methodize: keep self before position-only params

i2/tests/test_partials_and_methodize.py

@@ -1,5 +1,5 @@
 """Tests for partialx(_rm_partialize), curried, methodize(default_from) and the
-docstring parser fixes of i2mint/i2#111, #112, #113 and #116."""
+docstring parser fixes of i2mint/i2#111, #112, #113, #116 and #118."""
 
 import asyncio
 import inspect
@@ -82,6 +82,27 @@ class K:
     assert (k.bar(3), k.bar(3, 10), k.bar(3, z=0)) == (7, 31, 1)
 
 
+def test_methodize_keeps_self_before_position_only_params():
+    """i2mint/i2#118: ``self`` used to land after the ``/``."""
+
+    def foo(a, /, b, *, c=3):
+        return (a, b, c)
+
+    @dataclass
+    class K:
+        a: int = 1
+        b: int = 7
+        bound = methodize(foo, bind="b")
+        defaulted = methodize(foo, default_from="b")
+
+    assert str(inspect.signature(K.bound)) == "(self, a, /, *, c=3)"
+    assert str(inspect.signature(K.defaulted)) == "(self, a, /, b=self.b, *, c=3)"
+    k = K()
+    assert k.bound(5) == (5, 7, 3)
+    assert (k.defaulted(5), k.defaulted(5, 0)) == ((5, 7, 3), (5, 0, 3))
+    assert methodize(foo, bind="a")(k, 2) == (1, 2, 3)
+
+
 def test_methodize_default_from_rejects_variadic_and_unknown_params():
     def foo(x, *args, **kwargs):
         return x

i2/wrapper.py

@@ -2042,7 +2042,7 @@ def func_to_method_func(
         method_sig = _with_instance_attr_defaults(
             method_sig, default_from, instance_arg_name=instance_arg_name
         )
-    method_sig = instance_arg_name + method_sig
+    method_sig = _prepend_instance_param(method_sig, instance_arg_name)
     # make the ingress function that will map method_sig's interface to sig's.
     ingress = Ingress(
         inner_sig=sig,
@@ -2062,6 +2062,26 @@ def func_to_method_func(
     return method_func
 
 
+def _prepend_instance_param(sig: Sig, instance_arg_name: str = "self") -> Sig:
+    """Put the instance param first in ``sig``.
+
+    If ``sig`` has position-only params, the instance param is position-only too, so
+    it stays before the ``/`` (i2mint/i2#118).
+
+    >>> def f(a, /, b): ...
+    >>> _prepend_instance_param(Sig(f))
+    <Sig (self, a, /, b)>
+    >>> _prepend_instance_param(Sig(lambda b, *, c: 0))
+    <Sig (self, b, *, c)>
+    """
+    if PO not in sig.kinds.values():
+        return instance_arg_name + sig
+    return Sig(
+        [Parameter(instance_arg_name, PO), *sig.params],
+        return_annotation=sig.return_annotation,
+    )
+
+
 def _with_instance_attr_defaults(
     sig: Sig, default_from: dict, *, instance_arg_name: str = "self"
 ) -> Sig: