Refactor optional dependency constants

regions/_utils/optional_deps.py

@@ -1,13 +1,25 @@
 # Licensed under a 3-clause BSD style license - see LICENSE.rst
+"""
+Checks for optional dependencies using lazy import from `PEP 562
+<https://www.python.org/dev/peps/pep-0562/>`_.
+"""
 
-try:
-    import matplotlib
-    HAS_MATPLOTLIB = True
-    MPL_VERSION = getattr(matplotlib, '__version__', None)
-    if MPL_VERSION is None:
-        MPL_VERSION = matplotlib._version.version
-    MPL_VERSION = MPL_VERSION.split('.')
-    MPL_VERSION = 10 * int(MPL_VERSION[0]) + int(MPL_VERSION[1])
-except ImportError:
-    HAS_MATPLOTLIB = False
-    MPL_VERSION = 0
+import importlib
+
+# This list is a duplicate of the dependencies in pyproject.toml "all".
+# Note that in some cases the package names are different from the
+# pip-install name (e.g.k scikit-image -> skimage).
+optional_deps = ['matplotlib', 'shapely']
+deps = {key.upper(): key for key in optional_deps}
+__all__ = [f'HAS_{pkg}' for pkg in deps]
+
+
+def __getattr__(name):
+    if name in __all__:
+        try:
+            importlib.import_module(deps[name[4:]])
+        except ImportError:
+            return False
+        return True
+
+    raise AttributeError(f'Module {__name__!r} has no attribute {name!r}.')