[blackboard] client exists() method implemented (#238)

CHANGELOG.rst

@@ -4,6 +4,7 @@ Release Notes
 Forthcoming
 -----------
 * [blackboard] global ``Blackboard.set`` staticmethod
+* [blackboard] client ``exists()`` method, `#238 <https://github.com/splintered-reality/py_trees/pull/238>`_
 
 1.3.x (2019-10-03)
 ------------------

py_trees/blackboard.py

@@ -712,6 +712,22 @@ def set(self, name: str, value: typing.Any, overwrite: bool=True) -> bool:
             except AttributeError:  # when the object doesn't have the attributes
                 return False
 
+    def exists(self, name: str) -> bool:
+        """
+        Check if the specified variable exists on the blackboard.
+
+        Args:
+            name: name of the variable to get, can be nested, e.g. battery.percentage
+
+        Raises:
+            AttributeError: if the client does not have read access to the variable
+        """
+        try:
+            unused_value = self.get(name)
+            return True
+        except KeyError:
+            return False
+
     def get(self, name: str) -> typing.Any:
         """
         Method based accessor to the blackboard variables (as opposed to simply using

tests/test_blackboard.py

@@ -132,6 +132,26 @@ def test_delayed_register_key():
         assert(bar.other == 1)
 
 
+def test_key_exists():
+    console.banner("Nested Read")
+    with create_blackboards() as (foo, unused_bar):
+        print("foo.exists('dude') [{}][{}]".format(foo.exists("dude"), True))
+        assert(foo.exists("dude"), True)
+        with nose.tools.assert_raises_regexp(AttributeError, "does not have read/write access"):
+            print("Expecting Attribute Error with substring 'does not have read/write access'")
+            print("foo.exists('dude_not_here') [{}][{}]".format(foo.exists("dude_not_here"), False))
+            assert(foo.exists("dude_not_here"), False)
+
+
+def test_nested_exists():
+    console.banner("Nested Read")
+    with create_blackboards() as (foo, unused_bar):
+        print("foo.exists('motley.nested') [{}][{}]".format(foo.exists("foo.nested"), True))
+        assert(foo.exists("motley.nested"), True)
+        print("foo.exists('motley.not_here') [{}][{}]".format(foo.exists("foo.not_here"), False))
+        assert(foo.exists("motley.not_here"), False)
+
+
 def test_nested_read():
     console.banner("Nested Read")
     with create_blackboards() as (foo, unused_bar):