ModuleNotFoundError: No module named 'flask'

Question

After reading title of this post, don't try to make duplicate first because herewith content may be asked in different way. Btw, I'm very new in python and start learning now for work requirement.

here are my dependencies

virtualenv --version => 15.0.2

pip --version => 19.0.3

flask --version => 1.0.2, Python 2.7.10 (default, Aug 17 2018, 19:45:58)

python --version => 3.7.1

And, here is my source code of main.py

from flask import Flask
app = Flask(__name__)

@app.route("/")
def home():
    return "Hello, World!"

Problem is following error encountered when I render like python main.py

Traceback (most recent call last): File "main.py", line 1, in from flask import Flask ModuleNotFoundError: No module named 'flask'

But when I render like FLASK_APP=main.py flask run , it was working. Please let me know how's difference between python... and FLASH_APP=...

Answer 1

pip can for some reason point to system-wide pip (which on many systems corresponds to Python 2.7). In order to use pip from the virtualenv, use python -m pip command. The following command will do the trick:

pip uninstall flask && python -m pip install flask

Another possibility is that you installed flask via apt and not pip . Here's the difference between the two: What is the difference between `sudo apt install python3-flask` and `pip3 install Flask`?

So now the flask command is available system-wide.

If this is the case, uninstalling flask with apt and installing it with pip should do the trick:

sudo apt remove python-flask
pip install flask

(this is my guess that the apt package is called python-flask .

Answer 2

As Pavel said; you might use python 2.7 instead of python 3 to run your project...

I had this issue and by using this command instead of Pavel's command, problem fixed

pip uninstall flask && python3.7 -m pip install flask

You can replace flask by any other lib and this will work

Hope it works for everyone sees this too

Answer 3

如果您使用带有 pipenv 的 Visual Studio Code IDE 并收到上述错误，则应检查此链接。

Answer 4

The previous answers are fine but you may need to be careful of the version of python you use to install flask this time. For me,

pip3 uninstall flask && python -m pip install flask

did not work, but

pip3 uninstall flask && python3 -m pip install flask

did work. The issue of having the right version when installing flask and its installation in the virtual environment also has consequences. Find out more in this article: ModuleNotFoundError .