ModuleNotFoundError: No module named 'flask'

Problem Summary:

We are encountering a persistent issue where the Flask module is not being found when trying to deploy a Python Flask application on The error we are consistently facing is a “ModuleNotFoundError” for Flask, despite it being installed and the application working as expected locally. The key challenge is ensuring that the dependencies, particularly Flask, are recognized and accessible in the deployment environment on Render.

Actions Taken:

  1. Local Environment Testing:
    The Flask application works perfectly in the local environment. The dependencies are installed and recognized, and the application is running as expected.

  2. Initial Deployment:
    Upon deploying to, we encountered a “ModuleNotFoundError” indicating that Flask is not installed or not found in the expected path.

  3. Dependency Installation:
    We tried installing dependencies using a virtual environment and without it. We ensured that the ‘requirements.txt’ file is up to date and the dependencies are installed during the build process.

  4. PYTHONPATH Modification:
    We modified the PYTHONPATH to include the path where the dependencies are installed. We ensured that this path is explicitly included during the application start command.

  5. Python Version:
    We ensured that the Python version is consistent. The user confirmed using Python 3.10.9. We even set the Python version explicitly in the environment variables on Render.

  6. Logging:
    We added logs to print the Python version and sys.path to understand the paths Python is referring to, and to ensure that the correct version of Python is being used.

Despite these efforts, the “ModuleNotFoundError” for Flask persists. The dependencies, especially Flask, appear not to be recognized in the path or the environment where the application is running on


Start with the basics:

  1. Ensure Flask is listed in your requirements.txt file.
  2. Install the necessary dependencies using pip install -r requirements.txt exclusively.
  3. Launch your Flask app—whether through gunicorn, ‘flask run’, or your preferred method.

Let me know how that goes.

Render Support, UTC+3

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