Explain default `_method` used for `url_for()` #5536

xArbisRox · 2024-07-29T09:42:08Z

xArbisRox
Jul 29, 2024

Hello folks,
I stumbled upon an unexpected behavior recently, which I assume might be worth to pick up in the documentation.

Let's take this simple application:

app.py

from flask import Flask, redirect, url_for, render_template

app = Flask(__name__)


@app.route('/')
def index():
    return render_template("index.html")


@app.route("/target", methods=["POST"])
@app.route("/target/<arg>", methods=["GET"])
def target(arg=None):
    return arg


@app.route("/redirecter/<arg>", methods=["GET", "POST"])
def redirecter(arg):
    return redirect(url_for('target', arg=arg))


if __name__ == '__main__':
    app.run()

index.html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Flask</title>
</head>
<body>
<form method="POST"
      action="{{ url_for('redirecter', arg='POSTFromForm') }}">
    <input type="submit" value="POST">
</form>
<form method="GET"
      action="{{ url_for('redirecter', arg='GETFromForm') }}">
    <input type="submit" value="GET">
</form>
</body>
</html>

I would see this as a typical scenario, having a route without path arguments for POST requests and another endpoint with path arguments for GET requests in the target route.

However, when I send a POST request to the redirecter, url_for("target", arg=arg) generates '/target?arg=xyz'. When I send a GET request to the redirecter, it will generate the expected '/target/xyz'.

I can fix the redirecter for the POST request by explicitely defining url_for("target", arg=arg, _method="GET").
So I could adjust the redirecter to something like:

@app.route("/redirecter/<arg>", methods=["GET", "POST"])
def redirecter(arg):
    if request.method == "POST":
        return redirect(url_for("target", arg=arg, _method="GET"))
    return redirect(url_for('target', arg=arg))

I do understand, why this is happening. For the POST method, there is no endpoint including <arg> defined, which is why url_for() cannot resolve it and appends it as query argument thereafter.

However, this is the point where I am going to. For me, it was not intuitive, that url_for() uses the method of the current request context to generate its output. Currently, the documentation states:

_method (str | None) – If given, generate the URL associated with this method for the endpoint.

Might this be worth to pick up in the documentation?

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Explain default `_method` used for `url_for()` #5536

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Pallets

Explain default _method used for url_for() #5536

Uh oh!

xArbisRox Jul 29, 2024

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Explain default `_method` used for `url_for()` #5536

xArbisRox
Jul 29, 2024