GH-43388: [Python] Give precedence to pycapsule interface in pa.schem…

…a(..) (#43486)

### Rationale for this change

When converting an object to a pyarrow.Schema with `pa.schema(..)`, first check the capsule interface before checking for mappings. This allows other schema-like objects that expose the capsule interface to also be mappings.

### Are these changes tested?
Yes

* GitHub Issue: #43388

Authored-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>
Signed-off-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>

python/pyarrow/tests/test_types.py

@@ -16,7 +16,7 @@
 # under the License.
 
 from collections import OrderedDict
-from collections.abc import Iterator
+from collections.abc import Iterator, Mapping
 from functools import partial
 import datetime
 import sys
@@ -1325,17 +1325,36 @@ def test_types_come_back_with_specific_type():
         assert type(type_back) is type(arrow_type)
 
 
-def test_schema_import_c_schema_interface():
-    class Wrapper:
-        def __init__(self, schema):
-            self.schema = schema
+class SchemaWrapper:
+    def __init__(self, schema):
+        self.schema = schema
+
+    def __arrow_c_schema__(self):
+        return self.schema.__arrow_c_schema__()
+
+
+class SchemaMapping(Mapping):
+    def __init__(self, schema):
+        self.schema = schema
+
+    def __arrow_c_schema__(self):
+        return self.schema.__arrow_c_schema__()
+
+    def __getitem__(self, key):
+        return self.schema[key]
+
+    def __iter__(self):
+        return iter(self.schema)
+
+    def __len__(self):
+        return len(self.schema)
 
-        def __arrow_c_schema__(self):
-            return self.schema.__arrow_c_schema__()
 
+@pytest.mark.parametrize("wrapper_class", [SchemaWrapper, SchemaMapping])
+def test_schema_import_c_schema_interface(wrapper_class):
     schema = pa.schema([pa.field("field_name", pa.int32())], metadata={"a": "b"})
     assert schema.metadata == {b"a": b"b"}
-    wrapped_schema = Wrapper(schema)
+    wrapped_schema = wrapper_class(schema)
 
     assert pa.schema(wrapped_schema) == schema
     assert pa.schema(wrapped_schema).metadata == {b"a": b"b"}

python/pyarrow/types.pxi

@@ -5347,14 +5347,15 @@ def schema(fields, metadata=None):
         Field py_field
         vector[shared_ptr[CField]] c_fields
 
-    if isinstance(fields, Mapping):
-        fields = fields.items()
-    elif hasattr(fields, "__arrow_c_schema__"):
+    if hasattr(fields, "__arrow_c_schema__"):
         result = Schema._import_from_c_capsule(fields.__arrow_c_schema__())
         if metadata is not None:
             result = result.with_metadata(metadata)
         return result
 
+    if isinstance(fields, Mapping):
+        fields = fields.items()
+
     for item in fields:
         if isinstance(item, tuple):
             py_field = field(*item)