Extend ComparisonExpression so we can get the value from a callable (#468)

Author: corot

py_trees/behaviours.py

@@ -570,10 +570,10 @@ def update(self) -> common.Status:
         """
         self.logger.debug("%s.update()" % self.__class__.__name__)
         try:
-            value = self.blackboard.get(self.key)
+            lhs_value = self.blackboard.get(self.key)
             if self.key_attributes:
                 try:
-                    value = operator.attrgetter(self.key_attributes)(value)
+                    lhs_value = operator.attrgetter(self.key_attributes)(lhs_value)
                 except AttributeError:
                     self.feedback_message = (
                         "blackboard key-value pair exists, but the value does not "
@@ -588,20 +588,21 @@ def update(self) -> common.Status:
             )
             return common.Status.FAILURE
 
-        success = self.check.operator(value, self.check.value)
+        rhs_value = self.check.value_generator()
+        success = self.check.operator(lhs_value, rhs_value)
 
         if success:
             self.feedback_message = "'%s' comparison succeeded [v: %s][e: %s]" % (
                 self.check.variable,
-                value,
-                self.check.value,
+                lhs_value,
+                rhs_value,
             )
             return common.Status.SUCCESS
         else:
             self.feedback_message = "'%s' comparison failed [v: %s][e: %s]" % (
                 self.check.variable,
-                value,
-                self.check.value,
+                lhs_value,
+                rhs_value,
             )
             return common.Status.FAILURE
 
@@ -722,7 +723,7 @@ def update(self) -> common.Status:
                     )
                 )
                 return common.Status.FAILURE
-            results.append(check.operator(value, check.value))
+            results.append(check.operator(value, check.value_generator()))
         if self.blackboard_results is not None:
             for counter in range(1, len(results) + 1):
                 self.blackboard_results.set(str(counter), results[counter - 1])

py_trees/common.py

@@ -195,7 +195,7 @@ class ComparisonExpression(object):
 
     Args:
         variable: name of the variable to compare
-        value: value to compare against
+        value: value or callable to compare against
         operator: a callable comparison operator
 
     .. tip::
@@ -206,7 +206,7 @@ class ComparisonExpression(object):
 
         check = ComparisonExpression(
             variable="foo",
-            value= 5,
+            value=5,
             operator=operator.eq
         )
         success = check.operator(blackboard[check.variable], check.value)
@@ -215,18 +215,18 @@ class ComparisonExpression(object):
     def __init__(
         self,
         variable: str,
-        value: ComparisonV,
+        value: typing.Union[ComparisonV, typing.Callable[[], ComparisonV]],
         operator: typing.Callable[[ComparisonV, ComparisonV], bool],
     ):
         """Initialise variables.
 
         Args:
             variable: the variable name to use
-            value: the value to use on the RHS of the expression
+            value: the value to use on the RHS of the expression (can also be a callable that returns the value)
             operator: a logical operator to enable comparisons
         """
         self.variable = variable
-        self.value = value
+        self.value_generator = value if callable(value) else lambda: value
         self.operator = operator