graph: don't open a blank image for an empty graph

* Write a message to stderr saying there are no tasks to display rather
  than just opening a blank image.
* Closes #6841

Author: oliver-sanders

.github/workflows/test_fast.yml

@@ -54,6 +54,7 @@ jobs:
             bash
             subversion
             sqlite
+            graphviz
 
       - name: Install Python Dependencies
         run: |

cylc/flow/scripts/graph.py

@@ -44,6 +44,8 @@
 from tempfile import NamedTemporaryFile
 from typing import Dict, List, Optional, TYPE_CHECKING, Tuple, Callable
 
+from ansimarkup import ansiprint as cprint
+
 from cylc.flow.config import WorkflowConfig
 from cylc.flow.exceptions import InputError, CylcError
 from cylc.flow.id import Tokens
@@ -323,7 +325,6 @@ def render_dot(dot_lines, filename, fmt):
 
 def open_image(filename):
     """Open an image file."""
-    print(f'Graph rendered to {filename}')
     try:
         from PIL import Image
     except ModuleNotFoundError:
@@ -352,6 +353,13 @@ def graph_render(opts, workflow_id, start, stop, flow_file) -> int:
         flow_file
     )
 
+    if not nodes:
+        msg = 'No tasks to display.'
+        if start or stop:
+            msg += ' Try changing the start and stop values.'
+        cprint(f'<red>{msg}</red>', file=sys.stderr)
+        return 1
+
     # format the graph in graphviz-dot format
     dot_lines = format_graphviz(opts, nodes, edges)
 
@@ -361,7 +369,11 @@ def graph_render(opts, workflow_id, start, stop, flow_file) -> int:
         try:
             fmt = filename.rsplit('.', 1)[1]
         except IndexError:
-            sys.exit('Output filename requires a format.')
+            cprint(
+                '<red>Output filename requires a format.</red>',
+                file=sys.stderr,
+            )
+            return 1
     else:
         filename = NamedTemporaryFile().name
         fmt = 'png'
@@ -376,9 +388,8 @@ def graph_render(opts, workflow_id, start, stop, flow_file) -> int:
     render_dot(dot_lines, filename, fmt)
 
     # notify the user / open the graph
-    if opts.output:
-        print(f'Graph rendered to {opts.output}')
-    else:
+    cprint(f'<green>Graph rendered to {opts.output or filename}</green>')
+    if not opts.output:
         open_image(filename)
     return 0
 
@@ -537,12 +548,11 @@ def main(
     start: Optional[str] = None,
     stop: Optional[str] = None
 ) -> None:
-    result = asyncio.run(_main(parser, opts, workflow_id, start, stop))
+    result = asyncio.run(_main(opts, workflow_id, start, stop))
     sys.exit(result)
 
 
 async def _main(
-    parser: COP,
     opts: 'Values',
     workflow_id: str,
     start: Optional[str] = None,

tests/integration/scripts/test_graph.py

@@ -0,0 +1,46 @@
+# THIS FILE IS PART OF THE CYLC WORKFLOW ENGINE.
+# Copyright (C) NIWA & British Crown (Met Office) & Contributors.
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+import pytest
+
+from cylc.flow.option_parsers import Options
+from cylc.flow.scripts.graph import _main, get_option_parser
+
+
+Opts = Options(get_option_parser())
+
+
+@pytest.fixture
+def disable_graph_open(monkeypatch):
+    """Prevent "cylc graph" from trying to pop open the image."""
+    monkeypatch.setattr(
+        'cylc.flow.scripts.graph.open_image',
+        lambda *_a, **_k: None,
+    )
+
+
+async def test_blank_graph(one, disable_graph_open, capsys):
+    """It should inform the user if there are no tasks to display."""
+    # graph with one task
+    await _main(Opts(color='never'), one.tokens.id)
+    out, err = capsys.readouterr()
+    assert 'Graph rendered to' in out
+
+    # graph with no tasks (the only task is in cycle "1")
+    await _main(Opts(color='never'), one.tokens.id, '5')
+    out, err = capsys.readouterr()
+    assert 'No tasks to display' in err
+    assert 'Try changing the start and stop values' in err

tests/integration/test_get_old_tvars.py

@@ -85,7 +85,7 @@ async def test_validate_with_old_tvars(
         opts.reference = True
 
     async with mod_start(_setup):
-        if function in {view, cylclist}:
+        if function in {view, cylclist, graph}:
             await function(opts, _setup.workflow_name)
         else:
             await function(parser, opts, _setup.workflow_name)