Fix conversion of non-mapping sequences into (i.e can be cast to ) du…

…e to returning true for sequences.

src/types/mapping.rs

@@ -101,7 +101,7 @@ impl<'v> PyTryFrom<'v> for PyMapping {
     fn try_from<V: Into<&'v PyAny>>(value: V) -> Result<&'v PyMapping, PyDowncastError<'v>> {
         let value = value.into();
         unsafe {
-            if ffi::PyMapping_Check(value.as_ptr()) != 0 {
+            if ffi::PyMapping_Check(value.as_ptr()) != 0 && ffi::PySequence_Check(value.as_ptr()) == 0 {
                 Ok(<PyMapping as PyTryFrom>::try_from_unchecked(value))
             } else {
                 Err(PyDowncastError::new(value, "Mapping"))
@@ -142,8 +142,8 @@ mod tests {
     use std::collections::HashMap;
 
     use crate::{
-        exceptions::PyKeyError,
-        types::{PyDict, PyTuple},
+        exceptions::{PyKeyError, PyTypeError},
+        types::{PyDict, PyTuple, PyString},
         Python,
     };
 
@@ -299,4 +299,12 @@ mod tests {
             assert_eq!(mapping_ref.get_refcnt(), 2);
         })
     }
+
+    #[test]
+    fn test_not_mapping() {
+        Python::with_gil(|py| {
+            let str = PyString::new(py, "hello, world");
+            str.downcast::<PyMapping>().unwrap_err();
+        });
+    }
 }