为什么我在 Flask 中收到 UnboundLocalError？

Question

this is my app.py code but i am receiving unbound local error after adding another path for delete and creating the function delete.

from crypt import methods
from email.policy import default
from flask import Flask, render_template, request, redirect
from flask_sqlalchemy import SQLAlchemy
from datetime import datetime

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///todo.db'
app.config['SQLALCHEMY_TRACK_MODIFICATIONS'] = False
db = SQLAlchemy(app)


class Todo(db.Model):
    sno = db.Column(db.Integer, primary_key=True)
    title = db.Column(db.String(200), nullable=False)
    desc = db.Column(db.String(200), nullable=False)
    date_created = db.Column(db.DateTime, default=datetime.utcnow)

    def __repr__(self) -> str:
        return f"{self.sno} - {self.title}"


@app.route('/', methods=['GET', 'POST'])
def hello_world():
    if request.method == 'POST':
        title = request.form["title"]
        desc = request.form["desc"]
    todo = Todo(title=title, desc=desc)
    db.session.add(todo)
    db.session.commit()
    mytodo = Todo.query.all()
    return render_template('index.html', mytodo=mytodo)


@app.route('/delete/<int:sno>')
def delete(sno):
    todo = Todo.query.filter_by(sno==sno).first()
    db.session.delete(todo)
    db.session.commit()
    return redirect('/')


if __name__ == "__main__":
    app.run(debug=True, port=8000)

Below i've added the screenshot of the errors i am getting even when i just visit the root url.

Answer 1

Problem

If the server receive a GET / , then you won't define te title variable, but would try to use it at Todo(title=title, desc=desc) so an error

---

Fix

All the creation code should be in the if POST branch

@app.route('/', methods=['GET', 'POST'])
def hello_world():
    if request.method == 'POST':
        title = request.form["title"]
        desc = request.form["desc"]
        todo = Todo(title=title, desc=desc)
        db.session.add(todo)
        db.session.commit()
    mytodo = Todo.query.all()
    return render_template('index.html', mytodo=mytodo)