目前，我正在学习Miguel Grinberg编写的Flask with Flask mega教程。我没有验证用户的新用户名来测试自定义错误页面。问题是服务器无法返回500个错误代码的自定义错误页面，但它可以处理404错误代码。在回溯中，我注意到Jinja2的current_user存在问题。结果，服务器返回标准500错误页面。
用户加载器：
@login.user_loader
def load_user(user_id):
cur_user = User.query.get(int(user_id))
return cur_user
用户模型：
class User(UserMixin, db.Model):
id = db.Column(db.Integer, primary_key=True)
username = db.Column(db.String(64), index=True, unique=True)
email = db.Column(db.String(120), index=True, unique=True)
password_hash = db.Column(db.String(128))
about_me = db.Column(db.String(140))
last_seen = db.Column(db.DateTime, default=datetime.utcnow)
查看功能：
@app.route('/edit_profile', methods=['GET', 'POST'])
def edit_profile():
form = EditProfileForm()
if form.validate_on_submit():
current_user.username = form.username.data
current_user.about_me = form.about_me.data
db.session.add(current_user)
db.session.commit()
flash('Your changes have been saved')
return redirect(url_for('edit_profile'))
elif request.method == 'GET':
form.username.data = current_user.username
form.about_me.data = current_user.about_me
return render_template('edit_profile.html', title='Edit Profile', form=form)
错误处理程序：
@app.errorhandler(500)
def internal_error(error):
return render_template('500.html'), 500
500.html
{% extends 'base.html' %}
{% block content %}
<h1>An unexpected error has occurred.</h1>
<p>The administrator has been notified. Sorry for the inconvenience.</p>
<p><a href="{{ url_for('index') }}">Back</a></p>
{% endblock %}
base.html
<html>
<head>
{% if title %}
<title>{{ title }} - Microblog</title>
{% else %}
<title>Welcome to Microblog</title>
{% endif %}
</head>
<body>
<div>
Microblog:
<a href="{{ url_for('index') }}">Home</a>
{% if current_user.is_anonymous %}
<a href="{{ url_for('login') }}">Login</a>
{% else %}
<a href="{{ url_for('user', username=current_user.username) }}">Profile</a>
<a href="{{ url_for('logout') }}">Logout</a>
{% endif %}
</div>
<hr>
{% with messages = get_flashed_messages() %}
{% if messages %}
<ul>
{% for message in messages %}
<li>{{ message }}</li>
{% endfor %}
</ul>
{% endif %}
{% endwith %}
{% block content %}{% endblock %}
</body>
</html>
追踪：
Traceback (most recent call last):
File "C:\Users\Georgiy\PycharmProjects\flask_microblog\venv\lib\site-packages\werkzeug\serving.py", line 303, in run_wsgi
execute(self.server.app)
File "C:\Users\Georgiy\PycharmProjects\flask_microblog\venv\lib\site-packages\werkzeug\serving.py", line 291, in execute
application_iter = app(environ, start_response)
File "C:\Users\Georgiy\PycharmProjects\flask_microblog\venv\lib\site-packages\flask\app.py", line 2463, in __call__
return self.wsgi_app(environ, start_response)
File "C:\Users\Georgiy\PycharmProjects\flask_microblog\venv\lib\site-packages\flask\app.py", line 2449, in wsgi_app
response = self.handle_exception(e)
File "C:\Users\Georgiy\PycharmProjects\flask_microblog\venv\lib\site-packages\flask\app.py", line 1878, in handle_exception
server_error = handler(server_error)
File "C:\Users\Georgiy\PycharmProjects\flask_microblog\app\errors.py", line 12, in internal_error
return render_template('500.html'), 500
File "C:\Users\Georgiy\PycharmProjects\flask_microblog\venv\lib\site-packages\flask\templating.py", line 140, in render_template
ctx.app,
File "C:\Users\Georgiy\PycharmProjects\flask_microblog\venv\lib\site-packages\flask\templating.py", line 120, in _render
rv = template.render(context)
File "C:\Users\Georgiy\PycharmProjects\flask_microblog\venv\lib\site-packages\jinja2\asyncsupport.py", line 76, in render
return original_render(self, *args, **kwargs)
File "C:\Users\Georgiy\PycharmProjects\flask_microblog\venv\lib\site-packages\jinja2\environment.py", line 1008, in render
return self.environment.handle_exception(exc_info, True)
File "C:\Users\Georgiy\PycharmProjects\flask_microblog\venv\lib\site-packages\jinja2\environment.py", line 780, in handle_exception
reraise(exc_type, exc_value, tb)
File "C:\Users\Georgiy\PycharmProjects\flask_microblog\venv\lib\site-packages\jinja2\_compat.py", line 37, in reraise
raise value.with_traceback(tb)
