Add inferred union variant names (#3442)

* Add inferred union variant names

* Fix union variant name CI checks

* Update prompt expected output

* Cover union variant name helpers

* style: auto-fix by pre-commit.ci

* Address union variant name review

* style: auto-fix by pre-commit.ci

* Cover union variant name fallback

* Satisfy CodeQL for variant helpers

* Keep union naming off hot path

* Avoid duplicate inferred variant names

* Clarify inferred union variant example

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: koxudaxi

docs/cli-reference/field-customization.md

@@ -14,6 +14,7 @@
 | [`--field-extra-keys-without-x-prefix`](#field-extra-keys-without-x-prefix) | Include schema extension keys in Field() without requiring '... |
 | [`--field-include-all-keys`](#field-include-all-keys) | Include all schema keys in Field() json_schema_extra. |
 | [`--field-type-collision-strategy`](#field-type-collision-strategy) | Rename type class instead of field when names collide (Pydan... |
+| [`--infer-union-variant-names`](#infer-union-variant-names) | Infer names for inline oneOf/anyOf object variants from lite... |
 | [`--no-alias`](#no-alias) | Disable Field alias generation for non-Python-safe property ... |
 | [`--original-field-name-delimiter`](#original-field-name-delimiter) | Specify delimiter for original field names when using snake-... |
 | [`--remove-special-field-name-prefix`](#remove-special-field-name-prefix) | Remove the special prefix from field names. |
@@ -1742,6 +1743,123 @@ to preserve the original field name, instead of renaming the field and adding an
 
 ---
 
+## `--infer-union-variant-names` {#infer-union-variant-names}
+
+Infer names for inline oneOf/anyOf object variants from literal fields.
+
+The `--infer-union-variant-names` flag uses branch-local `const` values or
+single-value enums on a shared discriminator-style property to name generated
+inline union models. This is useful when inline union branches would otherwise
+receive position-based names such as `Event` and `Event1`.
+
+The literal value can also come from a single-value enum or an internal `$ref`.
+Existing generated output is preserved unless this option is enabled.
+
+**Related:** [`--use-title-as-name`](field-customization.md#use-title-as-name)
+
+!!! tip "Usage"
+
+    ```bash
+    datamodel-codegen --input schema.json --infer-union-variant-names # (1)!
+    ```
+
+    1. :material-arrow-left: `--infer-union-variant-names` - the option documented here
+
+??? example "Examples"
+
+    **Input Schema:**
+
+    ```json
+    {
+      "$schema": "http://json-schema.org/draft-07/schema#",
+      "title": "WebhookEnvelope",
+      "type": "object",
+      "required": ["event"],
+      "properties": {
+        "event": {
+          "oneOf": [
+            {
+              "type": "object",
+              "required": ["type", "id"],
+              "properties": {
+                "type": {"const": "message.created"},
+                "id": {"type": "string"}
+              }
+            },
+            {
+              "type": "object",
+              "required": ["type", "reason"],
+              "properties": {
+                "type": {"const": "message.failed"},
+                "reason": {"type": "string"}
+              }
+            }
+          ]
+        }
+      }
+    }
+    ```
+
+    **Output:**
+
+    === "With Option"
+
+        ```python
+        # generated by datamodel-codegen:
+        #   filename:  infer_union_variant_names.json
+        #   timestamp: 2019-07-26T00:00:00+00:00
+
+        from __future__ import annotations
+
+        from typing import Literal
+
+        from pydantic import BaseModel
+
+
+        class EventMessageCreated(BaseModel):
+            type: Literal['message.created']
+            id: str
+
+
+        class EventMessageFailed(BaseModel):
+            type: Literal['message.failed']
+            reason: str
+
+
+        class WebhookEnvelope(BaseModel):
+            event: EventMessageCreated | EventMessageFailed
+        ```
+
+    === "Without Option"
+
+        ```python
+        # generated by datamodel-codegen:
+        #   filename:  infer_union_variant_names.json
+        #   timestamp: 2019-07-26T00:00:00+00:00
+
+        from __future__ import annotations
+
+        from typing import Literal
+
+        from pydantic import BaseModel
+
+
+        class Event(BaseModel):
+            type: Literal['message.created']
+            id: str
+
+
+        class Event1(BaseModel):
+            type: Literal['message.failed']
+            reason: str
+
+
+        class WebhookEnvelope(BaseModel):
+            event: Event | Event1
+        ```
+
+---
+
 ## `--no-alias` {#no-alias}
 
