tensorflow: tf.enable_eager_execution must be called at program startup.

from __future__ import absolute_import, division, print_function
import os
import matplotlib.pyplot as plt

import tensorflow as tf
print(tf.VERSION)   #  => 1.7.0
import tensorflow.contrib.eager as tfe

tf.enable_eager_execution()

ValueError                                Traceback (most recent call last)
<ipython-input-59-0c62cac7517d> in <module>()
      7 import tensorflow.contrib.eager as tfe
      8 
----> 9 tf.enable_eager_execution()

D:\Anaconda3\envs\tensorflow\lib\site-packages\tensorflow\python\framework\ops.py in enable_eager_execution(config, device_policy)
   5242     if graph_mode_has_been_used:
   5243       raise ValueError(
-> 5244           "tf.enable_eager_execution must be called at program startup.")
   5245   context._default_mode = context.EAGER_MODE
   5246   if context._context is None:

ValueError: tf.enable_eager_execution must be called at program startup.