Feature: Custom Param Decorators (@Body, @Param, @Query, @Headers, createParamDecorator)

[ItayTheDar]

Overview

PyNest route handlers today rely entirely on FastAPI's implicit parameter binding — function signature type hints are parsed by FastAPI directly. This means there is no PyNest-native way to:

1. Explicitly declare where a parameter comes from (@Body, @Param, @Query)
2. Build reusable domain-specific decorators like @CurrentUser() or @TenantId()
3. Transform or validate individual parameters before they reach the handler

This feature request proposes a full suite of built-in param decorators and a public createParamDecorator factory API.

---

Motivation

```python

Today — implicit and ambiguous:

@post('/users')
def create_user(self, user_data: CreateUserDto, token: str = Header(None)):
...

With this feature — explicit and composable:

@post('/users')
def create_user(self, @Body() user_data: CreateUserDto, @currentuser() user: User):
...
```

Explicit param decorators also enable:

• Cleaner OpenAPI docs (FastAPI knows exactly what goes where)
• Param-level pipes and transforms (prerequisite for Feature add docker file if db is sqlite (in the future i will add support in … #5)
• Domain decorators reusable across the entire app

---

Built-in Param Decorators

@Body(key?: str)

```python
@post('/users')
def create_user(self, @Body() body: CreateUserDto):
# body is the full request body, validated against CreateUserDto
...

@post('/upload')
def upload(self, @Body('file') file: UploadFile):
...
```

@Param(name?: str)

```python
@get('/:user_id/posts/:post_id')
def get_post(self, @PARAM('user_id') uid: int, @PARAM('post_id') pid: int):
...

@get('/:user_id')
def get_user(self, @PARAM() params: dict):
# params = {"user_id": "123"}
...
```

@Query(name?: str)

```python
@get('/search')
def search(self, @query('q') query: str, @query('limit') limit: int = 20):
...
```

@Headers(name?: str)

```python
@get('/me')
def get_me(self, @headers('authorization') auth: str):
...

@get('/debug')
def debug(self, @headers() all_headers: dict):
...
```

@Req() and @Res()

```python
@get('/raw')
def raw(self, @Req() request: Request, @res() response: Response):
...
```

@Ip() and @HostParam()

```python
@post('/audit')
def audit(self, @ip() ip: str):
...
```

---

createParamDecorator — Custom Param Decorators

The real power: a public factory for building reusable param decorators with arbitrary extraction logic.

```python
from nest.common.decorators import createParamDecorator, ExecutionContext

Define once:

CurrentUser = createParamDecorator(
lambda data, ctx: ctx.switch_to_http().get_request().state.user
)

TenantId = createParamDecorator(
lambda data, ctx: ctx.switch_to_http().get_request().headers.get("x-tenant-id")
)

Pagination = createParamDecorator(
lambda data, ctx: {
"page": int(ctx.switch_to_http().get_request().query_params.get("page", 1)),
"limit": int(ctx.switch_to_http().get_request().query_params.get("limit", 20)),
}
)

Use anywhere:

@get('/posts')
def list_posts(self, @currentuser() user: User, @Pagination() pagination: dict):
return self.service.get_posts(user.id, **pagination)

@delete('/:id')
def delete_item(self, @PARAM('id') item_id: int, @TenantID() tenant: str):
...
```

data argument — parametrized decorators

```python
UserProperty = createParamDecorator(
lambda data, ctx: getattr(ctx.switch_to_http().get_request().state.user, data, None)
)

@get('/profile')
def profile(self, @userproperty('email') email: str):
...
```

---

Integration with Pipes (Feature #5)

Param decorators should accept an optional pipe argument for per-parameter transformation:

```python
@get('/:id')
def get(self, @PARAM('id', ParseIntPipe) id: int):
...

@post('/')
def create(self, @Body(ValidationPipe) body: CreateDto):
...
```

---

Implementation Approach

Since Python does not have native parameter-level decorators, this should be implemented via:

1. A ParamMetadata descriptor or a sentinel object returned by each decorator
2. The controller decorator (@Controller) or a route processing wrapper that inspects function signatures for these sentinels
3. FastAPI Depends() under the hood for actual extraction and injection

---

Acceptance Criteria

• @Body(key?), @Param(name?), @Query(name?), @Headers(name?) built-in decorators
• @Req(), @Res(), @Ip(), @HostParam() built-in decorators
• createParamDecorator(factory) public API
• Custom decorators support a data argument for parameterized use
• Param decorators compose with pipes (@Param('id', ParseIntPipe))
• ExecutionContext with switch_to_http() available inside factory functions
• Full backward compatibility — existing routes without param decorators still work
• Unit tests for all built-in decorators
• Documentation page with createParamDecorator examples

---

Related

• NestJS docs: https://docs.nestjs.com/custom-decorators
• Feature: Pipes & ValidationPipe (prerequisite for pipe argument on param decorators)

[ItayTheDar]

Resolved by #129, which was merged on 2026-05-12.

Implemented scope:

• Added HTTP parameter decorator markers: Body, Param, Query, Headers, Req, Res, Ip, HostParam, createParamDecorator, and ExecutionContext.
• Updated route registration so marked parameters are rewritten into FastAPI dependencies while preserving existing FastAPI-style implicit binding for undecorated routes.
• Added support for custom parameter decorators with data arguments and pipe-style transformations.
• Added request/response injection coverage and backwards compatibility coverage.
• Added documentation in docs/param_decorators.md and wired it into mkdocs.yml.

Important syntax note: Python does not support NestJS-style parameter decorators inside a function parameter list, so the implementation uses PyNest marker defaults instead, for example:

def get_user(self, user_id: int = Param("id")):
    ...

Validation from #129:

• uv run --group test python -m pytest -q -> 162 passed, 8 warnings
• uv run --group docs mkdocs build --strict -> completed successfully, with only the existing MkDocs/Material notice and existing nav warning noted in the PR

Closing this as completed.