def apply_step(self, variables, deltas):
if len(variables) != len(deltas):<EOL><INDENT>raise TensorForceError("<STR_LIT>")<EOL><DEDENT>return tf.group(<EOL>*(tf.assign_add(ref=variable, value=delta) for variable, delta in zip(variables, deltas))<EOL>)<EOL>
Applies the given (and already calculated) step deltas to the variable values.

Args:
    variables: List of variables.
    deltas: List of deltas of same length.

Returns:
    The step-applied operation. A tf.group of tf.assign_add ops.