init: fix top-level module reloading

tensorboard/BUILD

@@ -71,6 +71,18 @@ py_library(
     ],
 )
 
+py_test(
+    name = "lib_test",
+    size = "small",
+    srcs = ["lib_test.py"],
+    srcs_version = "PY2AND3",
+    visibility = ["//tensorboard:internal"],
+    deps = [
+        ":lib",
+        "@org_pythonhosted_six",
+    ],
+)
+
 py_library(
     name = "manager",
     srcs = ["manager.py"],

tensorboard/__init__.py

@@ -22,19 +22,62 @@
 from tensorboard import lazy
 
 
+# Please be careful when changing the structure of this file.
+#
+# The lazy imports in this file must use `importlib.import_module`, not
+# `import tensorboard.foo` or `from tensorboard import foo`, or it will
+# be impossible to reload the TensorBoard module without breaking these
+# top-level public APIs. This has to do with the gory details of
+# Python's module system. Take `tensorboard.notebook` as an example:
+#
+#   - When the `tensorboard` module (that's us!) is initialized, its
+#     `notebook` attribute is initialized to a new LazyModule. The
+#     actual `tensorboard.notebook` submodule is not loaded.
+#
+#   - When the `tensorboard.notebook` submodule is first loaded, Python
+#     _reassigns_ the `notebook` attribute on the `tensorboard` module
+#     object to point to the underlying `tensorboard.notebook` module
+#     object, rather than its former LazyModule value. This occurs
+#     whether the module is loaded via the lazy module or directly as an
+#     import:
+#
+#       - import tensorboard; tensorboard.notebook.start(...)  # one way
+#       - from tensorboard import notebook  # other way; same effect
+#
+#   - When the `tensorboard` module is reloaded, its `notebook`
+#     attribute is once again bound to a (new) LazyModule, while the
+#     `tensorboard.notebook` module object is unaffected and still
+#     exists in `sys.modules`. But then...
+#
+#   - When the new LazyModule is forced, it must resolve to the existing
+#     `tensorboard.notebook` module object rather than itself (which
+#     just creates a stack overflow). If the LazyModule load function
+#     uses `import tensorboard.notebook; return tensorboard.notebook`,
+#     then the first statement will do _nothing_ because the
+#     `tensorboard.notebook` module is already loaded, and the second
+#     statement will return the LazyModule itself. The same goes for the
+#     `from tensorboard import notebook` form. We need to ensure that
+#     the submodule is loaded and then pull the actual module object out
+#     of `sys.modules`... which is exactly what `importlib` handles for
+#     us.
+#
+# See <https://github.com/tensorflow/tensorboard/issues/1989> for
+# additional discussion.
+
+
 @lazy.lazy_load('tensorboard.notebook')
 def notebook():
-  import tensorboard.notebook as module  # pylint: disable=g-import-not-at-top
-  return module
+  import importlib  # pylint: disable=g-import-not-at-top
+  return importlib.import_module('tensorboard.notebook')
 
 
 @lazy.lazy_load('tensorboard.program')
 def program():
-  import tensorboard.program as module  # pylint: disable=g-import-not-at-top
-  return module
+  import importlib  # pylint: disable=g-import-not-at-top
+  return importlib.import_module('tensorboard.program')
 
 
 @lazy.lazy_load('tensorboard.summary')
 def summary():
-  import tensorboard.summary as module  # pylint: disable=g-import-not-at-top
-  return module
+  import importlib  # pylint: disable=g-import-not-at-top
+  return importlib.import_module('tensorboard.summary')

tensorboard/lib_test.py

@@ -0,0 +1,43 @@
+# Copyright 2019 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.
+
+from __future__ import absolute_import
+from __future__ import division
+from __future__ import print_function
+
+from six import moves
+import sys
+import unittest
+
+
+class ReloadTensorBoardTest(unittest.TestCase):
+
+  def test_functional_after_reload(self):
+    self.assertNotIn("tensorboard", sys.modules)
+    import tensorboard as tensorboard  # it makes the Google sync happy
+    submodules = ["notebook", "program", "summary"]
+    dirs_before = {
+        module_name: dir(getattr(tensorboard, module_name))
+        for module_name in submodules
+    }
+    tensorboard = moves.reload_module(tensorboard)
+    dirs_after = {
+        module_name: dir(getattr(tensorboard, module_name))
+        for module_name in submodules
+    }
+    self.assertEqual(dirs_before, dirs_after)
+
+
+if __name__ == '__main__':
+  unittest.main()