Python: Add a compat.py with a constant to help with

maintaining forward compatibility of Python API calls.

PiperOrigin-RevId: 202618021

tensorflow/python/compat/BUILD

+licenses(["notice"])  # Apache 2.0
+
+exports_files(["LICENSE"])
+
+py_library(
+    name = "compat",
+    srcs = ["compat.py"],
+    srcs_version = "PY2AND3",
+    visibility = ["//tensorflow:internal"],
+)

tensorflow/python/compat/compat.py

+# Copyright 2018 The TensorFlow Authors. All Rights Reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+# ==============================================================================
+"""Utilities for API compatibility between TensorFlow release versions.
+
+See
+@{$guide/version_compat#backward_and_partial_forward_compatibility}
+"""
+
+from __future__ import absolute_import
+from __future__ import division
+from __future__ import print_function
+
+import datetime
+
+_FORWARD_COMPATIBILITY_HORIZON = datetime.date(2018, 8, 1)
+
+
+def forward_compatible(year, month, day):
+  """Return true if the forward compatibility window has expired.
+
+  Forward-compatibility refers to scenarios where the producer of a TensorFlow
+  model (a GraphDef or SavedModel) is compiled against a version of the
+  TensorFlow library newer than what the consumer was compiled against. The
+  "producer" is typically a Python program that constructs and trains a model
+  while the "consumer" is typically another program that loads and serves the
+  model.
+
+  TensorFlow has been supporting a 3 week forward-compatibility window for
+  programs compiled from source at HEAD.
+
+  For example, consider the case where a new operation `MyNewAwesomeAdd` is
+  created with the intent of replacing the implementation of an existing Python
+  wrapper - `tf.add`.  The Python wrapper implementation should change from
+  something like:
+
+  ```python
+  def add(inputs, name=None):
+    return gen_math_ops.add(inputs, name)
+  ```
+
+  to:
+
+  ```python
+  from tensorflow.python.compat import compat
+
+  def add(inputs, name=None):
+    if compat.forward_compatible(year, month, day):
+      # Can use the awesome new implementation.
+      return gen_math_ops.my_new_awesome_add(inputs, name)
+    # To maintain forward compatibiltiy, use the old implementation.
+    return gen_math_ops.add(inputs, name)
+  ```
+
+  Where `year`, `month`, and `day` specify the date beyond which binaries
+  that consume a model are expected to have been updated to include the
+  new operations. This date is typically at least 3 weeks beyond the date
+  the code that adds the new operation is committed.
+
+  Args:
+    year:  A year (e.g., 2018).
+    month: A month (1 <= month <= 12) in year.
+    day:   A day (1 <= day <= 31, or 30, or 29, or 28) in month.
+
+  Returns:
+    True if the caller can expect that serialized TensorFlow graphs produced
+    can be consumed by programs that are compiled with the TensorFlow library
+    source code after (year, month, day).
+  """
+  return _FORWARD_COMPATIBILITY_HORIZON > datetime.date(year, month, day)