Introduce ModuleAvailableCache (#86)

src/lightning_utilities/core/imports.py

@@ -121,6 +121,42 @@ def __repr__(self) -> str:
         return self.__str__()
 
 
+class ModuleAvailableCache:
+    """Boolean-like class for check of module availability.
+
+    >>> ModuleAvailableCache("torch")
+    Module 'torch' available
+    >>> bool(ModuleAvailableCache("torch"))
+    True
+    >>> bool(ModuleAvailableCache("unknown_package"))
+    False
+    """
+
+    def __init__(self, module: str) -> None:
+        self.module = module
+
+    def _check_requirement(self) -> None:
+        if hasattr(self, "available"):
+            return
+
+        self.available = module_available(self.module)
+        if self.available:
+            self.message = f"Module {self.module!r} available"
+        else:
+            self.message = f"Module not found: {self.module!r}. HINT: Try running `pip install -U {self.module}`"
+
+    def __bool__(self) -> bool:
+        self._check_requirement()
+        return self.available
+
+    def __str__(self) -> str:
+        self._check_requirement()
+        return self.message
+
+    def __repr__(self) -> str:
+        return self.__str__()
+
+
 def get_dependency_min_version_spec(package_name: str, dependency_name: str) -> str:
     """Returns the minimum version specifier of a dependency of a package.
 

tests/unittests/core/test_imports.py

@@ -8,6 +8,7 @@
     get_dependency_min_version_spec,
     lazy_import,
     module_available,
+    ModuleAvailableCache,
     RequirementCache,
     requires,
 )
@@ -52,6 +53,12 @@ def test_requirement_cache():
     assert "pip install -U '-'" in str(RequirementCache("-"))
 
 
+def test_module_available_cache():
+    assert ModuleAvailableCache("pytest")
+    assert not ModuleAvailableCache("this_module_is_not_installed")
+    assert "pip install -U this_module_is_not_installed" in str(ModuleAvailableCache("this_module_is_not_installed"))
+
+
 def test_get_dependency_min_version_spec():
     attrs_min_version_spec = get_dependency_min_version_spec("pytest", "attrs")
     assert re.match(r"^>=[\d.]+$", attrs_min_version_spec)