changing `assert` to `AssertionError`

This `assert` statement does not provide any error message making debugging this case hard. Moreover, `assert` is eliminated when using optimized python code (https://docs.python.org/2.7/reference/simple_stmts.html#the-assert-statement), introducing a potential bug into the system. Explicitly checking and raising an AssertionError will prevent this from happening.

changing `assert` to `AssertionError`
b97a7b0e · jotterbach · ad4e2a09 · b97a7b0e
--- a/theano/gradient.py
+++ b/theano/gradient.py
@@ -431,7 +431,8 @@ def grad(cost, wrt, consider_constant=None,
        from theano import tensor
    if cost is None:
-        assert known_grads is not None
+        if known_grads is None:
+            raise AssertionError("cost and known_grads can't both be None.")
    if cost is not None and isinstance(cost.type, NullType):
        raise ValueError("Can't differentiate a NaN cost."