How to use url_for() to pass path and query data to a route using FastAPI and Jinja2?

Question

I would like to call a FastAPI route from within Jinja2 template, and pass both path and query data (parameters) to that route. I tried in the Jinja2 template something like this:

{{ url_for('function1', uustr=data.uustr, interval=1) }}

Here is the FastAPI route that I would like to call (syntax has been simplified for demo purposes):

@app.get("/updates/data/{uustr}",response_class=HTMLResponse)
async def function1(request: Request, uustr:str, interval:int):

  return"""
<html>
    <head>
        <title>{{ uustr }}</title>
    </head>
    <body>
        <h1>{{ interval }}</h1>
    </body>
</html>
"""

I get this error:

raise ValueError('context must include a "request" key') 
ValueError: context must include a "request" key

Does anybody have an idea?

Have you taken a look at the FastAPI documentation for jinja templates? Looks like they do it in a very different way to yours: https://fastapi.tiangolo.com/advanced/templates/#using-jinja2templates — Jason Rebelo Neves, Mar 05 '21 at 16:33
Yes, I saw that, and you are right. The usage there is different. In the documentation they use `url_for()` for locating a folder. I want to use it to call a route. At least in flask something like this is possible. I hope also in fastapi — Mips, Mar 05 '21 at 16:50

Chris · Answer 1 · 2022-09-13T18:06:02.027

This is not FastAPI's issue, but rather Starlette's issue (i.e., request.url_for() receives path parameters, not query parameters). So, nspired by #560 and #1385, I have created the following working example for calling FastAPI routes from within Jinja2 templates, and passing query params (alone or along with path params as well).

Please note that this is a feature that is probably about to be introduced into the next version of Starlette #1385. Thus, best to use that one, when it is out.

app.py

import uvicorn
from fastapi import FastAPI, Response
from fastapi.templating import Jinja2Templates
from fastapi import Request
from fastapi.responses import HTMLResponse
import urllib

app = FastAPI()

 
class CustomURLProcessor:
    def __init__(self):  
        self.path = "" 
        self.request = None

    def url_for(self, request: Request, name: str, **params: str):
        self.path = request.url_for(name, **params)
        self.request = request
        return self
    
    def include_query_params(self, **params: str):
        parsed = list(urllib.parse.urlparse(self.path))
        parsed[4] = urllib.parse.urlencode(params)
        return urllib.parse.urlunparse(parsed)


templates = Jinja2Templates(directory='templates')
templates.env.globals['CustomURLProcessor'] = CustomURLProcessor


@app.get('/updates/page/{page_no}/item/{item_id}')
def updates(request: Request, page_no: int, item_id: int, user: str, msg: str):
    return templates.TemplateResponse("item.html", {"request": request, "page_no": page_no, "item_id":item_id, "user": user, "msg": msg})


@app.get('/updates_query_only')
def updates_query_only(request: Request, user: str, msg: str):
    return templates.TemplateResponse("item.html", {"request": request, "user": user, "msg": msg})

@app.get('/')
def index(request: Request):
    return templates.TemplateResponse("index.html",  {"request": request})

if __name__ == '__main__':
    uvicorn.run(app, host='127.0.0.1', port=8000, debug=True)

templates/index.html

<!DOCTYPE html>
<html>
    <body>
        <div class="container">
            {% set cu = CustomURLProcessor() %}
            {% set _url = cu.url_for(request, 'updates', page_no=5, item_id=3).include_query_params(user='foo', msg='bar') %}
            <!-- if only query params required, use as follows: -->
            {# {% set _url = cu.url_for(request, 'updates_query_only').include_query_params(user='foo', msg='bar') %} #}
            
            <iframe src="{{ _url }}" width = 300 height = 300 style= "border: none;"></iframe>
        </div>
    </body>
</html>

templates/item.html

<!DOCTYPE html>
<html>
    <body>
        <h1>Page No {{ page_no }}</h1>
        <h2>Item {{ item_id }}</h2>
        <h3>{{ user }}</h3>
        <h4>{{ msg }}</h4>
    </body>
</html>

Update

You can now use Starlette's starlette.datastructures.URL, which provides a method to include_query_params. Example below:

In app.py import the URL class and make it accessible from Jinja2 templates:

from starlette.datastructures import URL

templates = Jinja2Templates(directory="templates")
templates.env.globals['URL'] = URL

In templates/item.html use as follows:

<!DOCTYPE html>
<html>
   <body>
      <div class="container">
         <iframe src="{{ URL(url_for('updates', page_no=5, item_id=3)).include_query_params(user='foo', msg='bar') }}" width = 300 height = 300 style= "border: none;"></iframe>
      </div>
   </body>
</html>

score -1 · Answer 2 · answered Mar 05 '21 at 18:08

I found a very easy solution without using url_for()

In case someone coming from flask world has a similar problem here my solution: I created a simple HTML button in my jinja2 template:

<button onclick="myFunction()">pass data to route</button>

and I created a very simple javascript function for passing data to route:

<script>
function myFunction() {
  window.location.href = "/updates/data/{{data.uustr}}?interval=2";
}
</script>

that's it.

How to use url_for() to pass path and query data to a route using FastAPI and Jinja2?

2 Answers2

Update

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