change CastError to describe the isinstance failure (#5693)

* change `CastError` to describe the `isinstance` failure

* special case `None` in cast error display

* newsfragment

* update error messages in tests

Author: davidhewitt

guide/src/conversions/traits.md

@@ -444,7 +444,7 @@ enum RustyEnum {
 # }
 ```
 
-If the input is neither a string nor an integer, the error message will be: `"'<INPUT_TYPE>' cannot be cast as 'str | int'"`.
+If the input is neither a string nor an integer, the error message will be: `"'<INPUT_TYPE>' is not an instance of 'str | int'"`.
 
 ### `#[derive(FromPyObject)]` Container Attributes
 

newsfragments/5693.changed.md

@@ -0,0 +1 @@
+Change `CastError` formatted message to directly describe the "is not an instance of" failure condition.

src/conversions/chrono.rs

@@ -780,37 +780,37 @@ mod tests {
             let none = py.None().into_bound(py);
             assert_eq!(
                 none.extract::<Duration>().unwrap_err().to_string(),
-                "TypeError: 'NoneType' object cannot be cast as 'timedelta'"
+                "TypeError: 'None' is not an instance of 'timedelta'"
             );
             assert_eq!(
                 none.extract::<FixedOffset>().unwrap_err().to_string(),
-                "TypeError: 'NoneType' object cannot be cast as 'tzinfo'"
+                "TypeError: 'None' is not an instance of 'tzinfo'"
             );
             assert_eq!(
                 none.extract::<Utc>().unwrap_err().to_string(),
                 "ValueError: expected datetime.timezone.utc"
             );
             assert_eq!(
                 none.extract::<NaiveTime>().unwrap_err().to_string(),
-                "TypeError: 'NoneType' object cannot be cast as 'time'"
+                "TypeError: 'None' is not an instance of 'time'"
             );
             assert_eq!(
                 none.extract::<NaiveDate>().unwrap_err().to_string(),
-                "TypeError: 'NoneType' object cannot be cast as 'date'"
+                "TypeError: 'None' is not an instance of 'date'"
             );
             assert_eq!(
                 none.extract::<NaiveDateTime>().unwrap_err().to_string(),
-                "TypeError: 'NoneType' object cannot be cast as 'datetime'"
+                "TypeError: 'None' is not an instance of 'datetime'"
             );
             assert_eq!(
                 none.extract::<DateTime<Utc>>().unwrap_err().to_string(),
-                "TypeError: 'NoneType' object cannot be cast as 'datetime'"
+                "TypeError: 'None' is not an instance of 'datetime'"
             );
             assert_eq!(
                 none.extract::<DateTime<FixedOffset>>()
                     .unwrap_err()
                     .to_string(),
-                "TypeError: 'NoneType' object cannot be cast as 'datetime'"
+                "TypeError: 'None' is not an instance of 'datetime'"
             );
         });
     }

src/conversions/jiff.rs

@@ -580,31 +580,31 @@ mod tests {
             let none = py.None().into_bound(py);
             assert_eq!(
                 none.extract::<Span>().unwrap_err().to_string(),
-                "TypeError: 'NoneType' object cannot be cast as 'timedelta'"
+                "TypeError: 'None' is not an instance of 'timedelta'"
             );
             assert_eq!(
                 none.extract::<Offset>().unwrap_err().to_string(),
-                "TypeError: 'NoneType' object cannot be cast as 'tzinfo'"
+                "TypeError: 'None' is not an instance of 'tzinfo'"
             );
             assert_eq!(
                 none.extract::<TimeZone>().unwrap_err().to_string(),
-                "TypeError: 'NoneType' object cannot be cast as 'tzinfo'"
+                "TypeError: 'None' is not an instance of 'tzinfo'"
             );
             assert_eq!(
                 none.extract::<Time>().unwrap_err().to_string(),
-                "TypeError: 'NoneType' object cannot be cast as 'time'"
+                "TypeError: 'None' is not an instance of 'time'"
             );
             assert_eq!(
                 none.extract::<Date>().unwrap_err().to_string(),
-                "TypeError: 'NoneType' object cannot be cast as 'date'"
+                "TypeError: 'None' is not an instance of 'date'"
             );
             assert_eq!(
                 none.extract::<DateTime>().unwrap_err().to_string(),
-                "TypeError: 'NoneType' object cannot be cast as 'datetime'"
+                "TypeError: 'None' is not an instance of 'datetime'"
             );
             assert_eq!(
                 none.extract::<Zoned>().unwrap_err().to_string(),
-                "TypeError: 'NoneType' object cannot be cast as 'datetime'"
+                "TypeError: 'None' is not an instance of 'datetime'"
             );
         });
     }

src/conversions/smallvec.rs

@@ -138,7 +138,7 @@ mod tests {
             let sv: PyResult<SmallVec<[u64; 8]>> = dict.extract();
             assert_eq!(
                 sv.unwrap_err().to_string(),
-                "TypeError: 'dict' object cannot be cast as 'Sequence'"
+                "TypeError: 'dict' object is not an instance of 'Sequence'"
             );
         });
     }