Repository: omni-us/jsonargparse (python)

Issue:
Add support for `typing.Protocol` classes in `CLI`
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## 🚀 Feature request

Add support for `typing.Protocol` classes in `CLI`.

### Motivation

Currently I cannot use protocol classes for type annotations. For example:

In `main.py`:

```Python
import typing as tp

from jsonargparse import CLI


@tp.runtime_checkable
class ModelType(tp.Protocol):
    def predict(self, items: tp.Sequence[tp.Any]) -> tp.Any:
        ...


class Model:

    def __init__(self, batch_size: int) -> None:
        self._batch_size = batch_size

    def predict(self, items: tp.Sequence[tp.Any]) -> tp.Any:
        print(f"Predicting {len(items)} items with a batch size {self._batch_size}")


def main(model: ModelType) -> None:
    print(model)
    model.predict([1, 2, 3])


if __name__ == "__main__":
    CLI(main)
```

In `config.yaml`:

```Yaml
model:
  class_path: __main__.Model
  init_args:
    batch_size: 1
```

In terminal:

`python -m main --config config.yaml`

Prints `<class '__main__.Model'>` and gives error: `TypeError: predict() missing 1 required positional argument: 'items'`

### Pitch

The above code should run without errors. `Model` instance should be printed, not the class.

### Alternatives

<!-- Any alternative solutions or features you've considered, if any. -->


Pull request description:
Support for Protocol types only accepting exact matching signature of public methods
## What does this PR do?

Fixes #520

## Before submitting

- [x] Did you read the [contributing guideline](https://github.com/omni-us/jsonargparse/blob/main/CONTRIBUTING.rst)?
- [x] Did you update **the documentation**? (readme and public docstrings)
- [x] Did you write **unit tests** such that there is 100% coverage on related code? (required for bug fixes and new features)
- [x] Did you verify that new and existing **tests pass locally**?
- [x] Did you make sure that all changes preserve **backward compatibility**?
- [x] Did you update **the CHANGELOG**? (not for typos, docs, test updates, or minor internal changes/refactors)


Patch:
diff --git a/CHANGELOG.rst b/CHANGELOG.rst
index e62c433..f23c37a 100644
--- a/CHANGELOG.rst
+++ b/CHANGELOG.rst
@@ -15,6 +15,12 @@ paths are considered internals and can change in minor and patch releases.
 v4.31.0 (2024-06-??)
 --------------------
 
+Added
+^^^^^
+- Support for ``Protocol`` types only accepting exact matching signature of
+  public methods (`#526
+  <https://github.com/omni-us/jsonargparse/pull/526>`__).
+
 Fixed
 ^^^^^
 - Resolving of import paths for some ``torch`` functions not working (`#535
diff --git a/DOCUMENTATION.rst b/DOCUMENTATION.rst
index d6ce5ad..cc56c13 100644
--- a/DOCUMENTATION.rst
+++ b/DOCUMENTATION.rst
@@ -421,6 +421,10 @@ Some notes about this support are:
   :py:meth:`.ArgumentParser.instantiate_classes` can be used to instantiate all
   classes in a config object. For more details see :ref:`sub-classes`.
 
+- ``Protocol`` types are also supported the same as sub-classes. The protocols
+  are not required to be ``runtime_checkable``. But the accepted classes must
+  match exactly the signature of the protocol's public methods.
+
 - ``dataclasses`` are supported even when nested. Final classes, attrs'
   ``define`` decorator, and pydantic's ``dataclass`` decorator and ``BaseModel``
   classes are supported and behave like standard dataclasses. For more details
diff --git a/jsonargparse/_postponed_annotations.py b/jsonargparse/_postponed_annotations.py
index a8a3ab3..e5a6279 100644
--- a/jsonargparse/_postponed_annotations.py
+++ b/jsonargparse/_postponed_annotations.py
@@ -334,7 +334,7 @@ def evaluate_postponed_annotations(params, component, parent, logger):
             param.annotation = param_type
 
