Add: core cli, lazy_group, example apps

isp_maintenance/core/cli/lazy_group.py

@@ -0,0 +1,39 @@
+import importlib
+import click
+
+
+class LazyGroup(click.Group):
+    def __init__(self, *args, lazy_subcommands=None, **kwargs):
+        super().__init__(*args, **kwargs)
+        # lazy_subcommands is a map of the form:
+        #
+        #   {command-name} -> {module-name}.{command-object-name}
+        #
+        self.lazy_subcommands = lazy_subcommands or {}
+
+    def list_commands(self, ctx):
+        base = super().list_commands(ctx)
+        lazy = sorted(self.lazy_subcommands.keys())
+        return base + lazy
+
+    def get_command(self, ctx, cmd_name):
+        if cmd_name in self.lazy_subcommands:
+            return self._lazy_load(cmd_name)
+        return super().get_command(ctx, cmd_name)
+
+    def _lazy_load(self, cmd_name):
+        # lazily loading a command,
+        # first get the module name and attribute name
+        import_path = self.lazy_subcommands[cmd_name]
+        modname, cmd_object_name = import_path.rsplit(".", 1)
+        # do the import
+        mod = importlib.import_module(modname)
+        # get the Command object from that module
+        cmd_object = getattr(mod, cmd_object_name)
+        # check the result to make debugging easier
+        if not isinstance(cmd_object, click.BaseCommand):
+            raise ValueError(
+                f"Lazy loading of {import_path} failed by returning "
+                "a non-command object"
+            )
+        return cmd_object