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ModuleNotFoundError: No module named 'flask'

Exact error

ModuleNotFoundError: No module named 'flask'

What it means

The Python interpreter running your code doesn't have the package installed.

Likely causes

• Package not installed, or installed into a different Python/virtualenv.
• Not listed in requirements.txt / pyproject.toml, so fresh environments miss it.

How to fix it

1. Install with the same interpreter that runs your code:

   python -m pip install flask

2. Record it so deploys install it too:

   echo "flask" >> requirements.txt

Prevent this next time

• Commit package-lock.json or requirements.txt so every environment installs the same dependencies.
• Read the port from `process.env.PORT` and bind to 0.0.0.0 in hosted environments.

Guide: catch outages before your users do.

Related errors

• Error: Cannot find module 'express'