When using Python, a common error you may encounter is modulenotfounderror: no module named ‘flask’. This error occurs when Python cannot detect the Flask library in your current environment. Flask does not come with the default Python installation. This tutorial goes through the exact steps to troubleshoot this error for the Windows, Mac and Linux operating systems.


ModuleNotFoundError: no module named ‘flask’

What is ModuleNotFoundError?

The ModuleNotFoundError occurs when the module you want to use is not present in your Python environment. There are several causes of the modulenotfounderror:

The module’s name is incorrect, in which case you have to check the name of the module you tried to import. Let’s try to import the re module with a double e to see what happens:

import ree
---------------------------------------------------------------------------
ModuleNotFoundError                       Traceback (most recent call last)
1 import ree

ModuleNotFoundError: No module named 'ree'

To solve this error, ensure the module name is correct. Let’s look at the revised code:

import re

print(re.__version__)
2.2.1

You may want to import a local module file, but the module is not in the same directory. Let’s look at an example package with a script and a local module to import. Let’s look at the following steps to perform from your terminal:

mkdir example_package

cd example_package

mkdir folder_1

cd folder_1

vi module.py

Note that we use Vim to create the module.py file in this example. You can use your preferred file editor, such as Emacs or Atom. In module.py, we will import the re module and define a simple function that prints the re version:

import re

def print_re_version():

    print(re.__version__)

Close the module.py, then complete the following commands from your terminal:

cd ../

vi script.py

Inside script.py, we will try to import the module we created.

import module

if __name__ == '__main__':

    mod.print_re_version()

Let’s run python script.py from the terminal to see what happens:

ModuleNotFoundError: No module named 'module'

To solve this error, we need to point to the correct path to module.py, which is inside folder_1. Let’s look at the revised code:

import folder_1.module as mod

if __name__ == '__main__':

    mod.print_re_version()

When we run python script.py, we will get the following result:

2.2.1

Lastly, you can encounter the modulenotfounderror when you import a module that is not installed in your Python environment.

What is Flask?

Flask is a lightweight web framework written in Python. It does not automatically come installed with Python. The simplest way to install Flask is to use the package manager for Python called pip. The following instructions to install Flask are for the major Python version 3.

How to install Flask on Windows Operating System

You can install pip on Windows by downloading the installation package, opening the command line and launching the installer. You can install pip via the CMD prompt by running the following command.

python get-pip.py

You may need to run the command prompt as administrator. Check whether the installation has been successful by typing.

pip --version

To install Flask with pip, run the following command from the command prompt.

pip3 install flask

How to install Flask on Mac Operating System

Open a terminal by pressing command (⌘) + Space Bar to open the Spotlight search. Type in terminal and press enter. To get pip, first ensure you have installed Python3:

python3 --version
Python 3.8.8

Download pip by running the following curl command:

curl https://bootstrap.pypa.io/get-pip.py -o get-pip.py

The curl command allows you to specify a direct download link. Using the -o option sets the name of the downloaded file.

Install pip by running:

python3 get-pip.py

From the terminal, use pip3 to install Flask:

pip3 install flask

How to install Flask on Linux Operating System

All major Linux distributions have Python installed by default. However, you will need to install pip. You can install pip from the terminal, but the installation instructions depend on the Linux distribution you are using. You will need root privileges to install pip. Open a terminal and use the commands relevant to your Linux distribution to install pip.

Installing pip for Ubuntu, Debian, and Linux Mint

sudo apt install python-pip3

Installing pip for CentOS 8 (and newer), Fedora, and Red Hat

sudo dnf install python-pip3

Installing pip for CentOS 6 and 7, and older versions of Red Hat

sudo yum install epel-release

sudo yum install python-pip3

Installing pip for Arch Linux and Manjaro

sudo pacman -S python-pip

Installing pip for OpenSUSE

sudo zypper python3-pip

Once you have installed pip, you can install flask using:

pip3 install flask

Check Flask Version

Once you have successfully installed Flask, you can use two methods to check the version of Flask. First, you can use pip show from your terminal.

pip show flask
Name: Flask
Version: 1.1.2
Summary: A simple framework for building complex web applications.
Home-page: https://palletsprojects.com/p/flask/
Author: Armin Ronacher
Author-email: [email protected]
License: BSD-3-Clause
Location: /Users/Yusufu.Shehu/opt/anaconda3/lib/python3.8/site-packages
Requires: Werkzeug, Jinja2, itsdangerous, click
Required-by: 

Second, within your python program, you can import Flask and then reference the __version__ attribute:

import flask

print(flask.__version__
1.1.2

Installing Flask Using Anaconda

Anaconda is a distribution of Python and R for scientific computing and data science. You can install Anaconda by going to the installation instructions. Once you have installed Anaconda, you can install flask using the following command:

conda install -c anaconda flask

Testing Flask

Once you install Flask, you can test it by writing a hello world script. To do this, first, create a file called flask_test.py and add the code below to the file:

from flask import Flask

app = Flask(__name__)


@app.route('/')

def hello_world():

    return 'Hello, World!'

if __name__ == '__main__':

    app.run()

Save and close the file, then run it from the command line using:

python flask_test.py

You will get something similar to the following output:

 * Serving Flask app "flask_test" (lazy loading)
 * Environment: production
   WARNING: This is a development server. Do not use it in a production deployment.
   Use a production WSGI server instead.
 * Debug mode: off
 * Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)

This output informs you that you can interact with your web application by going to the above URL. Go to http://127.0.0.1:5000/, and “Hello, World!” will appear on the page.

Summary

Congratulations on reading to the end of this tutorial. The modulenotfounderror occurs if you misspell the module name, incorrectly point to the module path or do not have the module installed in your Python environment. If you do not have the module installed in your Python environment, you can use pip to install the package. However, you must ensure you have pip installed on your system. You can also install Anaconda on your system and use the conda install command to install the Flask library.

For further reading on installing data science and machine learning libraries, you can go to the articles:

Go to the online courses page on Python to learn more about Python for data science and machine learning.

Have fun and happy researching!