GH-47602: [Python] Make Schema hashable even when it has metadata

python/pyarrow/tests/test_schema.py

@@ -482,6 +482,28 @@ def test_schema_set_field():
     assert s3.field(0).nullable is False
 
 
+def test_schema_hash_metadata():
+    fields = [
+        pa.field("foo", pa.int32()),
+    ]
+
+    schema1 = pa.schema(fields, metadata={b'foo': b'bar'})
+    schema2 = pa.schema(fields, metadata={b'foo': b'bar'})
+    schema3 = pa.schema(fields, metadata={b'foo_different': b'bar'})
+    schema4 = pa.schema(fields, metadata={b'foo': b'bar_different'})
+
+    assert hash(schema1) == hash(schema2)
+    assert hash(schema1) != hash(schema3)
+    assert hash(schema1) != hash(schema4)
+    assert hash(schema3) != hash(schema4)
+
+    schema_empty1 = pa.schema(fields, metadata={})
+    schema_empty2 = pa.schema(fields, metadata=None)
+
+    assert hash(schema_empty1) == hash(schema_empty2)
+    assert hash(schema_empty1) != hash(schema1)
+
+
 def test_schema_equals():
     fields = [
         pa.field('foo', pa.int32()),

python/pyarrow/types.pxi

@@ -2918,7 +2918,8 @@ cdef class Schema(_Weakrefable):
         return schema, (list(self), self.metadata)
 
     def __hash__(self):
-        return hash((tuple(self), self.metadata))
+        metadata = frozenset(self.metadata.items() if self.metadata else {})
+        return hash((tuple(self), metadata))
 
     def __sizeof__(self):
         size = 0