What are hourglass imports and why can they be avoided in the codebase?

I've seen some commits in the Python codebase removing hourglass imports. I've never seen this term before, and I can't find anything about it via Python documentation or web search.

What is an hourglass import, and when to use or not to use one? My best guess is that removing them makes submodules easier to find , but are there any other reasons?

An example of changing an hourglass import change from one of the linked commits:

diff --git a/tensorflow/contrib/slim/python/slim/nets/vgg.py b/tensorflow/contrib/slim/python/slim/nets/vgg.py
index 3c29767f2..d4eb43cbb 100644
--- a/tensorflow/contrib/slim/python/slim/nets/vgg.py
+++ b/tensorflow/contrib/slim/python/slim/nets/vgg.py
@@ -37,13 +37,20 @@ Usage:
 @@vgg_16
 @@vgg_19
 """
+
 from __future__ import absolute_import
 from __future__ import division
 from __future__ import print_function

-import tensorflow as tf
-
-slim = tf.contrib.slim
+from tensorflow.contrib import layers
+from tensorflow.contrib.framework.python.ops import arg_scope
+from tensorflow.contrib.layers.python.layers import layers as layers_lib
+from tensorflow.contrib.layers.python.layers import regularizers
+from tensorflow.contrib.layers.python.layers import utils
+from tensorflow.python.ops import array_ops
+from tensorflow.python.ops import init_ops
+from tensorflow.python.ops import nn_ops
+from tensorflow.python.ops import variable_scope


 def vgg_arg_scope(weight_decay=0.0005):

      

        
        
        
      

    

The top-level tensor __init__.py

exports symbols from submodules.

# tensorflow/python/__init__.py
...
from tensorflow.python.ops.standard_ops import *
...

      

        
        
        
      

    

# tensorflow/python/ops/standard_ops.py
...
from tensorflow.python.ops.array_ops import *
from tensorflow.python.ops.check_ops import *
from tensorflow.python.ops.clip_ops import *
...