title: ImportError: cannot import name 'backend'
body: <p>I encounter an ImportError when importing <code>backend</code> from <code>keras</code></p>
  
  <pre><code>from keras import backend
  </code></pre>
  
  <p>The output is</p>
  
  <pre><code>Using TensorFlow backend.
  Traceback (most recent call last):
    File "&lt;stdin&gt;", line 1, in &lt;module&gt;
    File "/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site-packages/keras/__init__.py", line 3, in &lt;module&gt;
      from . import utils
    File "/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site-packages/keras/utils/__init__.py", line 6, in &lt;module&gt;
      from . import conv_utils
    File "/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site-packages/keras/utils/conv_utils.py", line 9, in &lt;module&gt;
      from .. import backend as K
    File "/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site-packages/keras/backend/__init__.py", line 89, in &lt;module&gt;
      from .tensorflow_backend import *
    File "/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site-packages/keras/backend/tensorflow_backend.py", line 5, in &lt;module&gt;
      import tensorflow as tf
    File "/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site-packages/tensorflow/__init__.py", line 24, in &lt;module&gt;
      from tensorflow.python import pywrap_tensorflow  # pylint: disable=unused-import
    File "/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site-packages/tensorflow/python/__init__.py", line 88, in &lt;module&gt;
      from tensorflow.python import keras
    File "/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site-packages/tensorflow/python/keras/__init__.py", line 25, in &lt;module&gt;
      from tensorflow.python.keras import applications
    File "/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site-packages/tensorflow/python/keras/applications/__init__.py", line 74, in &lt;module&gt;
      from tensorflow.python.keras.applications.densenet import DenseNet121
    File "/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site-packages/tensorflow/python/keras/applications/densenet/__init__.py", line 21, in &lt;module&gt;
      from tensorflow.python.keras._impl.keras.applications.densenet import decode_predictions
    File "/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site-packages/tensorflow/python/keras/_impl/keras/applications/densenet.py", line 30, in     module&gt;
      from tensorflow.python.keras._impl.keras import backend as K
  ImportError: cannot import name 'backend'
  </code></pre>
  
  <p>I don't understand which may be the problem.</p>