 
-def get_return_type(component, logger):
+def get_return_type(component, logger=None):
     return_type = inspect.signature(component).return_annotation
     if type_requires_eval(return_type):
         global_vars = vars(import_module(component.__module__))
@@ -343,6 +343,7 @@ def get_return_type(component, logger):
             if isinstance(return_type, ForwardRef):
                 return_type = resolve_forward_refs(return_type.__forward_arg__, global_vars, logger)
         except Exception as ex:
-            logger.debug(f"Unable to evaluate types for {component}", exc_info=ex)
+            if logger:
+                logger.debug(f"Unable to evaluate types for {component}", exc_info=ex)
             return None
     return return_type
diff --git a/jsonargparse/_typehints.py b/jsonargparse/_typehints.py
index afcc291..5d25709 100644
--- a/jsonargparse/_typehints.py
+++ b/jsonargparse/_typehints.py
@@ -977,7 +977,7 @@ def adapt_typehints(
 
     # Subclass
     elif not hasattr(typehint, "__origin__") and inspect.isclass(typehint):
-        if isinstance(val, typehint):
+        if is_instance_or_supports_protocol(val, typehint):
             if serialize:
                 val = serialize_class_instance(val)
             return val
@@ -985,9 +985,9 @@ def adapt_typehints(
             return val
 
         val_input = val
-        if prev_val is None and not inspect.isabstract(typehint):
+        if prev_val is None and not inspect.isabstract(typehint) and not is_protocol(typehint):
             with suppress(ValueError):
-                prev_val = Namespace(class_path=get_import_path(typehint))
+                prev_val = Namespace(class_path=get_import_path(typehint))  # implicit class_path
         val = subclass_spec_as_namespace(val, prev_val)
         if not is_subclass_spec(val):
             raise_unexpected_value(
@@ -1000,20 +1000,20 @@ def adapt_typehints(
 
         try:
             val_class = import_object(resolve_class_path_by_name(typehint, val["class_path"]))
-            if isinstance(val_class, typehint):
-                return val_class
+            if is_instance_or_supports_protocol(val_class, typehint):
+                return val_class  # importable instance
             not_subclass = False
-            if not is_subclass(val_class, typehint):
+            if not is_subclass_or_implements_protocol(val_class, typehint):
                 not_subclass = True
                 if not inspect.isclass(val_class) and callable(val_class):
                     from ._postponed_annotations import get_return_type
 
                     return_type = get_return_type(val_class, logger)
-                    if is_subclass(return_type, typehint):
+                    if is_subclass_or_implements_protocol(return_type, typehint):
                         not_subclass = False
             if not_subclass:
                 raise_unexpected_value(
-                    f'Import path {val["class_path"]} does not correspond to a subclass of {typehint}'
+                    f"Import path {val['class_path']} does not correspond to a subclass of {typehint.__name__}"
                 )
             val["class_path"] = get_import_path(val_class)
             val = adapt_class_type(val, serialize, instantiate_classes, sub_add_kwargs, prev_val=prev_val)
@@ -1029,6 +1029,46 @@ def adapt_typehints(
     return val
 
 
+def implements_protocol(value, protocol) -> bool:
+    from jsonargparse._parameter_resolvers import get_signature_parameters
+    from jsonargparse._postponed_annotations import get_return_type
+
+    if not inspect.isclass(value):
+        return False
+    members = 0
+    for name, _ in inspect.getmembers(protocol, predicate=inspect.isfunction):
+        if name.startswith("_"):
+            continue
+        if not hasattr(value, name):
+            return False
+        members += 1
+        proto_params = get_signature_parameters(protocol, name)
+        value_params = get_signature_parameters(value, name)
+        if [(p.name, p.annotation) for p in proto_params] != [(p.name, p.annotation) for p in value_params]:
+            return False
+        proto_return = get_return_type(inspect.getattr_static(protocol, name))
+        value_return = get_return_type(inspect.getattr_static(value, name))
+        if proto_return != value_return:
+            return False
+    return True if members else False
+
+
+def is_protocol(class_type) -> bool:
+    return getattr(class_type, "_is_protocol", False)
+
+
+def is_subclass_or_implements_protocol(value, class_type) -> bool:
+    if is_protocol(class_type):
+        return implements_protocol(value, class_type)
+    return is_subclass(value, class_type)
+
+
+def is_instance_or_supports_protocol(value, class_type):
+    if is_protocol(class_type):
+        return is_subclass_or_implements_protocol(value.__class__, class_type)
+    return isinstance(value, class_type)
+
+
 def is_subclass_spec(val):
     is_class = isinstance(val, (dict, Namespace)) and "class_path" in val
     if is_class:


