gradient.py 文件源码

def grad_undefined(op, x_pos, x, comment=""):
    """
    Return an un-computable symbolic variable of type `x.type`.

    If any call to tensor.grad results in an expression containing this
    un-computable variable, an exception (GradUndefinedError) will be
    raised indicating that the gradient on the
    `x_pos`'th input of `op` is mathematically undefined. Likewise if
    any call to theano.function involves this variable.

    Optionally adds a comment to the exception explaining why this
    gradient is not defined.
    """

    return (NullType(
        (
            "This variable is Null because the grad method for "
            "input %s (%s) of the %s op is mathematically undefined. %s"
        ) % (x_pos, x, op, comment)))()