 Disable Field alias generation for non-Python-safe property names.

docs/cli-reference/index.md

@@ -10,7 +10,7 @@ This documentation is auto-generated from test cases.
 |----------|---------|-------------|
 | 📁 [Base Options](base-options.md) | 11 | Input/output configuration |
 | 🔧 [Typing Customization](typing-customization.md) | 30 | Type annotation and import behavior |
-| 🏷️ [Field Customization](field-customization.md) | 26 | Field naming and docstring behavior |
+| 🏷️ [Field Customization](field-customization.md) | 27 | Field naming and docstring behavior |
 | 🏗️ [Model Customization](model-customization.md) | 41 | Model generation behavior |
 | 🎨 [Template Customization](template-customization.md) | 22 | Output formatting and custom rendering |
 | 📘 [OpenAPI-only Options](openapi-only-options.md) | 8 | OpenAPI-specific features |
@@ -118,6 +118,7 @@ This documentation is auto-generated from test cases.
 - [`--ignore-enum-constraints`](typing-customization.md#ignore-enum-constraints)
 - [`--ignore-pyproject`](general-options.md#ignore-pyproject)
 - [`--include-path-parameters`](openapi-only-options.md#include-path-parameters)
+- [`--infer-union-variant-names`](field-customization.md#infer-union-variant-names)
 - [`--input`](base-options.md#input)
 - [`--input-file-type`](base-options.md#input-file-type)
 - [`--input-model`](base-options.md#input-model)

docs/cli-reference/quick-reference.md

@@ -77,6 +77,7 @@ datamodel-codegen [OPTIONS]
 | [`--field-extra-keys-without-x-prefix`](field-customization.md#field-extra-keys-without-x-prefix) | Include schema extension keys in Field() without requiring 'x-' prefix. |
 | [`--field-include-all-keys`](field-customization.md#field-include-all-keys) | Include all schema keys in Field() json_schema_extra. |
 | [`--field-type-collision-strategy`](field-customization.md#field-type-collision-strategy) | Rename type class instead of field when names collide (Pydantic v2 only). |
+| [`--infer-union-variant-names`](field-customization.md#infer-union-variant-names) | Infer names for inline oneOf/anyOf object variants from literal fields. |
 | [`--no-alias`](field-customization.md#no-alias) | Disable Field alias generation for non-Python-safe property names. |
 | [`--original-field-name-delimiter`](field-customization.md#original-field-name-delimiter) | Specify delimiter for original field names when using snake-case conversion. |
 | [`--remove-special-field-name-prefix`](field-customization.md#remove-special-field-name-prefix) | Remove the special prefix from field names. |
@@ -299,6 +300,7 @@ All options sorted alphabetically:
 - [`--ignore-enum-constraints`](typing-customization.md#ignore-enum-constraints) - Ignore enum constraints and use base string type instead of ...
 - [`--ignore-pyproject`](general-options.md#ignore-pyproject) - Ignore pyproject.toml configuration file.
 - [`--include-path-parameters`](openapi-only-options.md#include-path-parameters) - Include OpenAPI path parameters in generated parameter model...
+- [`--infer-union-variant-names`](field-customization.md#infer-union-variant-names) - Infer names for inline oneOf/anyOf object variants from lite...
 - [`--input`](base-options.md#input) - Specify the input schema file path.
 - [`--input-file-type`](base-options.md#input-file-type) - Specify the input file type for code generation.
 - [`--input-model`](base-options.md#input-model) - Import a Python type or dict schema from a module.

