TensorFlow tf.group 忽略依赖关系？

继从先前的问题 https://stackoverflow.com/questions/44244275/tensorflow-fifoqueue-not-fifo， 它似乎tf.group确实忽略了依赖关系。这是一个简单的独立示例（我已在 Python 2.7 和 TensorFlow 1.1 上运行它）：

import tensorflow as tf
from tensorflow.python.ops import control_flow_ops

xs = [tf.constant(x) for x in range(10)]
xs = [tf.Print(x, [x]) for x in xs]
dependency = None
dxs = []

for x in xs:
    if dependency is None:
        dependency = x
    else:
        dependency = control_flow_ops.with_dependencies([dependency], x)

    dxs.append(dependency)

print_all_op = tf.group(*dxs)

with tf.Session() as session:
    session.run(print_all_op)

预期输出：

2017-05-29 15:11:53.961221: I tensorflow/core/kernels/logging_ops.cc:79] [0]
2017-05-29 15:11:53.961236: I tensorflow/core/kernels/logging_ops.cc:79] [1]
2017-05-29 15:11:53.961255: I tensorflow/core/kernels/logging_ops.cc:79] [2]
2017-05-29 15:11:53.961237: I tensorflow/core/kernels/logging_ops.cc:79] [3]
2017-05-29 15:11:53.961262: I tensorflow/core/kernels/logging_ops.cc:79] [4]
2017-05-29 15:11:53.961263: I tensorflow/core/kernels/logging_ops.cc:79] [5]
2017-05-29 15:11:53.961268: I tensorflow/core/kernels/logging_ops.cc:79] [6]
2017-05-29 15:11:53.961272: I tensorflow/core/kernels/logging_ops.cc:79] [7]
2017-05-29 15:11:53.961274: I tensorflow/core/kernels/logging_ops.cc:79] [8]
2017-05-29 15:11:53.961221: I tensorflow/core/kernels/logging_ops.cc:79] [9]

实际输出（每次运行代码时都会不同）：

2017-05-29 15:16:26.279655: I tensorflow/core/kernels/logging_ops.cc:79] [0]
2017-05-29 15:16:26.279655: I tensorflow/core/kernels/logging_ops.cc:79] [9]
2017-05-29 15:16:26.279697: I tensorflow/core/kernels/logging_ops.cc:79] [3]
2017-05-29 15:16:26.279660: I tensorflow/core/kernels/logging_ops.cc:79] [1]
2017-05-29 15:16:26.279711: I tensorflow/core/kernels/logging_ops.cc:79] [8]
2017-05-29 15:16:26.279713: I tensorflow/core/kernels/logging_ops.cc:79] [4]
2017-05-29 15:16:26.279723: I tensorflow/core/kernels/logging_ops.cc:79] [5]
2017-05-29 15:16:26.279663: I tensorflow/core/kernels/logging_ops.cc:79] [2]
2017-05-29 15:16:26.279724: I tensorflow/core/kernels/logging_ops.cc:79] [7]
2017-05-29 15:16:26.279728: I tensorflow/core/kernels/logging_ops.cc:79] [6]

里面什么都没有tf.group https://www.tensorflow.org/api_docs/python/tf/group说明为什么依赖项被忽略的文档。

有没有替代方案tf.group这确实考虑了依赖关系吗？

切换使用tf.control_dependencies代替tensorflow.python.ops.control_flow_ops.with_dependencies没有帮助：

import tensorflow as tf

xs = [tf.constant(x) for x in range(10)]
xs = [tf.Print(x, [x]) for x in xs]
dependency = None
dxs = []

for x in xs:
    if dependency is None:
        dependency = x
    else:
        with tf.control_dependencies([dependency]):
            dependency = x

    dxs.append(dependency)

print_all_op = tf.group(*dxs)

with tf.Session() as session:
    session.run(print_all_op)

Using tf.control_dependencies正确地解决了这个问题：

import tensorflow as tf

xs = [tf.constant(x) for x in range(10)]
dependency = None
dxs = []

for x in xs:
    if dependency is None:
        dependency = tf.Print(x, [x])
    else:
        with tf.control_dependencies([dependency]):
            dependency = tf.Print(x, [x])

    dxs.append(dependency)

print_all_op = tf.group(*dxs)

with tf.Session() as session:
    session.run(print_all_op)

请注意，Print操作需要在tf.control_dependencies上下文管理器。

我仍然不清楚为什么control_flow_ops.with_dependencies版本失败。