ImportError: flask中出现错误，无法从'flask_sqlalchemy‘导入名称'SQLALchemy’

from flask_sqlalchemy import SQLAlchemy

pip show flask_sqlalchemy

pip install Flask-SQLAlchemy

pip cache purge
pip install --no-cache-dir Flask-SQLAlchemy

from flask import Flask
from flask_sqlalchemy import SQLAlchemy

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

class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(80), unique=True, nullable=False)

@app.route('/')
def index():
    return "Hello, Flask with SQLAlchemy!"

if __name__ == '__main__':
    db.create_all()
    app.run(debug=True)