, active_items, context)
 
-            # Return graph to show in jupyter
-            elif output_fmt == 'jupyter':
-                return G
+        # Return graph to show in jupyter
+        elif output_fmt == 'jupyter':
+            return G
 
-            elif output_fmt == 'gv':
-                return G
+        elif output_fmt == 'gv':
+            return G
 
-            elif output_fmt == 'source':
-                return G.source
+        elif output_fmt == 'source':
+            return G.source
 
-            elif not output_fmt:
-                return None
+        elif not output_fmt:
+            return None
 
-            else:
-                raise ShowGraphError(f"Bad arg in call to {composition.name}.show_graph: '{output_fmt}'.")
-
-        except ShowGraphError as e:
-            # raise ShowGraphError(str(e.error_value))
-            raise ShowGraphError(str(e.error_value)) from e
-        # except:
-        #     raise ShowGraphError(f"Problem displaying graph for {composition.name}")
-        except Exception as e:
-            raise ShowGraphError(f"Problem displaying graph for {composition.name}: {e}") from e
+        else:
+            raise ShowGraphError(f"Bad arg in call to {composition.name}.show_graph: '{output_fmt}'.")
 
     def _is_composition_controller(self, mech, context, enclosing_comp=None):
         # FIX 6/12/20: REPLACE WITH TEST FOR NodeRole.CONTROLLER ONCE THAT IS IMPLEMENTED
@@ -2918,6 +2923,8 @@ def _animate_execution(self, active_items, context):
                                )
 
     def _generate_gifs(self, G, active_items, context):
+        # graphviz is currently only imported within methods
+        from graphviz.backend.execute import ExecutableNotFound
 
         composition = self.composition
 
@@ -2969,11 +2976,15 @@ def create_time_string(time, spec):
         index = repr(composition._component_animation_execution_count)
         image_filename = '-'.join([repr(run_num), repr(trial_num), index])
         image_file = pathlib.Path(composition._animation_directory, image_filename + '.gif')
-        G.render(filename=image_filename,
-                 directory=composition._animation_directory,
-                 cleanup=True,
-                 # view=True
-                 )
+        try:
+            G.render(
+                filename=image_filename,
+                directory=composition._animation_directory,
+                cleanup=True,
+                # view=True
+            )
+        except ExecutableNotFound as e:
+            raise ShowGraphError(_gv_executable_not_found_error_msg) from e
         # Append gif to composition._animation
         image = Image.open(image_file)
         # TBI?

tests/composition/test_show_graph.py

@@ -1,3 +1,8 @@
+import os
+import pathlib
+import sys
+import uuid
+
 import numpy as np
 import pytest
 
@@ -15,14 +20,24 @@
 from psyneulink.core.components.ports.modulatorysignals.controlsignal import ControlSignal
 from psyneulink.core.components.projections.pathway.mappingprojection import MappingProjection
 from psyneulink.core.compositions.composition import Composition, NodeRole
+from psyneulink.core.compositions.showgraph import (
+    ShowGraphError,
+    _gv_executable_not_found_error_msg,
+)
 from psyneulink.core.globals.keywords import ALL, INSET, INTERCEPT, NESTED, NOISE, SLOPE
 from psyneulink.library.components.mechanisms.modulatory.control.agt.lccontrolmechanism import LCControlMechanism
 from psyneulink.library.components.mechanisms.processing.integrator.ddm import DDM
 from psyneulink.library.components.mechanisms.processing.integrator.episodicmemorymechanism import \
     EpisodicMemoryMechanism, VALUE_INPUT, VALUE_OUTPUT, KEY_INPUT, KEY_OUTPUT
 
+graphviz_executables = {
+    'dot', 'neato', 'twopi', 'circo', 'fdp', 'sfdp', 'patchwork', 'osage'
+}
+
+
 """These test various elaborate forms of Composition configuration and nesting, in addition to show_graph itself"""
 
+
 class TestSimpleCompositions:
     def test_process(self):
         a = TransferMechanism(name="a", default_variable=[0, 0, 0])
@@ -819,3 +834,59 @@ def test_projections_from_nested_comp_to_ocm_or_obj_mech(self, show_graph_kwargs
     #                                 assert gv == repr(expected_gv_correct)
     #                             except AssertionError:
     #                                 assert gv == repr(expected_gv_incorrect)
+
+
+def _mock_gv_missing_executable(monkeypatch_context):
+    """
+    replace directory of graphviz executable with a uuid that shouldn't
+    exist, so that the executable won't be found by subprocess system
+    calls
+    """
+    if sys.platform == 'win32':
+        import _winapi
+        orig_CreateProcess = _winapi.CreateProcess
+
+        def mock_CreateProcess_gv_fail(executable, args, *a, **kwargs):
+            if any(args.startswith(f'{ex} ') for ex in graphviz_executables):
+                args = f'{uuid.uuid4()}\\{args}'
+            return orig_CreateProcess(executable, args, *a, **kwargs)
+
+        monkeypatch_context.setattr(_winapi, 'CreateProcess', mock_CreateProcess_gv_fail)
+    else:
+        orig_dirname = os.path.dirname
+
+        def mock_dirname_gv_fail(p):
+            q = p
+            try:
+                q = q.decode('utf-8')
+            except AttributeError:
+                # may be str or bytes
+                pass
+
+            if pathlib.Path(q).name in graphviz_executables:
+                return uuid.uuid4()
+
+            return orig_dirname(p)
+
+        monkeypatch_context.setattr(os.path, 'dirname', mock_dirname_gv_fail)
+
+
+def _test_graphviz_not_found(monkeypatch, comp_func, **kwargs):
+    a = TransferMechanism()
+    b = TransferMechanism()
+    comp = Composition([a, b])
+
+    with monkeypatch.context() as m:
+        _mock_gv_missing_executable(m)
+        with pytest.raises(ShowGraphError, match=_gv_executable_not_found_error_msg):
+            getattr(comp, comp_func)(**kwargs)
+
+
+# other output_fmt do not fail on static show_graph
+@pytest.mark.parametrize('output_fmt', ['pdf'])
+def test_graphviz_not_found_static(monkeypatch, output_fmt):
+    _test_graphviz_not_found(monkeypatch, 'show_graph', output_fmt=output_fmt)
+
+
+def test_graphviz_not_found_animated(monkeypatch):
+    _test_graphviz_not_found(monkeypatch, 'run', animate=True)