fix(python-fastapi): map binary response type file to bytes (#24285)

Fixes invalid `-> file` / `"model": file` for octet-stream responses (#20775).

Author: JerrySLau

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/PythonFastAPIServerCodegen.java

@@ -101,6 +101,11 @@ public PythonFastAPIServerCodegen() {
         languageSpecificPrimitives.add("Dict");
         typeMapping.put("array", "List");
         typeMapping.put("map", "Dict");
+        // Binary response body: map OAS file/binary to built-in bytes (not the invalid Py2 type `file`).
+        // Multipart upload fields remain UploadFile via overrideFileFormParamTyping (#23793).
+        // See https://github.com/OpenAPITools/openapi-generator/issues/20775
+        typeMapping.put("file", "bytes");
+        typeMapping.put("binary", "bytes");
 
         outputFolder = "generated-code" + File.separator + NAME;
         modelTemplateFiles.put("model.mustache", ".py");

modules/openapi-generator/src/test/java/org/openapitools/codegen/python/PythonFastAPIServerCodegenTest.java

@@ -136,4 +136,23 @@ public void testBinaryMultipartFieldUsesUploadFile() throws IOException {
         assertFileContains(baseApi, "csv_file: UploadFile,");
         assertFileContains(baseApi, "image: Optional[UploadFile],");
     }
+
+    @Test(description = "binary response body is typed as bytes, not invalid file (#20775)")
+    public void testBinaryResponseUsesBytesNotFile() throws IOException {
+        final DefaultCodegen codegen = new PythonFastAPIServerCodegen();
+        final String outputPath = generateFiles(codegen, "src/test/resources/3_0/issue_20775.yaml");
+        final Path api = Paths.get(outputPath + "src/openapi_server/apis/resource_api.py");
+        final Path baseApi = Paths.get(outputPath + "src/openapi_server/apis/resource_api_base.py");
+
+        assertFileExists(api);
+        assertFileExists(baseApi);
+
+        assertFileContains(api, "-> bytes");
+        assertFileContains(api, "\"model\": bytes");
+        assertFileNotContains(api, "-> file");
+        assertFileNotContains(api, "\"model\": file");
+
+        assertFileContains(baseApi, "-> bytes");
+        assertFileNotContains(baseApi, "-> file");
+    }
 }

modules/openapi-generator/src/test/java/org/openapitools/codegen/python/PythonFastapiCodegenTest.java

@@ -119,4 +119,32 @@ public void testRegexPatternCheckedForArrayItems() throws IOException {
         TestUtils.assertFileContains(Paths.get(output + "/src/openapi_server/models/test_object.py"),
                 "raise ValueError(r\"must validate the regular expression /^[A-Z0-9_\\- ]+$/\")");
     }
+
+    @Test
+    public void testBinaryResponseUsesBytesNotFile() throws IOException {
+        // Regression for https://github.com/OpenAPITools/openapi-generator/issues/20775
+        // Repro: src/test/resources/3_0/issue_20775.yaml
+        File output = Files.createTempDirectory("test").toFile();
+        output.deleteOnExit();
+
+        final CodegenConfigurator configurator = new CodegenConfigurator()
+                .setGeneratorName("python-fastapi")
+                .setOutputDir(output.getAbsolutePath().replace("\\", "/"))
+                .setInputSpec("src/test/resources/3_0/issue_20775.yaml");
+
+        DefaultGenerator generator = new DefaultGenerator();
+        List<File> files = generator.opts(configurator.toClientOptInput()).generate();
+        files.forEach(File::deleteOnExit);
+
+        final String apiFile = output + "/src/openapi_server/apis/resource_api.py";
+        final String baseApiFile = output + "/src/openapi_server/apis/resource_api_base.py";
+
+        TestUtils.assertFileContains(Paths.get(apiFile), "-> bytes");
+        TestUtils.assertFileContains(Paths.get(apiFile), "\"model\": bytes");
+        TestUtils.assertFileNotContains(Paths.get(apiFile), "-> file");
+        TestUtils.assertFileNotContains(Paths.get(apiFile), "\"model\": file");
+
+        TestUtils.assertFileContains(Paths.get(baseApiFile), "-> bytes");
+        TestUtils.assertFileNotContains(Paths.get(baseApiFile), "-> file");
+    }
 }

modules/openapi-generator/src/test/resources/3_0/issue_20775.yaml

@@ -0,0 +1,37 @@
+openapi: 3.0.1
+info:
+  title: binary response typing
+  version: 1.0.0
+paths:
+  /resource:
+    get:
+      tags:
+        - resource
+      description: Binary response body (full or partial)
+      operationId: resourceInResponse
+      parameters:
+        - name: Range
+          in: header
+          required: false
+          schema:
+            type: string
+      responses:
+        "200":
+          description: Full binary body
+          content:
+            application/octet-stream:
+              schema:
+                type: string
+                format: binary
+        "206":
+          description: Partial binary body
+          headers:
+            Content-Range:
+              required: true
+              schema:
+                type: string
+          content:
+            application/octet-stream:
+              schema:
+                type: string
+                format: binary