Add DefaultPlaceholder and is_placeholder_default; InstanceAttrDefault is one

i2/__init__.py

@@ -10,6 +10,8 @@
     double_up_as_factory,
     NotSet,  # Sentinel for "no value given" (distinct from None and Parameter.empty)
     is_not_set,  # Test whether a value (e.g. a signature default) is NotSet
+    DefaultPlaceholder,  # Base for signature defaults that stand for a value
+    is_placeholder_default,  # NotSet or a DefaultPlaceholder: "no concrete default"
 )
 
 from i2.signatures import (

i2/deco.py

@@ -86,6 +86,36 @@ def is_not_set(x) -> bool:
     return x is NotSet
 
 
+class DefaultPlaceholder:
+    """Base class for signature defaults that stand for a value rather than being one.
+
+    A default whose value is only known at call time (e.g. ``methodize``'s
+    ``default_from``, whose default is read from the instance) is shown in the
+    signature as an instance of a subclass. It is not a concrete value, so code that
+    serializes signature defaults (form generators, OpenAPI/JSON schemas) should treat
+    it as "no default": see :func:`is_placeholder_default`.
+    """
+
+
+def is_placeholder_default(x) -> bool:
+    """Return ``True`` if ``x`` is a signature default that is not a concrete value.
+
+    That is, the ``NotSet`` sentinel or a :class:`DefaultPlaceholder` (such as
+    ``i2.wrapper.InstanceAttrDefault``). Signature consumers that need a concrete,
+    serializable default (UI or schema generators) can treat these like
+    ``inspect.Parameter.empty``:
+
+    >>> from inspect import Parameter
+    >>> class Later(DefaultPlaceholder):
+    ...     pass
+    >>> is_placeholder_default(NotSet), is_placeholder_default(Later())
+    (True, True)
+    >>> is_placeholder_default(None), is_placeholder_default(Parameter.empty)
+    (False, False)
+    """
+    return is_not_set(x) or isinstance(x, DefaultPlaceholder)
+
+
 # ---------------------------------------------------------------------------------------
 # TODO: https://github.com/i2mint/i2/issues/48
 class FuncFactory:

i2/tests/test_partials_and_methodize.py

@@ -82,6 +82,28 @@ class K:
     assert (k.bar(3), k.bar(3, 10), k.bar(3, z=0)) == (7, 31, 1)
 
 
+def test_methodize_default_from_json_dump_of_signature_defaults():
+    """A default_from default is a placeholder: JSON consumers skip it, like NotSet."""
+    import json
+
+    from i2 import NotSet, is_placeholder_default
+
+    def foo(x, y, z, w=NotSet, v=1):
+        return x + y * z
+
+    @dataclass
+    class K:
+        x: int
+        zoo: int = 2
+        bar = methodize(foo, bind="x", default_from=[("z", "zoo")])
+
+    defaults = Sig(K(1).bar).defaults
+    with pytest.raises(TypeError):
+        json.dumps(defaults)  # what a naive consumer would meet
+    concrete = {k: v for k, v in defaults.items() if not is_placeholder_default(v)}
+    assert json.loads(json.dumps(concrete)) == {"v": 1}
+
+
 def test_methodize_keeps_self_before_position_only_params():
     """i2mint/i2#118: ``self`` used to land after the ``/``."""
 

i2/wrapper.py

@@ -95,7 +95,7 @@
 )
 from i2.errors import OverwritesNotAllowed
 from i2.multi_object import Pipe
-from i2.deco import double_up_as_factory
+from i2.deco import double_up_as_factory, DefaultPlaceholder
 from i2.util import deprecation_of
 
 # ---------------------------------------------------------------------------------------
@@ -1854,14 +1854,19 @@ def pairs():
     return param_to_attr
 
 
-class InstanceAttrDefault:
+class InstanceAttrDefault(DefaultPlaceholder):
     """The default of a method param that is sourced from an instance attribute.
 
     Shown in the method's signature as ``self.<attr>`` (see ``methodize``'s
-    ``default_from``).
+    ``default_from``). It is a ``DefaultPlaceholder``, not a value: its value depends
+    on the instance, which a schema or form generator does not have, so such code
+    should treat it as "no default" (``i2.is_placeholder_default``).
 
     >>> InstanceAttrDefault('zoo')
     self.zoo
+    >>> from i2 import is_placeholder_default
+    >>> is_placeholder_default(InstanceAttrDefault('zoo'))
+    True
     """
 
     def __init__(self, attr: str, *, instance_arg_name: str = "self"):
@@ -2175,8 +2180,8 @@ def methodize(
     16
 
     The signature default is an ``InstanceAttrDefault`` marker (shown ``self.zoo``),
-    not a value: code that serializes signature defaults (e.g. to JSON) should not
-    expect a JSON-able one there.
+    not a value: code that serializes signature defaults (e.g. to JSON) should skip
+    it, as it skips ``NotSet``, by testing ``i2.is_placeholder_default``.
     """
     return func_to_method_func(
         func,