tensorflow中tf.Variable(tf.constant)变量吗？为什么可以这么写？

tf.Variable()是变量
tf.constant()是常量
但是书上用tf.Variable(tf.constant(0.1, shape=[OUTPUT_NODE]))来表示偏置项,为什么这么表示
为什么不直接用tf.Variable(0.1, shape=[OUTPUT_NODE])来表示

看源码：

  def __init__(self,
               initial_value=None,
               trainable=None,
               validate_shape=True,
               caching_device=None,
               name=None,
               variable_def=None,
               dtype=None,
               import_scope=None,
               constraint=None,
               synchronization=VariableSynchronization.AUTO,
               aggregation=VariableAggregation.NONE,
               shape=None):
    """Creates a new variable with value `initial_value`.

    Args:
      initial_value: A `Tensor`, or Python object convertible to a `Tensor`,
        which is the initial value for the Variable. The initial value must have
        a shape specified unless `validate_shape` is set to False. Can also be a
        callable with no argument that returns the initial value when called. In
        that case, `dtype` must be specified. (Note that initializer functions
        from init_ops.py must first be bound to a shape before being used here.)
    """

Variable的构造函数第一个参数说了是一个Tensor对象，不能是值

但是在设置global_step的时候是这么写的
global_step = tf.Variable(0, trainable=False)，第一个参数直接用的0
代码在这个链接 https://www.jianshu.com/p/9f35d452e81c https://www.jianshu.com/p/9f35d452e81c