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I have created a python script inside the virtualenv and tried to execute that file. This is my code:

(env) C:\Users\amitayadav\Documents\Amita_projects\Python\flask_project>python3 app.py
Traceback (most recent call last):
  File "app.py", line 1, in <module>
    from flask import Flask
ModuleNotFoundError: No module named 'flask'

It throws an error. However, when I run this command it runs fine:

(env) C:\Users\amitayadav\Documents\Amita_projects\Python\flask_project>python app.py
 * Serving Flask app "app" (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: on
 * Restarting with stat
 * Debugger is active!
 * Debugger PIN: 500-586-454
 * Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)

I don't understand why this is happening. What is the problem with the python3 command?

petezurich
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Amita Yadav
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  • because flask is installed for python (python2 ) not for python3. install pip3("sudo apt-get -y install python3-pip"), and install flask using "sudo pip3 install flask" and try – venkata krishnan Jan 13 '20 at 07:55

3 Answers3

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The packages are installed only for the Python version that the virtual environment is configured to use, which is generally just run as python.

When the virtual environment is created, the interpreter is copied for use within that environment. By running the python3 interpreter, you might end up with an interpreter outside your environment, that doesn't have the required dependencies installed.

If you want to have the virtual environment use the Python 3 version, you might need to create the virtualenv with the optional --python flag to indicate that:

virtualenv --python=python3 my_virtual_env_directory

Then you just again install the required packages as before:

pip install flask

Then you just run as usual:

$ python
Python 3.6.7 (default, Oct 29 2018, 11:42:59) 
[GCC 5.4.0 20160609] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import flask
>>>

The correct versions of pip and the interpreter python are used implicitly to install dependencies and to run your application.

moooeeeep
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Your global packages does not have flask library installed. the code that is executing successfully is being run inside a virtual environment which has the flask dependency installed.

Run the command pip install flask outside the virtual environment and it should start the flask application.

Yayati Sule
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Did you install flask?

pip install flask

Also check this answer, it may help you.