Fix lazy_instance not working for callable classes

CHANGELOG.rst

@@ -29,6 +29,8 @@ Fixed
 - Account for breaking change in ``argparse.ArgumentParser._parse_optional``
   affecting python ``3.11.9`` and likely ``>3.13`` (`#484
   <https://github.com/omni-us/jsonargparse/issues/484>`__).
+- ``lazy_instance`` not working for callable classes (`#473 comment
+  <https://github.com/omni-us/jsonargparse/issues/481#issuecomment-2030932435>`__).
 
 
 v4.27.7 (2024-03-21)

jsonargparse/_typehints.py

@@ -1295,6 +1295,7 @@ class LazyInitBaseClass:
     def __init__(self, class_type: Type, lazy_kwargs: dict):
         assert not issubclass(class_type, LazyInitBaseClass)
         check_lazy_kwargs(class_type, lazy_kwargs)
+        self._lazy = type(self)
         self._lazy_class_type = class_type
         self._lazy_kwargs = lazy_kwargs
         self._lazy_methods = {}
@@ -1308,17 +1309,22 @@ def __init__(self, class_type: Type, lazy_kwargs: dict):
             if id(member) in seen_methods:
                 self.__dict__[name] = seen_methods[id(member)]
             else:
-                self.__dict__[name] = partial(self._lazy_init_then_call_method, name)
-                seen_methods[id(member)] = self.__dict__[name]
+                lazy_method = partial(self._lazy_init_then_call_method, name)
+                self.__dict__[name] = lazy_method
+                if name == "__call__":
+                    self._lazy.__call__ = lazy_method  # type: ignore[method-assign]
+                seen_methods[id(member)] = lazy_method
 
     def _lazy_init(self):
         for name in self._lazy_methods:
+            if name == "__call__":
+                self._lazy.__call__ = self._lazy_methods[name]
             del self.__dict__[name]
         super().__init__(**self._lazy_kwargs)
 
     def _lazy_init_then_call_method(self, method_name, *args, **kwargs):
         self._lazy_init()
-        return getattr(self, method_name)(*args, **kwargs)
+        return self._lazy_methods[method_name](*args, **kwargs)
 
     def lazy_get_init_args(self) -> Namespace:
         return Namespace(self._lazy_kwargs)

jsonargparse_tests/test_typehints.py

@@ -884,6 +884,21 @@ def test_lazy_instance_pickleable():
     assert reloaded.lazy_get_init_data() == instance1.lazy_get_init_data()
 
 
+class OptimizerCallable:
+    def __init__(self, lr: float = 0.1):
+        self.lr = lr
+
+    def __call__(self, params) -> SGD:
+        return SGD(params, lr=self.lr)
+
+
+def test_lazy_instance_callable():
+    lazy_optimizer = lazy_instance(OptimizerCallable, lr=0.2)
+    optimizer = lazy_optimizer([1, 2])
+    assert optimizer.lr == 0.2
+    assert optimizer.params == [1, 2]
+
+
 # other tests