Added property to display the expressions of the PDE class

TODO.md

@@ -1,5 +1,6 @@
 TODO
 ====
+* Add the JOSS citation so people can copy it conveniently
 * Count the number of compilations and store it in the info field of the simulation
     - raise a warning when this number became too large in a simulation?
 * Think about interface for changing boundary values in numba

pde/pdes/pde.py

@@ -118,6 +118,12 @@ def __init__(self,
                         'operators_detected': self.operators}
         self._cache: Dict[str, Any] = {}
 
+
+    @property
+    def expressions(self) -> Dict[str, str]:
+        """ show the expressions of the PDE """
+        return {k: v.expression for k, v in self._rhs_expr.items()}
+
 
     def _prepare(self, state: FieldBase) -> None:
         """ prepare the expression by setting internal variables in the cache

pde/pdes/tests/test_pde.py

@@ -19,13 +19,15 @@ def test_pde_wrong_input():
 
     grid = UnitGrid([4])
     eq = PDE({'u': 1})
+    assert eq.expressions == {'u': '1.0'}
     with pytest.raises(ValueError):
         eq.evolution_rate(FieldCollection.scalar_random_uniform(2, grid))
 
     eq = PDE({'u': 1, 'v': 2})
+    assert eq.expressions == {'u': '1.0', 'v': '2.0'}
     with pytest.raises(ValueError):
         eq.evolution_rate(ScalarField.random_uniform(grid))
-
+        
 
 
 def test_pde_scalar():