1 files changed, 68 insertions, 0 deletions
diff --git a/lib/sqlalchemy/util/_preloaded.py b/lib/sqlalchemy/util/_preloaded.py
new file mode 100644
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--- /dev/null
+++ b/lib/sqlalchemy/util/_preloaded.py
@@ -0,0 +1,68 @@
+# util/_preloaded.py
+# Copyright (C) 2005-2022 the SQLAlchemy authors and contributors
+# <see AUTHORS file>
+#
+# This module is part of SQLAlchemy and is released under
+# the MIT License: https://www.opensource.org/licenses/mit-license.php
+
+"""supplies the "preloaded" registry to resolve circular module imports at
+runtime.
+
+"""
+
+import sys
+
+from . import compat
+
+
+class _ModuleRegistry:
+    """Registry of modules to load in a package init file.
+
+    To avoid potential thread safety issues for imports that are deferred
+    in a function, like https://bugs.python.org/issue38884, these modules
+    are added to the system module cache by importing them after the packages
+    has finished initialization.
+
+    A global instance is provided under the name :attr:`.preloaded`. Use
+    the function :func:`.preload_module` to register modules to load and
+    :meth:`.import_prefix` to load all the modules that start with the
+    given path.
+
+    While the modules are loaded in the global module cache, it's advisable
+    to access them using :attr:`.preloaded` to ensure that it was actually
+    registered. Each registered module is added to the instance ``__dict__``
+    in the form `<package>_<module>`, omitting ``sqlalchemy`` from the package
+    name. Example: ``sqlalchemy.sql.util`` becomes ``preloaded.sql_util``.
+    """
+
+    def __init__(self, prefix="sqlalchemy."):
+        self.module_registry = set()
+        self.prefix = prefix
+
+    def preload_module(self, *deps):
+        """Adds the specified modules to the list to load.
+
+        This method can be used both as a normal function and as a decorator.
+        No change is performed to the decorated object.
+        """
+        self.module_registry.update(deps)
+        return lambda fn: fn
+
+    def import_prefix(self, path):
+        """Resolve all the modules in the registry that start with the
+        specified path.
+        """
+        for module in self.module_registry:
+            if self.prefix:
+                key = module.split(self.prefix)[-1].replace(".", "_")
+            else:
+                key = module
+            if (
+                not path or module.startswith(path)
+            ) and key not in self.__dict__:
+                compat.import_(module, globals(), locals())
+                self.__dict__[key] = sys.modules[module]
+
+
+preloaded = _ModuleRegistry()
+preload_module = preloaded.preload_module