#858 add VariableDot

pybamm/__init__.py

@@ -107,7 +107,7 @@ def version(formatted=False):
     ones_like,
 )
 from .expression_tree.scalar import Scalar
-from .expression_tree.variable import Variable, ExternalVariable
+from .expression_tree.variable import Variable, ExternalVariable, VariableDot
 from .expression_tree.independent_variable import (
     IndependentVariable,
     Time,

pybamm/expression_tree/variable.py

@@ -48,8 +48,38 @@ def _evaluate_for_shape(self):
         )
 
 
+class VariableDot(Variable):
+    """
+    A node in the expression tree represending the time derviative of a dependent
+    variable
+
+    This node will be discretised by :class:`.Discretisation` and converted
+    to a :class:`pybamm.StateVectorDot` node.
+
+    Parameters
+    ----------
+
+    name : str
+        name of the node
+    domain : iterable of str
+        list of domains that this variable is valid over
+    auxiliary_domains : dict
+        dictionary of auxiliary domains ({'secondary': ..., 'tertiary': ...}). For
+        example, for the single particle model, the particle concentration would be a
+        Variable with domain 'negative particle' and secondary auxiliary domain 'current
+        collector'. For the DFN, the particle concentration would be a Variable with
+        domain 'negative particle', secondary domain 'negative electrode' and tertiary
+        domain 'current collector'
+
+    *Extends:* :class:`Symbol`
+    """
+
+    def __init__(self, name, domain=None, auxiliary_domains=None):
+        super().__init__(name, domain=domain, auxiliary_domains=auxiliary_domains)
+
+
 class ExternalVariable(Variable):
-    """A node in the expression tree represending an external variable variable
+    """A node in the expression tree representing an external variable variable
 
     This node will be discretised by :class:`.Discretisation` and converted
     to a :class:`.Vector` node.

tests/unit/test_expression_tree/test_variable.py

@@ -25,6 +25,24 @@ def test_variable_id(self):
         self.assertNotEqual(a1.id, a3.id)
         self.assertNotEqual(a1.id, a4.id)
 
+class TestVariableDot(unittest.TestCase):
+    def test_variable_init(self):
+        a = pybamm.VariableDot("a'")
+        self.assertEqual(a.name, "a'")
+        self.assertEqual(a.domain, [])
+        a = pybamm.VariableDot("a", domain=["test"])
+        self.assertEqual(a.domain[0], "test")
+        self.assertRaises(TypeError, pybamm.Variable("a", domain="test"))
+
+    def test_variable_id(self):
+        a1 = pybamm.VariableDot("a", domain=["negative electrode"])
+        a2 = pybamm.VariableDot("a", domain=["negative electrode"])
+        self.assertEqual(a1.id, a2.id)
+        a3 = pybamm.VariableDot("b", domain=["negative electrode"])
+        a4 = pybamm.VariableDot("a", domain=["positive electrode"])
+        self.assertNotEqual(a1.id, a3.id)
+        self.assertNotEqual(a1.id, a4.id)
+
 
 class TestExternalVariable(unittest.TestCase):
     def test_external_variable_scalar(self):