Merge pull request #4 from stefanv/better-error

Simplify delayed exception handling and improve reporting

lazy_loader/__init__.py

@@ -6,10 +6,13 @@
 """
 import importlib
 import importlib.util
+import inspect
 import os
 import sys
 import types
 
+__all__ = ["attach", "load"]
+
 
 def attach(package_name, submodules=None, submod_attrs=None):
     """Attach lazily loaded submodules, functions, or other attributes.
@@ -83,6 +86,24 @@ def __dir__():
     return __getattr__, __dir__, list(__all__)
 
 
+class DelayedImportErrorModule(types.ModuleType):
+    def __init__(self, frame_data, *args, **kwargs):
+        self.__frame_data = frame_data
+        super().__init__(*args, **kwargs)
+
+    def __getattr__(self, x):
+        if x in ("__class__", "__file__", "__frame_data"):
+            super().__getattr__(x)
+        else:
+            fd = self.__frame_data
+            raise ModuleNotFoundError(
+                f"No module named '{fd['spec']}'\n\n"
+                "This error is lazily reported, having originally occured in\n"
+                f'  File {fd["filename"]}, line {fd["lineno"]}, in {fd["function"]}\n\n'
+                f'----> {"".join(fd["code_context"]).strip()}'
+            )
+
+
 def load(fullname, error_on_import=False):
     """Return a lazily imported proxy for a module.
 
@@ -138,13 +159,18 @@ def myfunc():
         if error_on_import:
             raise ModuleNotFoundError(f"No module named '{fullname}'")
         else:
-            spec = importlib.util.spec_from_loader(fullname, loader=None)
-            module = importlib.util.module_from_spec(spec)
-            tmp_loader = importlib.machinery.SourceFileLoader(module, path=None)
-            loader = DelayedImportErrorLoader(tmp_loader)
-            loader.exec_module(module)
-            # dont add to sys.modules. The module wasn't found.
-            return module
+            try:
+                parent = inspect.stack()[1]
+                frame_data = {
+                    "spec": fullname,
+                    "filename": parent.filename,
+                    "lineno": parent.lineno,
+                    "function": parent.function,
+                    "code_context": parent.code_context,
+                }
+                return DelayedImportErrorModule(frame_data, "DelayedImportErrorModule")
+            finally:
+                del parent
 
     module = importlib.util.module_from_spec(spec)
     sys.modules[fullname] = module
@@ -153,20 +179,3 @@ def myfunc():
     loader.exec_module(module)
 
     return module
-
-
-class DelayedImportErrorLoader(importlib.util.LazyLoader):
-    def exec_module(self, module):
-        super().exec_module(module)
-        module.__class__ = DelayedImportErrorModule
-
-
-class DelayedImportErrorModule(types.ModuleType):
-    def __getattribute__(self, attr):
-        """Trigger a ModuleNotFoundError upon attribute access"""
-        spec = super().__getattribute__("__spec__")
-        # allows isinstance and type functions to work without raising error
-        if attr in ["__class__"]:
-            return super().__getattribute__("__class__")
-
-        raise ModuleNotFoundError(f"No module named '{spec.name}'")