implement ability to load external extractor classes

- -X/--extractors
- extractor.module-sources

gallery_dl/extractor/__init__.py

@@ -1,11 +1,12 @@
 # -*- coding: utf-8 -*-
 
-# Copyright 2015-2022 Mike Fährmann
+# Copyright 2015-2023 Mike Fährmann
 #
 # This program is free software; you can redistribute it and/or modify
 # it under the terms of the GNU General Public License version 2 as
 # published by the Free Software Foundation.
 
+import sys
 import re
 
 modules = [
@@ -217,20 +218,33 @@ def extractors():
 # --------------------------------------------------------------------
 # internals
 
-_cache = []
-_module_iter = iter(modules)
-
 
 def _list_classes():
-    """Yield all available extractor classes"""
+    """Yield available extractor classes"""
     yield from _cache
 
-    globals_ = globals()
-    for module_name in _module_iter:
-        module = __import__(module_name, globals_, None, (), 1)
+    for module in _module_iter:
         yield from add_module(module)
 
-    globals_["_list_classes"] = lambda : _cache
+    globals()["_list_classes"] = lambda : _cache
+
+
+def _modules_internal():
+    globals_ = globals()
+    for module_name in modules:
+        yield __import__(module_name, globals_, None, (), 1)
+
+
+def _modules_path(path, files):
+    sys.path.insert(0, path)
+    try:
+        return [
+            __import__(name[:-3])
+            for name in files
+            if name.endswith(".py")
+        ]
+    finally:
+        del sys.path[0]
 
 
 def _get_classes(module):
@@ -240,3 +254,7 @@ def _get_classes(module):
             hasattr(cls, "pattern") and cls.__module__ == module.__name__
         )
     ]
+
+
+_cache = []
+_module_iter = _modules_internal()