Easier disabling of Table.LOCKING in add-on tests

This commit allows the add-ons to set Table.LOCKING to False
independent of the order of imports.

Orange/canvas/tests/__init__.py

@@ -1,3 +1,4 @@
 from Orange.data import Table
 
-Table.LOCKING = True
+if Table.LOCKING is None:
+    Table.LOCKING = True

Orange/data/table.py

@@ -359,7 +359,14 @@ def __init__(self, source, destination):
 
 # noinspection PyPep8Naming
 class Table(Sequence, Storage):
-    LOCKING = False
+
+    LOCKING = None
+    """ If the class attribute LOCKING is True, tables will throw exceptions
+    on in-place modifications unless unlocked explicitly. LOCKING is supposed
+    to be set to True for testing to help us find bugs. If set to False
+    or None, no safeguards are in place. Two different values are used for
+    the same behaviour to distinguish the unchanged default (None) form
+    explicit deactivation (False) that some add-ons might need. """
 
     __file__ = None
     name = "untitled"

Orange/tests/__init__.py

@@ -12,7 +12,8 @@
 import numpy as np
 import Orange
 
-Orange.data.Table.LOCKING = True
+if Orange.data.Table.LOCKING is None:
+    Orange.data.Table.LOCKING = True
 
 
 @contextmanager

Orange/widgets/tests/__init__.py

@@ -4,8 +4,8 @@
 import Orange
 import Orange.widgets
 
-
-Orange.data.Table.LOCKING = True
+if Orange.data.Table.LOCKING is None:
+    Orange.data.Table.LOCKING = True
 
 
 def load_tests(loader, tests, pattern):