SQLAlchemy create_all() does not create tables

You should put your model class before create_all() call, like this:

from flask import Flask
from flask_sqlalchemy import SQLAlchemy

app = Flask(__name__)

app.config['SQLALCHEMY_DATABASE_URI'] = 'postgresql+psycopg2://login:pass@localhost/flask_app'
db = SQLAlchemy(app)

class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(80), unique=True)
    email = db.Column(db.String(120), unique=True)

    def __init__(self, username, email):
        self.username = username
        self.email = email

    def __repr__(self):
        return '<User %r>' % self.username

with app.app_context():
    db.create_all()

    db.session.add(User('admin', 'admin@example.com'))
    db.session.add(User('guest', 'guest@example.com'))
    db.session.commit()

    users = User.query.all()
    print(users)

If your models are declared in a separate module, import them before calling create_all().

Say, the User model is in a file called models.py,

from flask import Flask
from flask_sqlalchemy import SQLAlchemy

app = Flask(__name__)

app.config['SQLALCHEMY_DATABASE_URI'] = 'postgresql+psycopg2://login:pass@localhost/flask_app'
db = SQLAlchemy(app)

# See important note below
from models import User

with app.app_context():
    db.create_all()

    db.session.add(User('admin', 'admin@example.com'))
    db.session.add(User('guest', 'guest@example.com'))
    db.session.commit()
    
    users = User.query.all()
    print(users)

Important note: It is important that you import your models after initializing the db object since, in your models.py you also need to import the db object from this module.