ENH: NodeSaver: suppress unhelpful vega CLI errors

altair_saver/savers/_node.py

@@ -1,7 +1,7 @@
 import functools
 import json
 import shutil
-from typing import Dict, List
+from typing import Callable, Dict, List, Optional
 import warnings
 
 from altair_saver.types import JSONDict, MimebundleContent
@@ -34,33 +34,49 @@ def exec_path(name: str) -> str:
     raise ExecutableNotFound(name)
 
 
-def vl2vg(spec: JSONDict) -> JSONDict:
+def vl2vg(
+    spec: JSONDict, stderr_filter: Optional[Callable[[str], bool]] = None
+) -> JSONDict:
     """Compile a Vega-Lite spec into a Vega spec."""
     vl2vg = exec_path("vl2vg")
     vl_json = json.dumps(spec).encode()
-    vg_json = check_output_with_stderr([vl2vg], input=vl_json)
+    vg_json = check_output_with_stderr(
+        [vl2vg], input=vl_json, stderr_filter=stderr_filter
+    )
     return json.loads(vg_json)
 
 
-def vg2png(spec: JSONDict) -> bytes:
+def vg2png(
+    spec: JSONDict, stderr_filter: Optional[Callable[[str], bool]] = None
+) -> bytes:
     """Generate a PNG image from a Vega spec."""
     vg2png = exec_path("vg2png")
     vg_json = json.dumps(spec).encode()
-    return check_output_with_stderr([vg2png], input=vg_json)
+    return check_output_with_stderr(
+        [vg2png], input=vg_json, stderr_filter=stderr_filter
+    )
 
 
-def vg2pdf(spec: JSONDict) -> bytes:
+def vg2pdf(
+    spec: JSONDict, stderr_filter: Optional[Callable[[str], bool]] = None
+) -> bytes:
     """Generate a PDF image from a Vega spec."""
     vg2pdf = exec_path("vg2pdf")
     vg_json = json.dumps(spec).encode()
-    return check_output_with_stderr([vg2pdf], input=vg_json)
+    return check_output_with_stderr(
+        [vg2pdf], input=vg_json, stderr_filter=stderr_filter
+    )
 
 
-def vg2svg(spec: JSONDict) -> str:
+def vg2svg(
+    spec: JSONDict, stderr_filter: Optional[Callable[[str], bool]] = None
+) -> str:
     """Generate an SVG image from a Vega spec."""
     vg2svg = exec_path("vg2svg")
     vg_json = json.dumps(spec).encode()
-    return check_output_with_stderr([vg2svg], input=vg_json).decode()
+    return check_output_with_stderr(
+        [vg2svg], input=vg_json, stderr_filter=stderr_filter
+    ).decode()
 
 
 class NodeSaver(Saver):
@@ -70,6 +86,12 @@ class NodeSaver(Saver):
         "vega-lite": ["pdf", "png", "svg", "vega"],
     }
 
+    _stderr_ignore = ["WARN Can not resolve event source: window"]
+
+    @classmethod
+    def _stderr_filter(cls, line: str) -> bool:
+        return line not in cls._stderr_ignore
+
     @classmethod
     def enabled(cls) -> bool:
         try:
@@ -87,15 +109,15 @@ def _serialize(self, fmt: str, content_type: str) -> MimebundleContent:
         spec = self._spec
 
         if self._mode == "vega-lite":
-            spec = vl2vg(spec)
+            spec = vl2vg(spec, self._stderr_filter)
 
         if fmt == "vega":
             return spec
         elif fmt == "png":
-            return vg2png(spec)
+            return vg2png(spec, self._stderr_filter)
         elif fmt == "svg":
-            return vg2svg(spec)
+            return vg2svg(spec, self._stderr_filter)
         elif fmt == "pdf":
-            return vg2pdf(spec)
+            return vg2pdf(spec, self._stderr_filter)
         else:
             raise ValueError(f"Unrecognized format: {fmt!r}")

altair_saver/savers/tests/test_node.py

@@ -6,10 +6,13 @@
 from PIL import Image
 from PyPDF2 import PdfFileReader
 import pytest
+from _pytest.capture import SysCapture
+from _pytest.monkeypatch import MonkeyPatch
 
 from altair_saver import NodeSaver
 from altair_saver._utils import fmt_to_mimetype
 from altair_saver.savers import _node
+from altair_saver.types import JSONDict
 
 
 def get_testcases() -> Iterator[Tuple[str, Dict[str, Any]]]:
@@ -30,6 +33,19 @@ def get_testcases() -> Iterator[Tuple[str, Dict[str, Any]]]:
         yield case, {"vega-lite": vl, "vega": vg, "svg": svg, "png": png, "pdf": pdf}
 
 
+@pytest.fixture
+def interactive_spec() -> JSONDict:
+    return {
+        "data": {"values": [{"x": 1, "y": 1}]},
+        "mark": "point",
+        "encoding": {
+            "x": {"field": "x", "type": "quantitative"},
+            "y": {"field": "y", "type": "quantitative"},
+        },
+        "selection": {"zoon": {"type": "interval", "bind": "scales"}},
+    }
+
+
 def get_modes_and_formats() -> Iterator[Tuple[str, str]]:
     for mode in ["vega", "vega-lite"]:
         for fmt in NodeSaver.valid_formats[mode]:
@@ -76,7 +92,7 @@ def test_node_mimebundle_fail(name: str, data: Any) -> None:
 
 
 @pytest.mark.parametrize("enabled", [True, False])
-def test_enabled(monkeypatch: Any, enabled: bool) -> None:
+def test_enabled(monkeypatch: MonkeyPatch, enabled: bool) -> None:
     def exec_path(name: str) -> str:
         if enabled:
             return name
@@ -85,3 +101,25 @@ def exec_path(name: str) -> str:
 
     monkeypatch.setattr(_node, "exec_path", exec_path)
     assert NodeSaver.enabled() is enabled
+
+
+@pytest.mark.parametrize("suppress_warnings", [True, False])
+def test_stderr_suppression(
+    interactive_spec: JSONDict,
+    suppress_warnings: bool,
+    monkeypatch: MonkeyPatch,
+    capsys: SysCapture,
+) -> None:
+    message = NodeSaver._stderr_ignore[0]
+
+    # Window resolve warnings are emitted by the vega CLI when an interactive chart
+    # is saved, and are suppressed by default.
+    if not suppress_warnings:
+        monkeypatch.setattr(NodeSaver, "_stderr_ignore", [])
+
+    NodeSaver(interactive_spec).save(fmt="png")
+    captured = capsys.readouterr()
+    if suppress_warnings:
+        assert message not in captured.err
+    else:
+        assert message in captured.err