cannot import name 'mydb' from partially initialized module 'connection' in Python

To answer the above question, we need to understand the problem of circular dependency.

To understand the circular dependency, I want to layout a simple example, in front of you.

I think every app needs to have the few basic blocks as follows:

+----------------+-------------------------------------------------------------------------------------------+
|    Filename    |                                        Description                                        |
+----------------+-------------------------------------------------------------------------------------------+
| app.py         | Creates the app and starts the server.                                                    |
| models.py      | Define what the entity will look like (e.g, UserModel has username, email, password etc.) |
| controllers.py | Fetches Data from database, generates HTML and sends the response to the user browser.    |
+----------------+-------------------------------------------------------------------------------------------+

Our simple example will also have three files

project/
    - app.py ( Creates and starts the server)
    - models.py ( Class to model a user)
    - controllers.py ( We will fetch data from database, and return html to user.)

The contents of the app.py file will look as follows:

# =============
#     app.py
# =============

# Define the application
app = Flask()

# Define the Database
db = SQLAlchemy(app)

# Register the Controller
from .controllers import auth_controller
app.register_blueprint(auth_controller)

The contents of the models.py file will look as follows:

# =============
#     models.py
# =============

from .app import db

# We will not focus on implementation
class User(db.Model):
    pass

The contents of the controllers.py file will look as follows:

# =============
#     controllers.py
# =============
from flask import Blueprint
from .models import User


# Create the auth app
auth = Blueprint('auth', __name__)

# Define the Rotues
@auth.route('/login')
def login():
    return "I will fetch some data and allow the user to login"

I think now, I have laid out a diagram of our app, now let’s proceed to understanding how the app will work.

1. The app starts from app.py
2. app variable inside the app.py file gets created in memory.
3. db variable inside the app.py gets created in memory.
4. Now, to import auth from controllers.py file we switch to “`controllers.py“ file
5. We import Blueprint from flask.
6. To import User, we switch to models.py file.
7. Now, inside models.py file we import db (We are able to import it because it was created in step 3)
8. And program continues so on and so on….

The most important import step in the above sequence is step 7, becuase it will cause the problem of circular dependency in our app, in just a moment.

Now we will try to change the app.py file to introduce the problem of circular dependency.

Now, as a developer, we might think that all our imports should be at the top of the file, doesn’t it make your code cleaner. Yes, of course! it does make the code cleaner.

# ============================
#       Refactored app.py file
# ============================
from .controllers import auth_controller

# ......
# Rest of the file is same, we only shifted this import at the top

Now, we have a problem of circular dependency in our app. Let me show you, how?

1. Our app starts from app.py file
2. First, we need to import auth_controller from controllers.py file
3. Let’s visit the controllers.py file, and process it.
4. From flask we import Blueprint
5. Let’s switch to models.py file to import User
6. Inside models.py file, we import db from app (But db does not exist yet.)

Now, I think you got the point, The problem if just saw, was an example of circular dependency. The same problem is causing the ImportError in your case.

The solution would be to inspect the import statements and put them at the right place. Sometimes, we use code formatter, and it refactors all the import statements at the top. This might be causing the problem for you.

I hope the this may answer your question!