Test patch:
diff --git a/jsonargparse_tests/test_subclasses.py b/jsonargparse_tests/test_subclasses.py
index 60bfb2a..2297bff 100644
--- a/jsonargparse_tests/test_subclasses.py
+++ b/jsonargparse_tests/test_subclasses.py
@@ -23,6 +23,8 @@ from jsonargparse import (
     Namespace,
     lazy_instance,
 )
+from jsonargparse._optionals import typing_extensions_import
+from jsonargparse._typehints import implements_protocol, is_instance_or_supports_protocol
 from jsonargparse.typing import final
 from jsonargparse_tests.conftest import (
     capture_logs,
@@ -32,6 +34,8 @@ from jsonargparse_tests.conftest import (
     source_unavailable,
 )
 
+Protocol = typing_extensions_import("Protocol")
+
 
 @pytest.mark.parametrize("type", [Calendar, Optional[Calendar]])
 def test_subclass_basics(parser, type):
@@ -1407,6 +1411,84 @@ def test_subclass_signature_instance_default(parser):
     assert "cal: Unable to serialize instance <calendar.Calendar " in dump
 
 
+# protocol tests
+
+
+class Interface(Protocol):  # type: ignore[valid-type,misc]
+    def predict(self, items: List[float]) -> List[float]: ...  # type: ignore[empty-body]
+
+
+class ImplementsInterface:
+    def __init__(self, batch_size: int):
+        self.batch_size = batch_size
+
+    def predict(self, items: List[float]) -> List[float]:
+        return items
+
+
+class NotImplementsInterface1:
+    def predict(self, items: str) -> List[float]:
+        return []
+
+
+class NotImplementsInterface2:
+    def predict(self, items: List[float], extra: int) -> List[float]:
+        return items
+
+
+class NotImplementsInterface3:
+    def predict(self, items: List[float]) -> None:
+        return
+
+
+@pytest.mark.parametrize(
+    "expected, value",
+    [
+        (True, ImplementsInterface),
+        (False, ImplementsInterface(1)),
+        (False, NotImplementsInterface1),
+        (False, NotImplementsInterface2),
+        (False, NotImplementsInterface3),
+        (False, object),
+    ],
+)
+@pytest.mark.skipif(not Protocol, reason="Requires Python 3.8+ or typing_extensions")
+def test_implements_protocol(expected, value):
+    assert implements_protocol(value, Interface) is expected
+
+
+@pytest.mark.parametrize(
+    "expected, value",
+    [
+        (False, ImplementsInterface),
+        (True, ImplementsInterface(1)),
+        (False, NotImplementsInterface1()),
+        (False, object),
+    ],
+)
+@pytest.mark.skipif(not Protocol, reason="Requires Python 3.8+ or typing_extensions")
+def test_is_instance_or_supports_protocol(expected, value):
+    assert is_instance_or_supports_protocol(value, Interface) is expected
+
+
+@pytest.mark.skipif(not Protocol, reason="Requires Python 3.8+ or typing_extensions")
+def test_parse_implements_protocol(parser):
+    parser.add_argument("--cls", type=Interface)
+    cfg = parser.parse_args([f"--cls={__name__}.ImplementsInterface", "--cls.batch_size=5"])
+    assert cfg.cls.class_path == f"{__name__}.ImplementsInterface"
+    assert cfg.cls.init_args == Namespace(batch_size=5)
+    init = parser.instantiate_classes(cfg)
+    assert isinstance(init.cls, ImplementsInterface)
+    assert init.cls.batch_size == 5
+    assert init.cls.predict([1.0, 2.0]) == [1.0, 2.0]
+    with pytest.raises(ArgumentError) as ctx:
+        parser.parse_args([f"--cls={__name__}.NotImplementsInterface1"])
+    ctx.match("does not correspond to a subclass of")
+    with pytest.raises(ArgumentError) as ctx:
+        parser.parse_args(['--cls={"batch_size": 5}'])
+    ctx.match("Not a valid subclass of Interface")
+
+
 # parameter skip tests