ModuleNotFoundError: No module named 'flask'

Posted in Python by Dirk - last update: Feb 05, 2024

Python raises the ModuleNotFoundError: No module named 'flask when it is unable to find the flask library. The most likely cause is that you didn’t install flask in the environment where you are running your code. Quick fix: install flask using: the pip install flask command

Possible causes and solutions

Flask is a lightweight and web framework written in Python. It is designed to be simple, easy to use, and flexible, making it an excellent choice for building web applications and APIs. Flask is considered a microframework because it focuses on providing the essentials needed for web development without imposing a specific project structure or dependencies.

Sample code:

from flask import Flask

app = Flask(__name__)

@app.route('/')
def hello_world():
    return 'Hello, World!'

if __name__ == '__main__':
    app.run(debug=True)

And when trying to run the code you get the ’no module named error’ - you try running your code, and there it is:

Traceback (most recent call last):
  File "myfile.py", line 1, in <module>
ModuleNotFoundError: No module named 'flask'

Luckily, it is easy to fix. Below you can find the most common causes and how to fix them:

‘flask’ Library is not installed

In your code you are importing the ‘flask’ module

from flask import Flask

which generates the ModuleNotFoundError: No module named 'flask’. The the most common cause for this error: the environment where you are running your code doesn’t have the flask library installed.

You can use the following command in the terminal or command prompt to check if the flask library is installed:

pip show flask

If flask is not installed you will get this result:

WARNING: Package(s) not found: flask

Solution:

pip install Flask

‘flask’ Module installed in the Wrong Directory

It is possible that you did install the flask module, but that the directory where you installed it is not in the Python path.

You can check the Python path in a few different ways:

1. Using sys module in Python:

Open a Python script or the Python interpreter and run the following code:

import sys
print(sys.path)

This will print a list of directories that make up the Python path.

2. Using Command Line

Open a command prompt or terminal and run:

Windows

echo %PYTHONPATH%

Unix/Linux/macOS

echo $PYTHONPATH

If the PYTHONPATH environment variable is set, it will display the directories in the Python path.

Solution: Move the flask module to a directory that is included in the Python path or add the flask module’s directory to the sys.path using sys.path.append().

‘flask’ not installed in Virtual Environment

Even if you have flask installed in on your computer, if you are using a virtual environment you cannot access it from this environment.

If you are using virtual environments, ensure that the virtual environment is activated, and that the flask modules is installed within this environment.

Installation is simple:

pip install flask

File Naming Conflicts:

It is not really likely - but if you happen to have a ```flask.py`` file in your environment and you also installed the standard flask library it will cause a conflict. Rename your script/module to avoid conflicts with existing module names.