Repository: amaranth-lang/amaranth (python)
Issue: `Signature.is_compliant()` should ensure that its argument is an interface object
It currently works with any Python object with a matching set of attributes.
Pull request description: lib.wiring: in `is_compliant(sig, obj)`, check that `obj` is an interface object with that signature
Fixes #935.

This behavior isn't clearly specified one way or another in the RFC, and the implementation in the PR feels more appropriate to me.

Patch:
diff --git a/amaranth/lib/wiring.py b/amaranth/lib/wiring.py
index bb8b4cb..624d98a 100644
--- a/amaranth/lib/wiring.py
+++ b/amaranth/lib/wiring.py
@@ -405,6 +405,21 @@ class Signature(metaclass=SignatureMeta):
             yield from iter_dimensions(value, dimensions=member.dimensions, path=path)
 
     def is_compliant(self, obj, *, reasons=None, path=("obj",)):
+        if not hasattr(obj, "signature"):
+            if reasons is not None:
+                reasons.append(f"{_format_path(path)} does not have an attribute 'signature'")
+            return False
+        if not isinstance(obj.signature, Signature):
+            if reasons is not None:
+                reasons.append(f"{_format_path(path + ('signature',))} is expected to be "
+                               f"a signature, but it is a {obj.signature!r}")
+            return False
+        if self != obj.signature:
+            if reasons is not None:
+                reasons.append(f"{_format_path(path + ('signature',))} is expected to be equal "
+                               f"to this signature, {self!r}, but it is a {obj.signature!r}")
+            return False
+
         def check_attr_value(member, attr_value, *, path):
             if member.is_port:
                 try:


Test patch:
diff --git a/tests/test_lib_wiring.py b/tests/test_lib_wiring.py
index c08f5dd..2ee96c2 100644
--- a/tests/test_lib_wiring.py
+++ b/tests/test_lib_wiring.py
@@ -416,7 +416,32 @@ class SignatureTestCase(unittest.TestCase):
             (("d", "s"), In (1), intf.d.s),
         ])
 
+    def test_is_compliant_signature(self):
+        sig = Signature({})
+
+        obj1 = NS()
+        self.assertFalse(sig.is_compliant(obj1))
+        reasons = []
+        self.assertFalse(sig.is_compliant(obj1, reasons=reasons))
+        self.assertEqual(reasons, ["'obj' does not have an attribute 'signature'"])
+
+        obj = NS(signature=1)
+        self.assertFalse(sig.is_compliant(obj))
+        reasons = []
+        self.assertFalse(sig.is_compliant(obj, reasons=reasons))
+        self.assertEqual(reasons, ["'obj.signature' is expected to be a signature, but it is a 1"])
+
+        obj = NS(signature=Signature({"a": In(1)}))
+        self.assertFalse(sig.is_compliant(obj))
+        reasons = []
+        self.assertFalse(sig.is_compliant(obj, reasons=reasons))
+        self.assertEqual(reasons, [
+            "'obj.signature' is expected to be equal to this signature, "
+            "Signature({}), but it is a Signature({'a': In(1)})"
+        ])
+
     def assertNotCompliant(self, reason_regex, sig, obj):
+        obj.signature = sig
         self.assertFalse(sig.is_compliant(obj))
         reasons = []
         self.assertFalse(sig.is_compliant(obj, reasons=reasons))
@@ -474,25 +499,32 @@ class SignatureTestCase(unittest.TestCase):
         self.assertNotCompliant(
             r"^'obj\.a' does not have an attribute 'b'$",
             sig=Signature({"a": Out(Signature({"b": In(1)}))}),
-            obj=NS(a=Signal()))
+            obj=NS(a=NS(signature=Signature({"b": In(1)}))))
         self.assertTrue(
             Signature({"a": In(1)}).is_compliant(
-                NS(a=Signal())))
+                NS(signature=Signature({"a": In(1)}),
+                   a=Signal())))
         self.assertTrue(
             Signature({"a": In(1)}).is_compliant(
-                NS(a=Const(1))))
+                NS(signature=Signature({"a": In(1)}),
+                   a=Const(1))))
         self.assertTrue( # list
             Signature({"a": In(1).array(2, 2)}).is_compliant(
-                NS(a=[[Const(1), Const(1)], [Signal(), Signal()]])))
+                NS(signature=Signature({"a": In(1).array(2, 2)}),
+                   a=[[Const(1), Const(1)], [Signal(), Signal()]])))
         self.assertTrue( # tuple
             Signature({"a": In(1).array(2, 2)}).is_compliant(
-                NS(a=((Const(1), Const(1)), (Signal(), Signal())))))
+                NS(signature=Signature({"a": In(1).array(2, 2)}),
+                   a=((Const(1), Const(1)), (Signal(), Signal())))))
         self.assertTrue( # mixed list and tuple
             Signature({"a": In(1).array(2, 2)}).is_compliant(
-                NS(a=[[Const(1), Const(1)], (Signal(), Signal())])))
+                NS(signature=Signature({"a": In(1).array(2, 2)}),
+                   a=[[Const(1), Const(1)], (Signal(), Signal())])))
         self.assertTrue(
             Signature({"a": Out(Signature({"b": In(1)}))}).is_compliant(
-                NS(a=NS(b=Signal()))))
+                NS(signature=Signature({"a": Out(Signature({"b": In(1)}))}),
+                   a=NS(signature=Signature({"b": In(1)}),
+                        b=Signal()))))
 
     def test_repr(self):
         sig = Signature({"a": In(1)})
@@ -933,9 +965,9 @@ class ConnectTestCase(unittest.TestCase):
         m = Module()
         connect(m,
                 p=NS(signature=Signature({"a": Out(Signature({"f": Out(1)}))}),
-                     a=NS(f=Signal(name='p__a'))),
+                     a=NS(signature=Signature({"f": Out(1)}), f=Signal(name='p__a'))),
                 q=NS(signature=Signature({"a": In(Signature({"f": Out(1)}))}),
-                     a=NS(f=Signal(name='q__a'))))
+                     a=NS(signature=Signature({"f": Out(1)}).flip(), f=Signal(name='q__a'))))
         self.assertEqual([repr(stmt) for stmt in m._statements], [
             '(eq (sig q__a) (sig p__a))'
         ])