File "C:\Users\Georgiy\PycharmProjects\flask_microblog\app\templates\500.html", line 1, in top-level template code
{% extends 'base.html' %}
File "C:\Users\Georgiy\PycharmProjects\flask_microblog\app\templates\base.html", line 16, in top-level template code
<a href="{{ url_for('user', username=current_user.username) }}">Profile</a>
File "C:\Users\Georgiy\PycharmProjects\flask_microblog\venv\lib\site-packages\jinja2\environment.py", line 430, in getattr
return getattr(obj, attribute)
File "C:\Users\Georgiy\PycharmProjects\flask_microblog\venv\lib\site-packages\sqlalchemy\orm\attributes.py", line 282, in __get__
return self.impl.get(instance_state(instance), dict_)
File "C:\Users\Georgiy\PycharmProjects\flask_microblog\venv\lib\site-packages\sqlalchemy\orm\attributes.py", line 705, in get
value = state._load_expired(state, passive)
File "C:\Users\Georgiy\PycharmProjects\flask_microblog\venv\lib\site-packages\sqlalchemy\orm\state.py", line 660, in _load_expired
self.manager.deferred_scalar_loader(self, toload)
File "C:\Users\Georgiy\PycharmProjects\flask_microblog\venv\lib\site-packages\sqlalchemy\orm\loading.py", line 979, in load_scalar_attributes
only_load_props=attribute_names,
File "C:\Users\Georgiy\PycharmProjects\flask_microblog\venv\lib\site-packages\sqlalchemy\orm\loading.py", line 208, in load_on_ident
identity_token=identity_token,
File "C:\Users\Georgiy\PycharmProjects\flask_microblog\venv\lib\site-packages\sqlalchemy\orm\loading.py", line 282, in load_on_pk_identity
return q.one()
File "C:\Users\Georgiy\PycharmProjects\flask_microblog\venv\lib\site-packages\sqlalchemy\orm\query.py", line 3292, in one
ret = self.one_or_none()
File "C:\Users\Georgiy\PycharmProjects\flask_microblog\venv\lib\site-packages\sqlalchemy\orm\query.py", line 3261, in one_or_none
ret = list(self)
File "C:\Users\Georgiy\PycharmProjects\flask_microblog\venv\lib\site-packages\sqlalchemy\orm\query.py", line 3334, in __iter__
return self._execute_and_instances(context)
File "C:\Users\Georgiy\PycharmProjects\flask_microblog\venv\lib\site-packages\sqlalchemy\orm\query.py", line 3356, in _execute_and_instances
querycontext, self._connection_from_session, close_with_result=True
File "C:\Users\Georgiy\PycharmProjects\flask_microblog\venv\lib\site-packages\sqlalchemy\orm\query.py", line 3371, in _get_bind_args
mapper=self._bind_mapper(), clause=querycontext.statement, **kw
File "C:\Users\Georgiy\PycharmProjects\flask_microblog\venv\lib\site-packages\sqlalchemy\orm\query.py", line 3349, in _connection_from_session
conn = self.session.connection(**kw)
File "C:\Users\Georgiy\PycharmProjects\flask_microblog\venv\lib\site-packages\sqlalchemy\orm\session.py", line 1124, in connection
execution_options=execution_options,
File "C:\Users\Georgiy\PycharmProjects\flask_microblog\venv\lib\site-packages\sqlalchemy\orm\session.py", line 1130, in _connection_for_bind
engine, execution_options
File "C:\Users\Georgiy\PycharmProjects\flask_microblog\venv\lib\site-packages\sqlalchemy\orm\session.py", line 408, in _connection_for_bind
self._assert_active()
File "C:\Users\Georgiy\PycharmProjects\flask_microblog\venv\lib\site-packages\sqlalchemy\orm\session.py", line 295, in _assert_active
code="7s2a",
sqlalchemy.exc.InvalidRequestError: This Session's transaction has been rolled back due to a previous exception during flush. To begin a new transaction with this Session, first issue Session.rollback(). Original exception was: (sqlite3.IntegrityError) UNIQUE constraint failed: user.username
[SQL: UPDATE user SET username=?, about_me=? WHERE user.id = ?]
[parameters: ('admin', "hello my name is Georgiy. I'm learning programming.", 2)]
(Background on this error at: http://sqlalche.me/e/gkpj) (Background on this error at: http://sqlalche.me/e/7s2a)
您需要将FLASK_DEBUG环境变量设置为0才能触发自定义500错误处理程序。否则，您将看到调试器的回溯。
"If you set FLASK_DEBUG=0 in your terminal session and then trigger the duplicate username bug one more time, you are going to see a slightly more friendly error page"
https://blog.miguelgrinberg.com/post/the-flask-mega-tutorial-part-vii-error-handling