Adds error message for missing TF dependency.

PiperOrigin-RevId: 258625168
Change-Id: I1112a934ed30a1a212f0c78f22aa9bd2782540e1

Author: Johannes Ballé

BUILD

@@ -35,6 +35,7 @@ py_binary(
         "//tensorflow_compression/python/layers:py_src",
         "//tensorflow_compression/python/ops:py_src",
     ],
+    python_version = "PY3",
 )
 
 py_binary(

tensorflow_compression/__init__.py

@@ -18,6 +18,15 @@
 from __future__ import division as _division
 from __future__ import print_function as _print_function
 
+try:
+  import tensorflow as _tensorflow
+  _tf_version = [int(v) for v in _tensorflow.version.VERSION.split(".")]
+  assert _tf_version[0] == 1 and _tf_version[1] >= 14
+except (ImportError, AssertionError):
+  raise RuntimeError("For tensorflow_compression, please install TensorFlow "
+                     "1.14 or above. TensorFlow 2 is not yet supported.")
+
+
 # pylint: disable=wildcard-import
 from tensorflow_compression.python.layers.entropy_models import *
 from tensorflow_compression.python.layers.gdn import *