RF: Split requirement check into function, prefer importlib.metadata

Author: effigies

lazy_loader/__init__.py

@@ -194,27 +194,15 @@ def myfunc():
     if not have_module:
         not_found_message = f"No module named '{fullname}'"
     elif require is not None:
-        import packaging.requirements
-
-        try:
-            import importlib_metadata
-        except ImportError:
-            import importlib.metadata as importlib_metadata
-
-        req = packaging.requirements.Requirement(require)
         try:
-            have_module = req.specifier.contains(
-                importlib_metadata.version(req.name),
-                prereleases=True,
-            )
-        except importlib_metadata.PackageNotFoundError as e:
+            have_module = _check_requirement(require)
+        except ModuleNotFoundError as e:
             raise ValueError(
                 f"Found module '{fullname}' but cannot test requirement '{require}'. "
                 "Requirements must match distribution name, not module name."
             ) from e
 
-        if not have_module:
-            not_found_message = f"No distribution can be found matching '{require}'"
+        not_found_message = f"No distribution can be found matching '{require}'"
 
     if not have_module:
         if error_on_import:
@@ -247,6 +235,37 @@ def myfunc():
     return module
 
 
+def _check_requirement(require: str) -> bool:
+    """Verify that a package requirement is satisfied
+
+    If the package is required, a ``ModuleNotFoundError`` is raised
+    by ``importlib.metadata``.
+
+    Parameters
+    ----------
+    require : str
+        A dependency requirement as defined in PEP-508
+
+    Returns
+    -------
+    satisfied : bool
+        True if the installed version of the dependency matches
+        the specified version, False otherwise.
+    """
+    import packaging.requirements
+
+    try:
+        import importlib.metadata as importlib_metadata
+    except ImportError:  # PY37
+        import importlib_metadata
+
+    req = packaging.requirements.Requirement(require)
+    return req.specifier.contains(
+        importlib_metadata.version(req.name),
+        prereleases=True,
+    )
+
+
 def have_module(module_like: types.ModuleType) -> bool:
     return not isinstance(module_like, DelayedImportErrorModule)
 

lazy_loader/tests/test_lazy_loader.py

@@ -7,13 +7,6 @@
 
 import lazy_loader as lazy
 
-try:
-    import importlib_metadata  # noqa
-
-    have_importlib_metadata = True
-except ImportError:
-    have_importlib_metadata = False
-
 
 def test_lazy_import_basics():
     math = lazy.load("math")
@@ -160,7 +153,8 @@ def test_stub_loading_errors(tmp_path):
 
 
 def test_require_kwarg():
-    dot = "_" if have_importlib_metadata else "."
+    have_importlib_metadata = importlib.util.find_spec("importlib.metadata") is not None
+    dot = "." if have_importlib_metadata else "_"
     # Test with a module that definitely exists, behavior hinges on requirement
     with mock.patch(f"importlib{dot}metadata.version") as version:
         version.return_value = "1.0.0"

pyproject.toml

@@ -23,7 +23,7 @@ classifiers = [
 description = "Makes it easy to load subpackages and functions on demand."
 dependencies = [
   "packaging",
-  "importlib_metadata; python_version < '3.9'",
+  "importlib_metadata; python_version < '3.8'",
 ]
 
 [project.optional-dependencies]