DOC: Warn when reloading numpy or using numpy in sub-interpreter

This adds a warning when the main NumPy module is reloaded
with the assumption that in this case objects such as `np.matrix`,
`np._NoValue` or exceptions may be cached internally.

It also gives a warning when NumPy is imported in a sub-interpreter.

1 files changed, 9 insertions, 5 deletions

diff --git a/numpy/tests/test_reloading.py b/numpy/tests/test_reloading.py
index 61ae91b00..5c4309f4a 100644
--- a/numpy/tests/test_reloading.py
+++ b/numpy/tests/test_reloading.py
@@ -1,4 +1,4 @@
-from numpy.testing import assert_raises, assert_, assert_equal
+from numpy.testing import assert_raises, assert_warns, assert_, assert_equal
 from numpy.compat import pickle
 
 import sys
@@ -16,13 +16,15 @@ def test_numpy_reloading():
     VisibleDeprecationWarning = np.VisibleDeprecationWarning
     ModuleDeprecationWarning = np.ModuleDeprecationWarning
 
-    reload(np)
+    with assert_warns(UserWarning):
+        reload(np)
     assert_(_NoValue is np._NoValue)
     assert_(ModuleDeprecationWarning is np.ModuleDeprecationWarning)
     assert_(VisibleDeprecationWarning is np.VisibleDeprecationWarning)
 
     assert_raises(RuntimeError, reload, numpy._globals)
-    reload(np)
+    with assert_warns(UserWarning):
+        reload(np)
     assert_(_NoValue is np._NoValue)
     assert_(ModuleDeprecationWarning is np.ModuleDeprecationWarning)
     assert_(VisibleDeprecationWarning is np.VisibleDeprecationWarning)
@@ -45,13 +47,15 @@ def test_full_reimport():
     # This is generally unsafe, especially, since we also reload the C-modules.
     code = textwrap.dedent(r"""
         import sys
+        from pytest import warns
         import numpy as np
 
         for k in list(sys.modules.keys()):
             if "numpy" in k:
                 del sys.modules[k]
 
-        import numpy as np
+        with warns(UserWarning):
+            import numpy as np
         """)
     p = subprocess.run([sys.executable, '-c', code])
-    assert p.returncode == 0
+