diff --git a/rust/hg-cpython/src/discovery.rs b/rust/hg-cpython/src/discovery.rs
--- a/rust/hg-cpython/src/discovery.rs
+++ b/rust/hg-cpython/src/discovery.rs
@@ -29,10 +29,14 @@
 py_class!(pub class PartialDiscovery |py| {
     data inner: RefCell<Box<CorePartialDiscovery<Index>>>;
 
+    // `_respectsize` is currently only here to replicate the Python API and
+    // will be used in future patches inside methods that are yet to be
+    // implemented.
     def __new__(
         _cls,
         index: PyObject,
-        targetheads: PyObject
+        targetheads: PyObject,
+        _respectsize: bool
     ) -> PyResult<PartialDiscovery> {
         Self::create_instance(
             py,
diff --git a/tests/test-rust-discovery.py b/tests/test-rust-discovery.py
--- a/tests/test-rust-discovery.py
+++ b/tests/test-rust-discovery.py
@@ -61,7 +61,7 @@
 
     def testaddcommonsmissings(self):
         idx = self.parseindex()
-        disco = PartialDiscovery(idx, [3])
+        disco = PartialDiscovery(idx, [3], True)
         self.assertFalse(disco.hasinfo())
         self.assertFalse(disco.iscomplete())
 
@@ -77,7 +77,7 @@
 
     def testaddmissingsstats(self):
         idx = self.parseindex()
-        disco = PartialDiscovery(idx, [3])
+        disco = PartialDiscovery(idx, [3], True)
         self.assertIsNone(disco.stats()['undecided'], None)
 
         disco.addmissings([2])
@@ -85,7 +85,7 @@
 
     def testaddinfocommonfirst(self):
         idx = self.parseindex()
-        disco = PartialDiscovery(idx, [3])
+        disco = PartialDiscovery(idx, [3], True)
         disco.addinfo([(1, True), (2, False)])
         self.assertTrue(disco.hasinfo())
         self.assertTrue(disco.iscomplete())
@@ -93,7 +93,7 @@
 
     def testaddinfomissingfirst(self):
         idx = self.parseindex()
-        disco = PartialDiscovery(idx, [3])
+        disco = PartialDiscovery(idx, [3], True)
         disco.addinfo([(2, False), (1, True)])
         self.assertTrue(disco.hasinfo())
         self.assertTrue(disco.iscomplete())