# Requests & Responses

This page covers the `Request` object in detail: how to construct requests, pass data between callbacks, control priority and deduplication, and use `response.follow()` for link-following.

## The Request Object

A `Request` represents a URL to be fetched. You create requests either directly or via `response.follow()`:

```python
from scrapling.spiders import Request

# Direct construction
request = Request(
    "https://example.com/page",
    callback=self.parse_page,
    priority=5,
)

# Via response.follow (preferred in callbacks)
request = response.follow("/page", callback=self.parse_page)
```

Here are all the arguments you can pass to `Request`:

| Argument      | Type       | Default    | Description                                                                                           |
|---------------|------------|------------|-------------------------------------------------------------------------------------------------------|
| `url`         | `str`      | *required* | The URL to fetch                                                                                      |
| `sid`         | `str`      | `""`       | Session ID - routes the request to a specific session (see [Sessions](sessions.md))                   |
| `callback`    | `callable` | `None`     | Async generator method to process the response. Defaults to `parse()`                                 |
| `priority`    | `int`      | `0`        | Higher values are processed first                                                                     |
| `dont_filter` | `bool`     | `False`    | If `True`, skip deduplication (allow duplicate requests)                                              |
| `meta`        | `dict`     | `{}`       | Arbitrary metadata passed through to the response                                                     |
| `**kwargs`    |            |            | Additional keyword arguments passed to the session's fetch method (e.g., `headers`, `method`, `data`) |

Any extra keyword arguments are forwarded directly to the underlying session. For example, to make a POST request:

```python
yield Request(
    "https://example.com/api",
    method="POST",
    data={"key": "value"},
    callback=self.parse_result,
)
```

## Response.follow()

`response.follow()` is the recommended way to create follow-up requests inside callbacks. It offers several advantages over constructing `Request` objects directly:

- **Relative URLs** are resolved automatically against the current page URL
- **Referer header** is set to the current page URL by default
- **Session kwargs** from the original request are inherited (headers, proxy settings, etc.)
- **Callback, session ID, and priority** are inherited from the original request if not specified

```python
async def parse(self, response: Response):
    # Minimal - inherits callback, sid, priority from current request
    yield response.follow("/next-page")

    # Override specific fields
    yield response.follow(
        "/product/123",
        callback=self.parse_product,
        priority=10,
    )

    # Pass additional metadata to
    yield response.follow(
        "/details",
        callback=self.parse_details,
        meta={"category": "electronics"},
    )
```

| Argument           | Type       | Default    | Description                                                |
|--------------------|------------|------------|------------------------------------------------------------|
| `url`              | `str`      | *required* | URL to follow (absolute or relative)                       |
| `sid`              | `str`      | `""`       | Session ID (inherits from original request if empty)       |
| `callback`         | `callable` | `None`     | Callback method (inherits from original request if `None`) |
| `priority`         | `int`      | `None`     | Priority (inherits from original request if `None`)        |
| `dont_filter`      | `bool`     | `False`    | Skip deduplication                                         |
| `meta`             | `dict`     | `None`     | Metadata (merged with existing response meta)              |
| **`referer_flow`** | `bool`     | `True`     | Set current URL as Referer header                          |
| `**kwargs`         |            |            | Merged with original request's session kwargs              |

### Disabling Referer Flow

By default, `response.follow()` sets the `Referer` header to the current page URL. To disable this:

```python
yield response.follow("/page", referer_flow=False)
```

## Callbacks

Callbacks are async generator methods on your spider that process responses. They must `yield` one of three types:

- **`dict`**: A scraped item, added to the results
- **`Request`**: A follow-up request, added to the queue
- **`None`**: Silently ignored

```python
class MySpider(Spider):
    name = "my_spider"
    start_urls = ["https://example.com"]

    async def parse(self, response: Response):
        # Yield items (dicts)
        yield {"url": response.url, "title": response.css("title::text").get("")}

        # Yield follow-up requests
        for link in response.css("a::attr(href)").getall():
            yield response.follow(link, callback=self.parse_page)

    async def parse_page(self, response: Response):
        yield {"content": response.css("article::text").get("")}
```

**Note:** All callback methods must be `async def` and use `yield` (not `return`). Even if a callback only yields items with no follow-up requests, it must still be an async generator.

## Request Priority

Requests with higher priority values are processed first. This is useful when some pages are more important to be processed first before others:

```python
async def parse(self, response: Response):
    # High priority - process product pages first
    for link in response.css("a.product::attr(href)").getall():
        yield response.follow(link, callback=self.parse_product, priority=10)

    # Low priority - pagination links processed after products
    next_page = response.css("a.next::attr(href)").get()
    if next_page:
        yield response.follow(next_page, callback=self.parse, priority=0)
```

When using `response.follow()`, the priority is inherited from the original request unless you specify a new one.

## Deduplication

The spider automatically deduplicates requests based on a fingerprint computed from the URL, HTTP method, request body, and session ID. If two requests produce the same fingerprint, the second one is silently dropped.

To allow duplicate requests (e.g., re-visiting a page after login), set `dont_filter=True`:

```python
yield Request("https://example.com/dashboard", dont_filter=True, callback=self.parse_dashboard)

# Or with response.follow
yield response.follow("/dashboard", dont_filter=True, callback=self.parse_dashboard)
```

You can fine-tune what goes into the fingerprint using class attributes on your spider:

| Attribute            | Default | Effect                                                                                                          |
|----------------------|---------|-----------------------------------------------------------------------------------------------------------------|
| `fp_include_kwargs`  | `False` | Include extra request kwargs (arguments you passed to the session fetch, like headers, etc.) in the fingerprint |
| `fp_keep_fragments`  | `False` | Keep URL fragments (`#section`) when computing fingerprints                                                     |
| `fp_include_headers` | `False` | Include request headers in the fingerprint                                                                      |

For example, if you need to treat `https://example.com/page#section1` and `https://example.com/page#section2` as different URLs:

```python
class MySpider(Spider):
    name = "my_spider"
    fp_keep_fragments = True
    # ...
```

## Request Meta

The `meta` dictionary lets you pass arbitrary data between callbacks. This is useful when you need context from one page to process another:

```python
async def parse(self, response: Response):
    for product in response.css("div.product"):
        category = product.css("span.category::text").get("")
        link = product.css("a::attr(href)").get()
        if link:
            yield response.follow(
                link,
                callback=self.parse_product,
                meta={"category": category},
            )

async def parse_product(self, response: Response):
    yield {
        "name": response.css("h1::text").get(""),
        "price": response.css(".price::text").get(""),
        # Access meta from the request
        "category": response.meta.get("category", ""),
    }
```

When using `response.follow()`, the meta from the current response is merged with the new meta you provide (new values take precedence).

The spider system also automatically stores some metadata. For example, the proxy used for a request is available as `response.meta["proxy"]` when proxy rotation is enabled.