fastapi jinja url_for with query params
=======================================

Out of the box Starlette does not support url_for with query params it throws the following error.

Date: April 5, 2024

Out of the box Starlette does not support url_for with query params. When
trying to use url_for with query params it throws the following error.

``` python
starlette.routing.NoMatchFound: No route exists for name "infinite" and params "page"
```

In my searching for this I found [starlette issue #560](https://github.com/encode/starlette/issues/560) quite helpful, but not complete, as it did not work for me.

``` python
import jinja2

if hasattr(jinja2, "pass_context"):
 pass_context = jinja2.pass_context
else:
 pass_context = jinja2.contextfunction

@pass_context
def url_for_query(context: dict, name: str, **params: dict) -> str:
 request = context["request"]
 url = str(request.url_for(name))
 if params == {}:
 return url
 from urllib.parse import parse_qs, urlencode, urlparse, urlunparse

 # Parse the URL
 parsed_url = urlparse(url)

 # Parse the query parameters
 query_params = parse_qs(parsed_url.query)

 # Update the query parameters with the new ones
 query_params.update(params)

 # Rebuild the query string
 updated_query_string = urlencode(query_params, doseq=True)

 # Rebuild the URL with the updated query string
 updated_url = urlunparse(
 (
 parsed_url.scheme,
 parsed_url.netloc,
 parsed_url.path,
 parsed_url.params,
 updated_query_string,
 parsed_url.fragment,
 )
 )

 if os.environ.get("ENV") in ["dev", "qa", "prod"]:
 updated_url = updated_url.replace("http", "https", 1)

 return updated_url

def get_templates():
 templates = Jinja2Templates(directory="templates")
 templates.env.globals["url_for"] = url_for_query
 return templates
```

!!! Note "https"
 If you want url_for to work in production you need some way to convert http
 to https. Here is how I make it work, for local development I `export
 ENV=local` then for each environment that I am running on a server I include
 it in the list and update `ENV` appropriately.

 ``` python
 if os.environ.get("ENV") in ["dev", "qa", "prod"]:
 updated_url = updated_url.replace("http", "https", 1)
 ```

The route might look something like this.

``` python
@infinite_router.get("/")
async def home(request: Request, page: int = 1, n: int = 10):
 ...
```

To access the home route using url_for in a jinja template you can use the
following, once you have applied the `url_for_query` function as your default
`url_for`

``` html
Home [1]
```

References:
[1]: {{ url_for(