Use method type annotation to determine schema type for method field

rest_framework/schemas/openapi.py

@@ -1,3 +1,4 @@
+import inspect
 import re
 import warnings
 from collections import OrderedDict
@@ -359,6 +360,16 @@ class AutoSchema(ViewInspector):
         return mapping
 
     def map_field(self, field):
+        if isinstance(field, serializers.SerializerMethodField) and field.parent and field.method_name:
+            return_type = inspect.signature(
+                getattr(field.parent, field.method_name)
+            ).return_annotation
+            if issubclass(return_type, bool):
+                return {'type': 'boolean'}
+            if issubclass(return_type, float):
+                return {'type': 'number'}
+            if issubclass(return_type, int):
+                return {'type': 'integer'}
 
         # Nested Serializers, `many` or not.
         if isinstance(field, serializers.ListSerializer):

tests/schemas/test_openapi.py

@@ -54,6 +54,25 @@ class TestFieldMapping(TestCase):
         uuid1 = uuid.uuid4()
         uuid2 = uuid.uuid4()
         inspector = AutoSchema()
+
+        class TestSerializer(serializers.Serializer):
+            unannotated = serializers.SerializerMethodField()
+            annotated_int = serializers.SerializerMethodField()
+            annotated_float = serializers.SerializerMethodField()
+            annotated_bool = serializers.SerializerMethodField()
+
+            def get_unannotated(self):
+                return 'blub'
+
+            def get_annotated_int(self) -> int:
+                return 1
+
+            def get_annotated_float(self) -> float:
+                return 1.0
+
+            def get_annotated_bool(self) -> bool:
+                return True
+
         cases = [
             (serializers.ListField(), {'items': {}, 'type': 'array'}),
             (serializers.ListField(child=serializers.BooleanField()), {'items': {'type': 'boolean'}, 'type': 'array'}),
@@ -83,6 +102,11 @@ class TestFieldMapping(TestCase):
                 {'items': {'enum': [1, 2, 3], 'type': 'integer'}, 'type': 'array'}),
             (serializers.IntegerField(min_value=2147483648),
              {'type': 'integer', 'minimum': 2147483648, 'format': 'int64'}),
+            (serializers.SerializerMethodField(), {'type': 'string'}),
+            (TestSerializer().fields['unannotated'], {'type': 'string'}),
+            (TestSerializer().fields['annotated_int'], {'type': 'integer'}),
+            (TestSerializer().fields['annotated_float'], {'type': 'number'}),
+            (TestSerializer().fields['annotated_bool'], {'type': 'boolean'}),
         ]
         for field, mapping in cases:
             with self.subTest(field=field):