Why am I getting a timeout exception with flask on googles app engine when trying to append a value to a wtforms validtor? - google-app-engine

Question

I'm having trouble deploying a flask app to the Google app engine. It works fine when I run it locally but as soon as it's deployed to the app engine it fails. With two exceptions (I'm always getting both, I'm unsure which one comes first).
Traceback (most recent call last):
File "/opt/python3.7/lib/python3.7/logging/handlers.py", line 1008, in emit
smtp = smtplib.SMTP(self.mailhost, port, timeout=self.timeout)
File "/opt/python3.7/lib/python3.7/smtplib.py", line 251, in __init__
(code, msg) = self.connect(host, port)
File "/opt/python3.7/lib/python3.7/smtplib.py", line 338, in connect
(code, msg) = self.getreply()
File "/opt/python3.7/lib/python3.7/smtplib.py", line 391, in getreply
+ str(e))
smtplib.SMTPServerDisconnected: Connection unexpectedly closed: timed out
and
Traceback (most recent call last):
File "/opt/python3.7/lib/python3.7/smtplib.py", line 387, in getreply
line = self.file.readline(_MAXLINE + 1)
File "/opt/python3.7/lib/python3.7/socket.py", line 589, in readinto
return self._sock.recv_into(b)
socket.timeout: timed out
It consistently happens within the __init__ function of a form. And specifically in there when I try to append a validator to a validators list.
self.password.validators.append(InputRequired(message='Please enter password!'))
It is not specific to this one validator, the same thing happens with others too. I am at a complete loss as to why and how appending validators would interact with SMTP or sockets.
One thing I noticed is that I had an issue with the Email validator which also worked fine locally but required me to install the module 'email_validator' to work. When I did that it complained that it couldn't use append on a tuple when I tried to append a validator to - what was a list in my local app - but for some reason turned out to be a tuple when I ran the exact same code on the app engine. I removed the Email validator (and the import/requirements) entirely and now I'm at the problem I've described above. It may or may not be related to the Email validator issue.
Update: The error message does indeed seem to be related to the validators.append() call seeing as by setting up the validators at object creation and avoiding all further changes via validators.append() fixed the problem for me. This is the requirements.txt that goes with the application:
blinker>=1.4
Click>=7.0
Flask>=1.1.1
Flask-Login>=0.5.0
Flask-Mail>=0.9.1
Flask-SQLAlchemy>=2.4.1
Flask-WTF>=0.14.3
itsdangerous>=1.1.0
Jinja2>=2.11.1
MarkupSafe>=1.1.1
PyJWT>=1.7.1
PyMySQL>=0.9.3
python-dotenv>=0.11.0
SQLAlchemy>=1.3.13
Werkzeug>=1.0.0
WTForms>=2.2.1
Instead of a full minimal, reproducible example I can offer a minimized version of the form that was causing the issues when instanciated on the GAE:
class ExampleForm(FlaskForm):
name = StringField('Name')
original_name = HiddenField('Original Name')
def add_validators(self):
names = [a.name for a in Users.query.all()]
if self.original_name.data:
names.remove(self.original_name.data)
self.name.validators.append(InputRequired(message='Please enter name!'))
self.name.validators.append(NoneOf(names, message='Username already exists!'))
def __init__(self):
super().__init__(*args, **kwargs)
self.add_validators()
As I'm writing this it occurs to me that maybe I need to always initialize an empty validators list on creation to ensure new entries can be appended to it (like with a normal list) like this name = StringField('Name', validators=[]). I'd still be confused as to why it works locally, but if it fixes the problem that'd be what really counts for me.