_string.py 文件源码

def _is_None(node):
    #
    # for some reason, 
    #
    #   > ast.dump(ast.parse(x[0:1:]))
    #   Module(body=[Expr(value=Subscript(value=Name(id='x', ctx=Load()), 
    #       slice=Slice(lower=Num(n=0), upper=None, 
    #       step=Name(id='None', ctx=Load())), ctx=Load()))])
    #
    # notice that the step value is not 'None' but the Name that contains 'None'. 
    # Need to special case this.
    #
    return node is None or (isinstance(node, ast.Name) and node.id == 'None')