def _gather_variables(stack_def):
    """Merges context provided & stack defined variables.

    If multiple stacks have a variable with the same name, we can specify the
    value for a specific stack by passing in the variable name as: `<stack
    name>::<variable name>`. This variable value will only be used for that
    specific stack.

    Order of precedence:
        - context defined stack specific variables (ie.
            SomeStack::SomeVariable)
        - context defined non-specific variables
        - variable defined within the stack definition

    Args:
        stack_def (dict): The stack definition being worked on.

    Returns:
        dict: Contains key/value pairs of the collected variables.

    Raises:
        AttributeError: Raised when the stack definitition contains an invalid
            attribute. Currently only when using old parameters, rather than
            variables.
    """
    variable_values = copy.deepcopy(stack_def.variables or {})
    return [Variable(k, v) for k, v in variable_values.items()]