Backport PR #12771: Do not rely on external stack frame to exist (#12782)

tacaswell · timhoffm · commit 1969c2153e5c · 2018-11-09T20:50:34.000+01:00
Conflicts:
	lib/matplotlib/cbook/__init__.py
           - also backported lgtm noqa flag
diff --git a/lib/matplotlib/cbook/__init__.py b/lib/matplotlib/cbook/__init__.py
@@ -2006,7 +2006,10 @@ def _warn_external(message, category=None):
     etc.).
     """
     frame = sys._getframe()
-    for stacklevel in itertools.count(1):
+    for stacklevel in itertools.count(1):  # lgtm[py/unused-loop-variable]
+        if frame is None:
+            # when called in embedded context may hit frame is None
+            break
         if not re.match(r"\A(matplotlib|mpl_toolkits)(\Z|\.)",
                         frame.f_globals["__name__"]):
             break
diff --git a/lib/matplotlib/tests/test_cbook.py b/lib/matplotlib/tests/test_cbook.py
@@ -2,6 +2,7 @@
 import pickle
 from weakref import ref
 import warnings
+from unittest.mock import patch, Mock
 
 from datetime import datetime
 
@@ -350,6 +351,15 @@ def test_normalize_kwargs_pass(inp, expected, kwargs_to_norm):
         assert len(w) == 0
 
 
+def test_warn_external_frame_embedded_python():
+    with patch.object(cbook, "sys") as mock_sys:
+        mock_sys._getframe = Mock(return_value=None)
+        with warnings.catch_warnings(record=True) as w:
+            cbook._warn_external("dummy")
+    assert len(w) == 1
+    assert str(w[0].message) == "dummy"
+
+
 def test_to_prestep():
     x = np.arange(4)
     y1 = np.arange(4)
