Incompatibility with pytest-flask

[azmeuk]

I found an incompatibility when using at the same time flask-babel 4.0.0 and pytest-flask 1.3.0. I am not sure whose responsability it is, maybe both, maybe neither, so I post this issue on both bugtrackers. Here is the link to the issue at pytest-flask.

In the following snippet, a dummy view translates a dummy string and returns the current lang code. The lang code is dynamically set by a request argument lang. In this test environment the first visit is successful and sets the lang to fr but the second visit fails and the lang is not set to uk but still is fr.

import pytest
from flask import Flask
from flask import render_template_string
from flask import request
from flask import g
from flask_babel import Babel

def locale_selector():
    if request.args.get("lang"):
        g.lang = request.args["lang"]

    return g.get("lang", "en")

@pytest.fixture
def app():
    app = Flask(__name__)
    Babel(app, locale_selector=locale_selector)

    @app.route("/")
    def index():
        render_template_string("{% trans %}foobar{% endtrans %}")
        return g.get("lang")

    return app

def test_foobar(app):
    client_app = app.test_client()

    res = client_app.get("/?lang=fr")
    assert res.text == "fr"

    res = client_app.get("/?lang=en")
    assert res.text == "en" # This assertion fails

Note that this test fails if pytest-flask is installed in the environment, but passes if pytest-flask is not installed. In production, the behavior is OK too, so there is going on with pytest-flask.

With a little debugging, I can see that the locale_selector method is only called once. That explains why the language stays to fr. Looking closer, it seems that the get_locale method from flask-babel saves the loaded lang in the current context and reuses it on following calls. On different requests the language would indeed be recomputed.

https://github.com/python-babel/flask-babel/blob/a754eade39d9850693dd2b645ae8a2545df7fdf7/flask_babel/__init__.py#L257-L258

However, as far as I understand, pytest-flask loads a context for the unit tests (so session and g are accessible without loading a new context). That probably makes flask-babel not recomputing the locale, because the same context is used, and leads to my test failing.

Please correct me if my analysis is wrong. I don't know if there is a mis-usage from my side or if the two libraries do not belong together. If they don't, I hope we can find a solution by bringing every one in the same room :)

What do you think?

[TkTech]

This seems like an issue with pytest-flask. We could probably add a teardown handler to make this explicit, but g persisting between requests is a break with normal flask behaviour and will cause issues with other plugins, not just flask-babel.

[azmeuk]

Thank you for your quick answer. Indeed, the test fails without flask-babel:

import pytest
from flask import Flask
from flask import request
from flask import g

@pytest.fixture
def app():
    app = Flask(__name__)

    @app.route("/")
    def index():
        if "lang" not in g:
            g.lang = request.args["lang"]
        return g.get("lang")

    return app

def test_foobar(app):
    client_app = app.test_client()

    res = client_app.get("/?lang=fr")
    assert res.text == "fr"

    res = client_app.get("/?lang=en")
    assert res.text == "en" # This assertion fails