Flask, SQLAlchemy : KeyError: 'SQLALCHEMY_TRACK_MODIFICATIONS'

Question

I am trying to follow the instructions from the following tutorial:

I downloaded the code from the following repo:

However when I run it locally and try to add something to the database, I get the following error:

builtins.KeyError
KeyError: 'SQLALCHEMY_TRACK_MODIFICATIONS'

When I tried to read the traceback, I realised that even if I add a variable SQLALCHEMY_TRACK_MODIFICATIONS to the config file, some python library file is unable to recognise it exists.

Looks like there is another answer to a similar question, but that was more like a quick fix, not why this is happening.

I would like to know why this is happening and how to fix it.Preferably without changing the whole structure.

Thanks a lot in advance.

Stephen Rauch · Accepted Answer · 2018-01-21 00:42:08Z

35

Having two app = Flask(__name__) in the code can cause this problem.

That was my case, I removed one and kept the one in the app's folder's __init__.py, and it worked

edited Jan 21, 2018 at 0:42

Stephen Rauch♦

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answered Jan 21, 2018 at 0:18

Javier PR

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1 Comment

Zaki Over a year ago

Another reason for this error is forgetting the db.init_app(app)

Rotem jackoby · Accepted Answer · 2019-05-18 07:10:51Z

8

I solved this problem by this way.
Remove the current one, and replace the old version.

pip3 uninstall flask-sqlalchemy
pip3 install flask-sqlalchemy==2.1.0

edited May 18, 2019 at 7:10

Rotem jackoby

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answered May 18, 2019 at 6:17

SeungWook Kim

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2 Comments

upendra Over a year ago

Wow. That worked like a charm. Never expected that changing the version would do such a wonder. Thanks a lot

Alex Over a year ago

I'm not sure I understand this solution. We are now on version 2.4.1 and this issue exists in scenarios.

vinit payal · Accepted Answer · 2018-05-22 06:34:11Z

3

@Javier's answer gave me a direction to problem as mentioned reason of error is having multiple flask apps.

Apart from creating app in __init__.py one more solution which worked was to use newly created app's context to run the query and boom!!! error was gone.

Below is code snippet for using newly created app's context:-

app = Flask(__name__)
app.config.from_pyfile('./config.py')
init_app(app)

def create():
    with app.app_context():
       #Below Tags is model class 
       tag = Tags(**data)
       db.session.add(tag)
       db.session.commit()
       return from_sql(tag)

answered May 22, 2018 at 6:34

vinit payal

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1 Comment

Idris Stack Over a year ago

Where is config.py ? please add it there

b26 · Accepted Answer · 2021-09-12 13:42:12Z

2

I had same problem .. i am using connexion

The solution which worked for me was removing one instance of connexion object in server.py. You need to init connex_app in the config file then load the object in server like this:

in my config.py

connex_app = connexion.App(__name__, specification_dir=basedir)

and in my server.py

import config

connex_app = config.connex_app

This way using the object which is already instantiated worked for me !!!

edited Sep 12, 2021 at 13:42

b26

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answered Aug 6, 2021 at 4:00

Lavanya Rani

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