skills/datamodel-code-generator/references/cli-options.md

@@ -69,6 +69,7 @@ Field naming, aliases, defaults, and constraints.
 - `--extra-fields`: Set the generated models to allow, forbid, or ignore extra fields. Choices: `allow`, `ignore`, `forbid`.
 - `--use-schema-description`: Use schema description to populate class docstring
 - `--use-title-as-name`: use titles as class names of models
+- `--infer-union-variant-names`: Infer inline oneOf/anyOf branch model names from literal discriminator-style fields
 - `--field-constraints`: Use field constraints and not con* annotations
 - `--set-default-enum-member`: Set enum members as default values for enum field
 - `--use-enum-values-in-discriminator`: Use enum member literals in discriminator fields instead of string literals

src/datamodel_code_generator/__main__.py

@@ -1113,6 +1113,7 @@ def run_generate_from_config(  # noqa: PLR0913, PLR0917
         graphql_no_typename=config.graphql_no_typename,
         wrap_string_literal=config.wrap_string_literal,
         use_title_as_name=config.use_title_as_name,
+        infer_union_variant_names=config.infer_union_variant_names,
         use_operation_id_as_name=config.use_operation_id_as_name,
         use_unique_items_as_set=config.use_unique_items_as_set,
         use_tuple_for_fixed_items=config.use_tuple_for_fixed_items,

src/datamodel_code_generator/_types/generate_config_dict.py

@@ -114,6 +114,7 @@ class BaseGenerateConfig(TypedDict):
     graphql_no_typename: NotRequired[bool]
     wrap_string_literal: NotRequired[bool | None]
     use_title_as_name: NotRequired[bool]
+    infer_union_variant_names: NotRequired[bool]
     use_operation_id_as_name: NotRequired[bool]
     use_unique_items_as_set: NotRequired[bool]
     use_tuple_for_fixed_items: NotRequired[bool]

src/datamodel_code_generator/_types/parser_config_dicts.py

@@ -113,6 +113,7 @@ class ParserConfigDict(TypedDict):
     model_extra_keys_without_x_prefix: NotRequired[set[str] | None]
     wrap_string_literal: NotRequired[bool | None]
     use_title_as_name: NotRequired[bool]
+    infer_union_variant_names: NotRequired[bool]
     use_operation_id_as_name: NotRequired[bool]
     use_unique_items_as_set: NotRequired[bool]
     use_tuple_for_fixed_items: NotRequired[bool]

src/datamodel_code_generator/arguments.py

@@ -492,6 +492,12 @@ def start_section(self, heading: str | None) -> None:
     action="store_true",
     default=None,
 )
+model_options.add_argument(
+    "--infer-union-variant-names",
+    help="Infer inline oneOf/anyOf branch model names from literal discriminator-style fields",
+    action="store_true",
+    default=None,
+)
 model_options.add_argument(
     "--use-pendulum",
     help="use pendulum instead of datetime",

src/datamodel_code_generator/cli_options.py

@@ -256,6 +256,7 @@ class CLIOptionMeta:
     "--no-alias": CLIOptionMeta(name="--no-alias", category=OptionCategory.FIELD),
     "--use-serialization-alias": CLIOptionMeta(name="--use-serialization-alias", category=OptionCategory.FIELD),
     "--use-title-as-name": CLIOptionMeta(name="--use-title-as-name", category=OptionCategory.FIELD),
+    "--infer-union-variant-names": CLIOptionMeta(name="--infer-union-variant-names", category=OptionCategory.FIELD),
     "--use-schema-description": CLIOptionMeta(name="--use-schema-description", category=OptionCategory.FIELD),
     "--use-field-description": CLIOptionMeta(name="--use-field-description", category=OptionCategory.FIELD),
     "--use-field-description-example": CLIOptionMeta(

src/datamodel_code_generator/config.py

@@ -137,6 +137,7 @@ class BaseGenerateConfig(BaseModel):
     graphql_no_typename: bool = False
     wrap_string_literal: bool | None = None
     use_title_as_name: bool = False
+    infer_union_variant_names: bool = False
     use_operation_id_as_name: bool = False
     use_unique_items_as_set: bool = False
     use_tuple_for_fixed_items: bool = False
@@ -299,6 +300,7 @@ class ParserConfig(BaseModel):
     model_extra_keys_without_x_prefix: set[str] | None = None
     wrap_string_literal: bool | None = None
     use_title_as_name: bool = False
+    infer_union_variant_names: bool = False
     use_operation_id_as_name: bool = False
     use_unique_items_as_set: bool = False
     use_tuple_for_fixed_items: bool = False