Change wording of PyDowncastError display implementation

Displays type(obj) instead of repr(obj) and uses `cannot` instead of
`can't`
to be more consistent with existing python error messages.

See discussion at #1212.

guide/src/conversions/traits.md

@@ -171,8 +171,7 @@ enum RustyEnum {
 ```
 
 If the input is neither a string nor an integer, the error message will be:
-`"Can't convert <INPUT> to Union[str, int]"`, where `<INPUT>` is replaced by the type name and
+`"'<INPUT_TYPE>' cannot be converted to 'Union[str, int]'"`.
-`repr()` of the input object.
 
 #### `#[derive(FromPyObject)]` Container Attributes
 - `pyo3(transparent)`

pyo3-derive-backend/src/from_pyobject.rs

@@ -73,11 +73,7 @@ impl<'a> Enum<'a> {
         quote!(
             #(#var_extracts)*
             let type_name = obj.get_type().name();
-            let from = obj
+            let err_msg = format!("'{}' object cannot be converted to '{}'", type_name, #error_names);
-                .repr()
-                .map(|s| format!("{} ({})", s.to_string_lossy(), type_name))
-                .unwrap_or_else(|_| type_name.to_string());
-            let err_msg = format!("Can't convert {} to {}", from, #error_names);
             Err(::pyo3::exceptions::PyTypeError::new_err(err_msg))
         )
     }

src/err/mod.rs

@@ -490,11 +490,8 @@ impl<'a> std::fmt::Display for PyDowncastError<'a> {
     fn fmt(&self, f: &mut std::fmt::Formatter) -> Result<(), std::fmt::Error> {
         write!(
             f,
-            "Can't convert {} to {}",
+            "'{}' object cannot be converted to '{}'",
-            self.from
+            self.from.get_type().name(),
-                .repr()
-                .map(|s| s.to_string_lossy())
-                .unwrap_or_else(|_| self.from.get_type().name()),
             self.to
         )
     }

tests/test_frompyobject.rs

@@ -296,6 +296,6 @@ fn test_err_rename() {
     assert!(f.is_err());
     assert_eq!(
         f.unwrap_err().to_string(),
-        "TypeError: Can't convert {} (dict) to Union[str, uint, int]"
+        "TypeError: 'dict' object cannot be converted to 'Union[str, uint, int]'"
     );
 }

tests/test_pyfunction.rs

@@ -139,14 +139,14 @@ fn test_conversion_error() {
         conversion_error,
         "conversion_error(None, None, None, None)",
         PyTypeError,
-        "argument 'str_arg': Can't convert None to PyString"
+        "argument 'str_arg': 'NoneType' object cannot be converted to 'PyString'"
     );
     py_expect_exception!(
         py,
         conversion_error,
         "conversion_error(100, None, None, None)",
         PyTypeError,
-        "argument 'str_arg': Can't convert 100 to PyString"
+        "argument 'str_arg': 'int' object cannot be converted to 'PyString'"
     );
     py_expect_exception!(
         py,
@@ -160,7 +160,7 @@ fn test_conversion_error() {
         conversion_error,
         "conversion_error('string1', -100, 'string2', None)",
         PyTypeError,
-        "argument 'tuple_arg': Can't convert 'string2' to PyTuple"
+        "argument 'tuple_arg': 'str' object cannot be converted to 'PyTuple'"
     );
     py_expect_exception!(
         py,