[Solved] ModuleNotFoundError: No module named 'flask'

In this tutorial, we'll address a common Python error: "ModuleNotFoundError: No module named 'flask'".

This error occurs when you try to import the Flask library in your Python script, but it's not installed or not found in your current Python environment.

Understanding the Flask ModuleNotFoundError

This error typically indicates one of two issues:

1. The Flask library is not installed in your Python environment.
2. Your Python script is running in an environment where Flask is not available.

Solution 1: Installing Flask

The most straightforward solution is to install Flask using pip, Python's package installer:

pip install flask

If you're using Python 3 specifically, you might need to use pip3:

pip3 install flask

After installation, try running your script again. If the error persists, try the following solutions.

Solution 2: Verifying the Installation

To confirm that Flask is correctly installed, you can use these Python commands:

import sys
print(sys.executable)

import flask
print(flask.__version__)

This will print the path to your Python interpreter and the version of Flask if it's installed correctly.

Solution 3: Using Virtual Environments

Virtual environments are particularly useful for Flask projects. Here's how to set one up and install Flask:

# Create a virtual environment
python -m venv flaskenv

# Activate the virtual environment
# On Windows:
flaskenv\Scripts\activate
# On macOS and Linux:
source flaskenv/bin/activate

# Install Flask in the virtual environment
pip install flask

After activation, run your Flask application within this environment.

Solution 4: Checking Python Path

Ensure that Flask is in your Python path:

import sys
print(sys.path)

# If needed, add the path where Flask is installed
import site
site.addsitedir("/path/to/site-packages")

Solution 5: Reinstalling Flask

If issues persist, try uninstalling and reinstalling Flask:

pip uninstall flask
pip install flask

Solution 6: Using Anaconda

If you're using Anaconda, install Flask with:

conda install flask

Ensure you're in the correct Anaconda environment where you want Flask installed.

Solution 7: Installing Flask-related Dependencies

Sometimes, you might need additional Flask-related packages. Install them with:

pip install flask-sqlalchemy
pip install flask-migrate
pip install flask-login
# Add other Flask extensions as needed

Solution 8: Upgrading Pip and Setuptools

Outdated pip or setuptools can sometimes cause installation issues. Upgrade them with:

pip install --upgrade pip
pip install --upgrade setuptools

Troubleshooting Tips

1. Check if you have multiple Python versions and ensure you're using the intended one.
2. On Windows, verify that the Python Scripts directory is in your system's PATH.
3. If using an IDE, restart it after installing Flask to ensure it recognizes the new installation.
4. Check for any error messages during the Flask installation process, as they might provide clues about specific issues.
5. Ensure you're not naming your own Python file 'flask.py', as this can conflict with the actual Flask module.

Conclusion

The "ModuleNotFoundError: No module named 'flask'" is a common issue when starting with Flask development.

By following these steps, you should be able to successfully install Flask and import it in your Python scripts.

Remember that using virtual environments is especially important for Flask projects to manage dependencies effectively and avoid conflicts between different projects.