How to pass data from one view to another one in Fastapi?

Question

I have a variable set in one view in Fastapi and want to pass it to another one :

from fastapi import APIRouter, Request, Response
from fastapi.templating import Jinja2Templates

templates = Jinja2Templates(directory="templates")
router = APIRouter()

@router.get("/my-first-view")
async def function1(request: Request) -> Response:
    """Display the home page."""
    my_variable = value
    return templates.TemplateResponse(
        "home.jinja",
        context={
           "my_variable": my_variable
        },
    )

@router.get("/my-second-view")
async def function2(request: Request, my_variable: str) -> Response:
    """Display the variable processing page."""
    
    return templates.TemplateResponse(
        "page.jinja"
    )

Normally, this would come to send my_variable from home.jinja to page.jinja. Thus, in home.jinja I have the following :

...
<a href="{{url_for('function2', my_variable=my_variable)}}" title="connect">Connect</a>
...

But this is throwing me an error : "starlette.routing.NoMatchFound: No route exists for name \"function2\" and params \"my_variable\".\n". I did some researches but I haven't found something really helpful

What is the proper way to do it with Fastapi ? What am I missing ?

Answer 1

Very little context information on this post, so I'll help out with fixes that change as little as possible.

First of all, to fix your code you need a place to save the changes that you've made. A variable that only exists in a function is deleted at the end of that function.

Now usually you'd give more information regarding your use case, and I'd give you the best choice and why. It could be to store data in a JSON file, or a Database, or an in memory object that would serve the purpose best.

Here there's no info so we'll just make a variable. Keep in mind that this variable will be reset every time you restart the shell. So if your API is not "always on" then this may no work.

from fastapi import APIRouter, Request, Response
from fastapi.templating import Jinja2Templates

VARIABLE = None
API_URL = "my_domain.com/api"

templates = Jinja2Templates(directory="templates")
router = APIRouter()

@router.get("/my-first-view")
async def function1(request: Request) -> Response:
    """Display the home page."""
    my_variable = value
    return templates.TemplateResponse(
        "home.jinja",
        context={
           "complete_url": f"{API_URL}/my-second-view?my_variable={VARIABLE}"
        },
    )

@router.get("/my-second-view")
async def function2(request: Request, my_variable: str) -> Response:
    """Display the variable processing page."""

    VARIABLE = my_variable
    
    return templates.TemplateResponse(
        "page.jinja"
    )

As far as the template goes, you haven't shared url_for, but I suppose it just creates the path. I'd just make home.jinja as follows.

<a href="{{ complete_url }}" title="connect">Connect</a>

Also I've replaced the url_for call with a simple f-string. Depending on what you pass in your VARIABLE you may need to encode it.

For an example of that, please look at the following thread: How to urlencode a querystring in Python?