BUG: Make sure numpy globals keep identity after reload.

Reloading currently causes problems because global classes defined in
numpy/__init__.py change their identity (a is b) after reload. The
solution taken here is to move those classes to a new non-reloadable
module numpy/_globals and import them into numpy from there.

Classes moved are ModuleDeprecationWarning, VisibleDeprecationWarning,
and _NoValue.

Closes #7844.

1 files changed, 62 insertions, 0 deletions

diff --git a/numpy/_globals.py b/numpy/_globals.py
new file mode 100644
index 000000000..64a84da96
--- /dev/null
+++ b/numpy/_globals.py
@@ -0,0 +1,62 @@
+"""
+Module defining global singleton classes.
+
+This module raises a RuntimeError if an attempt to reload it is made. In that
+way the identities of the classes defined here are fixed and will remain so
+even if numpy itself is reloaded. In particular, a function like the following
+will still work correctly after numpy is reloaded::
+
+    def foo(arg=np._NoValue):
+        if arg is np._NoValue:
+            ...
+
+That was not the case when the singleton classes were defined in the numpy
+``__init__.py`` file. See gh-7844 for a discussion of the reload problem that
+motivated this module.
+
+"""
+from __future__ import division, absolute_import, print_function
+
+
+__ALL__ = [
+    'ModuleDeprecationWarning', 'VisibleDeprecationWarning', '_NoValue'
+    ]
+
+
+# Disallow reloading this module so as to preserve the identities of the
+# classes defined here.
+if '_is_loaded' in globals():
+    raise RuntimeError('Reloading numpy._globals is not allowed')
+_is_loaded = True
+
+
+class ModuleDeprecationWarning(DeprecationWarning):
+    """Module deprecation warning.
+
+    The nose tester turns ordinary Deprecation warnings into test failures.
+    That makes it hard to deprecate whole modules, because they get
+    imported by default. So this is a special Deprecation warning that the
+    nose tester will let pass without making tests fail.
+
+    """
+    pass
+
+
+class VisibleDeprecationWarning(UserWarning):
+    """Visible deprecation warning.
+
+    By default, python will not show deprecation warnings, so this class
+    can be used when a very visible warning is helpful, for example because
+    the usage is most likely a user bug.
+
+    """
+    pass
+
+
+class _NoValue:
+    """Special keyword value.
+
+    This class may be used as the default value assigned to a deprecated
+    keyword in order to check if it has been given a user defined value.
+    """
+    pass