Do not rely on external stack frame to exist

When using matplotlib in an embedded Python interpreter (such as in
Julia package PyCall.jl), stack frame may not exist outside
matplotlib.  Therefore, it is better to not assume `frame.f_back` to
be an existing call frame.

See: JuliaPy/PyPlot.jl#409

Author: tkf

lib/matplotlib/cbook/__init__.py

@@ -1990,6 +1990,8 @@ def _warn_external(message, category=None):
     """
     frame = sys._getframe()
     for stacklevel in itertools.count(1):  # lgtm[py/unused-loop-variable]
+        if frame is None:
+            break
         if not re.match(r"\A(matplotlib|mpl_toolkits)(\Z|\.)",
                         # Work around sphinx-gallery not setting __name__.
                         frame.f_globals.get("__name__", "")):

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)