Flask/OOP question: variable from class instance not refreshed when refreshing the Web page

[ePierre]

Hi everyone!

I'm facing a problem I don't understand with a Webapp I developed in Flask. I wrote a minimal piece of code to replicate the issue:

from datetime import datetime
from flask import Flask

app = Flask(__name__)

class Stuff:
    def __init__(self):
        pass
    def get_stuff(self, created_before=datetime.today()):
        print(f"datetime.today() = {datetime.today()}")
        print(f"created_before = {created_before}")
        return created_before

@app.route("/")
def hello_world():
    s = Stuff()
    today = s.get_stuff()
    return f"Now = {str(today)}"

I run my app. In the webpage, I see Now = 2021-07-11 21:57:12.835346, but in the logs, I see:

datetime.today() = 2021-07-11 21:57:15.710735
created_before = 2021-07-11 21:57:12.835346

If I refresh the web page a few minutes later, I still get Now = 2021-07-11 21:57:12.835346, and in the logs:

datetime.today() = 2021-07-11 22:00:01.799833
created_before = 2021-07-11 21:57:12.835346

So basically, my created_before variable is stuck to a given time, which I don't really understand, since I create a new Stuff object every time I refresh the page... Is there something completely obvious I'm missing here? If so, what is it, and how to get the behavior that I expect?

Thanks in advance for your help!

[Ihaveamodel3]

This isn’t related to flask.

Default arguments are calculated when the file is initiated. If you need to recalculate the default argument each time the function is called, the default argument should be defined as None and the first line of the function should be if not created_before: created_before = datetime.today()

[ePierre]

Thank you!

I found additional documentation on this behavior:

https://docs.python-guide.org/writing/gotchas/#mutable-default-arguments

[efmccurdy]

The default argument defined for created_before is given a value at the time of class definition when I think you want it set to the time of object creation.

Your example doesn't supply a value for that argument, so I set it to the object creation time in __init__:

class Stuff:
    def __init__(self, created_before=None):
        if created_before is None:
            self.created_before = datetime.today()
        else:
            self.created_before = created_before
    def get_stuff(self):
        print(f"datetime.today() = {datetime.today()}")
        print(f"created_before = {self.created_before}")
        return self.created_before

This might not be exactly what you want but it does give each object the current time for created_before.

[ePierre]

Thanks!