fix build in Travis

.travis.yml

@@ -2,20 +2,41 @@ language: python
 
 sudo: required
 
-python:
-    - '2.7'
-    - '3.6'
+matrix:
+  include:
+    #- python: 2.7
+    #  dist: xenial  # Ubuntu 16.04
+    #  env: TOXENV=py27
+    - python: 3.5
+      dist: xenial  # Ubuntu 16.04
+      env: TF_VERSION="tensorflow<2.0"
+    - python: 3.6
+      dist: bionic  # Ubuntu 18.04
+      env: TF_VERSION="tensorflow<2.0"
+    - python: 3.7
+      dist: bionic  # Ubuntu 18.04
+      env: TF_VERSION="tensorflow<2.0"
+    - python: 3.6
+      dist: bionic  # Ubuntu 18.04
+      env: TF_VERSION="tensorflow>=2.0"
+    - python: 3.7
+      dist: bionic  # Ubuntu 18.04
+      env: TF_VERSION="tensorflow>=2.0"
+
+cache: pip
 
 install:
+    # see: https://github.com/fizyr/keras-retinanet/issues/1123#issuecomment-541566486
+    - pip install $TF_VERSION
     - pip install -r requirements.txt
     - pip install -r tests/requirements.txt
-
-cache: pip
+    - pip list
 
 script:
     - python setup.py check -m -s
     - python setup.py build_ext --inplace
-    - travis_wait coverage run --source keras_retinanet -m py.test keras_retinanet tests --doctest-modules --forked --flake8
+    - python setup.py install --dry-run
+    - coverage run --source keras_retinanet -m py.test keras_retinanet tests --doctest-modules --forked --flake8
 
 after_success:
     - coverage xml

keras_retinanet/backend/__init__.py

@@ -1,2 +1,15 @@
+import tensorflow
+
+# parse the tensorflow version
+tf_version = [int(i) for i in tensorflow.__version__.split('.')]
+if tf_version[0] >= 2:
+    # Disable Tensorflow 2.x behavior as we experience issues with it.
+    # see: https://www.tensorflow.org/api_docs/python/tf/compat/v1/disable_v2_behavior
+    tensorflow.compat.v1.disable_v2_behavior()
+    # for cases of using `x.numpy()`
+    # Aiming: NotImplementedError: numpy() is only available when eager execution is enabled.
+    # see: https://www.tensorflow.org/api_docs/python/tf/compat/v1/enable_eager_execution
+    tensorflow.compat.v1.enable_eager_execution()
+
 from .dynamic import *  # noqa: F401,F403
 from .common import *   # noqa: F401,F403

tests/models/test_densenet.py

@@ -48,4 +48,4 @@ def test_backbone(backbone):
         },
         optimizer=keras.optimizers.adam(lr=1e-5, clipnorm=0.001))
 
-    model.fit(inputs, targets, batch_size=1)
+    model.fit(inputs, targets, batch_size=1, epochs=1)