Add PyDowncastErrorArguments to delay formatting downcast errors.

CHANGELOG.md

@@ -11,6 +11,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Changed
 
 - Default to "m" ABI tag when choosing `libpython` link name for CPython 3.7 on Unix. [#2288](https://github.com/PyO3/pyo3/pull/2288)
+- Improved performance of failing calls to `FromPyObject::extract` which is common when functions accept multiple distinct types. [#2279](https://github.com/PyO3/pyo3/pull/2279)
 
 ## [0.16.3] - 2022-04-05
 

src/err/mod.rs

@@ -663,10 +663,34 @@ impl<'a> IntoPy<PyObject> for &'a PyErr {
     }
 }
 
+struct PyDowncastErrorArguments {
+    from: Py<PyType>,
+    to: Cow<'static, str>,
+}
+
+impl PyErrArguments for PyDowncastErrorArguments {
+    fn arguments(self, py: Python<'_>) -> PyObject {
+        format!(
+            "'{}' object cannot be converted to '{}'",
+            self.from
+                .as_ref(py)
+                .name()
+                .unwrap_or("<failed to extract type name>"),
+            self.to
+        )
+        .to_object(py)
+    }
+}
+
 /// Convert `PyDowncastError` to Python `TypeError`.
 impl<'a> std::convert::From<PyDowncastError<'a>> for PyErr {
     fn from(err: PyDowncastError<'_>) -> PyErr {
-        exceptions::PyTypeError::new_err(err.to_string())
+        let args = PyDowncastErrorArguments {
+            from: err.from.get_type().into(),
+            to: err.to,
+        };
+
+        exceptions::PyTypeError::new_err(args)
     }
 }