Message covariant type annotation

[adanaja]

@davidism I noticed something which led me to the PR for adding static type annotations #221

This modified version of the simple example shown in the README (notice the added recipients = ["recipient@example.com"]):

from flask import Flask
from flask_mail import Mail, Message

app = Flask(__name__)
app.config["MAIL_SERVER"] = "your_mail_server"
app.config["MAIL_PORT"] = 587
app.config["MAIL_USE_TLS"] = True
app.config["MAIL_USE_SSL"] = False
app.config["MAIL_USERNAME"] = "your_username"
app.config["MAIL_PASSWORD"] = "your_password"
app.config["MAIL_DEFAULT_SENDER"] = "your_email@example.com"

mail = Mail(app)


@app.route("/")
def send_email() -> str:
    recipients = ["recipient@example.com"]

    msg = Message(
        "Hello",
        recipients=recipients,
        body="This is a test email sent from Flask-Mail!",
    )
    mail.send(msg)
    return "Email sent succesfully!"

Makes mypy output this error:

error: Argument "recipients" to "Message" has incompatible type "list[str]"; expected "list[str | tuple[str, str]] | None"  [arg-type]
note: "list" is invariant -- see https://mypy.readthedocs.io/en/stable/common_issues.html#variance
note: Consider using "Sequence" instead, which is covariant

This is because the recipients variable is considered of type list[str] and not list[str | tuple[str, str]] by mypy.

Now it could be easy to fix the error by explicitly setting the type:

    recipients: list[str | tuple[str, str]] = ["recipient@example.com"]

    msg = Message(
        "Hello",
        recipients=recipients,
        body="This is a test email sent from Flask-Mail!",
    )

But doing that is not really safe as the type should be modified if someday Message changes the accepted type. Ideally it should work transparently with list[str].

So I don't know if it could be possible to make Message accept a Sequence as mypy suggests, or maybe list[str] | list[tuple[str, str]]?

I used:

• mypy 1.16.1
• flask 3.1.1
• flask_mail 0.10.0