Merge pull request #1803 from Manoj-Kumar-S/1996_issue

Incorrect Exception in dsolve() : First implementation

sympy/solvers/ode.py

@@ -529,7 +529,12 @@ def dsolve(eq, func=None, hint="default", simplify=True, prep=True, **kwargs):
 
     if not hints['default']:
         # classify_ode will set hints['default'] to None if no hints match.
-        raise NotImplementedError("dsolve: Cannot solve " + str(eq))
+        if hint not in allhints and hint != 'default':
+            raise ValueError("Hint not recognized: " + hint)
+        elif hint not in hints['ordered_hints'] and hint != 'default':
+            raise ValueError("ODE " + str(eq) + " does not match hint " + hint)
+        else:
+            raise NotImplementedError("dsolve: Cannot solve " + str(eq))
 
     if hint == 'default':
         return dsolve(eq, func, hint=hints['default'], simplify=simplify,

sympy/solvers/tests/test_ode.py

@@ -1427,3 +1427,8 @@ def test_nth_order_linear_euler_eq_homogeneous():
     assert our_hint in classify_ode(eq)
     assert dsolve(eq, y(t), hint=our_hint).rhs in (sol, sols)
     assert checkodesol(eq, sol, order=2, solve_for_func=False)[0]
+
+def test_issue_1996():
+    f = Function('f')
+    raises(ValueError, lambda: dsolve(f(x).diff(x)**2, f(x), 'seperable'))
+    raises(ValueError, lambda: dsolve(f(x).diff(x)**2, f(x), 'fdsjf'))