Add a mechanism for user to know the availability of cucim.CuImage (#107)

- Add `cucim.is_available()` method

Users can query if a specific module is available or not.

For example, if cucim.clara module which requires C shared library is not available:
```
>>> import cucim
>>> cucim.is_available()
False
>>> cucim.is_available("skimage")
True
>>> cucim.is_available("core")
True
>>> cucim.is_available("clara")
False
```

Resolves #104
Supports Project-MONAI/MONAI#2987

Authors:
  - Gigon Bae (https://github.com/gigony)

Approvers:
  - Gregory R. Lee (https://github.com/grlee77)
  - https://github.com/jakirkham

URL: #107

Author: gigony

python/cucim/src/cucim/__init__.py

@@ -30,20 +30,49 @@
     Functions from scikit-image.
 
 """
+_is_cupy_available = False
+_is_clara_available = False
 
 # Try to import cupy first.
 # If cucim.clara package is imported first, you may see the following error when running on CUDA 10.x (#44)
 #   python3: Relink `/usr/lib/x86_64-linux-gnu/libnccl.so.2.8.3' with `/lib/x86_64-linux-gnu/librt.so.1' for IFUNC symbol `clock_gettime'
 #   Segmentation fault
 try:
     import cupy
+    _is_cupy_available = True
 except ImportError:
     pass
 
 try:
     from .clara import CuImage, __version__, cli
+    _is_clara_available = True
 except ImportError:
     from ._version import get_versions
     __version__ = get_versions()['version']
     del get_versions
     del _version
+
+
+def is_available(module_name: str = "") -> bool:
+    """Check if a specific module is available.
+
+    If module_name is not specified, returns True if all of the modules are
+    available.
+
+    Parameters
+    ----------
+    module_name : str
+        Name of the module to check. (e.g. "skimage", "core", and "clara")
+
+    Returns
+    -------
+    bool
+        True if the module is available, False otherwise.
+
+    """
+    if module_name in ("skimage", "core"):
+        return _is_cupy_available
+    elif module_name == 'clara':
+        return _is_clara_available
+    else:
+        return _is_cupy_available and _is_clara_available

python/cucim/tests/unit/test_init.py

@@ -0,0 +1,45 @@
+#
+# Copyright (c) 2021, NVIDIA CORPORATION.
+# 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 unittest.mock import patch
+
+
+def test_is_available():
+    with patch('cucim._is_cupy_available', False):
+        with patch('cucim._is_clara_available', False):
+            import cucim
+            assert cucim.is_available() is False
+            assert cucim.is_available("skimage") is False
+            assert cucim.is_available("clara") is False
+            assert cucim.is_available("unknown") is False
+        with patch('cucim._is_clara_available', True):
+            import cucim
+            assert cucim.is_available() is False
+            assert cucim.is_available("skimage") is False
+            assert cucim.is_available("clara") is True
+            assert cucim.is_available("unknown") is False
+
+    with patch('cucim._is_cupy_available', True):
+        with patch('cucim._is_clara_available', False):
+            import cucim
+            assert cucim.is_available() is False
+            assert cucim.is_available("skimage") is True
+            assert cucim.is_available("clara") is False
+            assert cucim.is_available("unknown") is False
+        with patch('cucim._is_clara_available', True):
+            import cucim
+            assert cucim.is_available() is True
+            assert cucim.is_available("skimage") is True
+            assert cucim.is_available("clara") is True
+            assert cucim.is_available("unknown") is True

run

@@ -721,7 +721,7 @@ test_python() {
 
     local testsuite=""
     local testsuite_unit_skimage="src"
-    local testsuite_unit_clara="tests/unit/clara"
+    local testsuite_unit_clara="tests/unit"
     local testsuite_performance="tests/performance"
 
     install_python_test_deps_