Mechanism: fix no-input execute causing shared default variable memory (

#3148)

Calling Mechanism.execute with no input uses the Mechanism's
defaults.variable as input. This was passed in without creating a copy,
which resulted in defaults.variable sharing memory with an
InputPort.variable, which could result in silent changes to the
Mechanism's defaults.variable.

psyneulink/core/components/mechanisms/mechanism.py

@@ -2539,7 +2539,7 @@ def execute(self,
 
                     # No input was specified, so use Mechanism's default variable
                     if input is None:
-                        input = self.defaults.variable
+                        input = copy_parameter_value(self.defaults.variable)
                     #     FIX:  this input value is sent to input CIMs when compositions are nested
                     #           variable should be based on afferent projections
                     variable = self._get_variable_from_input(input, context)

tests/mechanisms/test_mechanisms.py

@@ -130,6 +130,14 @@ def test_noise_change_warning_to_function(self, noise):
         ):
             t.parameters.noise.set(pnl.NormalDist)
 
+    def test_execute_no_input_doesnt_change_default_variable(self):
+        m = pnl.ProcessingMechanism(default_variable=1)
+        assert m.defaults.variable == [[1]]
+        m.execute()
+        assert m.defaults.variable == [[1]]
+        m.execute(2)
+        assert m.defaults.variable == [[1]]
+
 
 class TestMechanismFunctionParameters:
     f = pnl.Linear()