Since this issue pops up for every Flask project you try—even a brand new Hello World one—it’s almost certainly not a problem with your code. Let’s walk through the most likely fixes, focused on PyCharm configurations and your Python environment:

1. Verify Your Python Interpreter & Flask Installation #

First, double-check that PyCharm is using the correct Python environment where Flask is properly installed. IDEs can sometimes default to a global interpreter or stale virtual environment without you noticing:

• Go to File > Settings > Project: [Your Project Name] > Python Interpreter
• Look for flask in the list of installed packages. If it’s missing, click the + button, search for Flask, and install it.
• If it’s present, try upgrading/downgrading to a stable version (like 2.0.x) in case there’s a compatibility issue with your Python version.

2. Fix PyCharm’s Run/Debug Configuration #

You mentioned you can’t modify the server port—this suggests your run configuration might be locked or misconfigured:

• Click the run configuration dropdown in the top-right corner of PyCharm, then select Edit Configurations...
• Find your Flask configuration (or create a new one if needed)
• Check the Parameters field—make sure there’s no forced --port 5000 entry overriding your code. To test a different port, add --port 5001 here, or modify your code to app.run(port=5001)
• Also verify the FLASK_APP environment variable is set correctly (it should point to your main script, like app.py)

3. Clear PyCharm’s Cache & Restart #

PyCharm’s cached files can get corrupted and cause weird, inconsistent behavior. A quick reset often fixes this:

• Go to File > Invalidate Caches...
• Check the box for Clear file system cache and local history, then click Invalidate and Restart
• Once PyCharm reboots, try running your Hello World project again.

4. Test Outside PyCharm (Terminal Run) #

To rule out PyCharm as the culprit, run your Flask project directly from the terminal:

• Navigate to your project directory in a terminal
• Activate your virtual environment (if using one: source venv/bin/activate on Mac/Linux, venv\Scripts\activate on Windows)
• Run either python app.py or flask run
• If this works without errors, the problem is definitely in PyCharm’s settings. If it still throws an internal server error:
  • Check if port 5000 is occupied by another process: use lsof -i :5000 (Mac/Linux) or netstat -ano | findstr :5000 (Windows) to see what’s using the port, then kill that process.
  • Temporarily disable your firewall/antivirus—sometimes these tools block Flask’s server requests unexpectedly.

5. Reinstall Flask & Dependencies #

If all else fails, a clean reinstall of Flask can fix hidden corruption:

• In your terminal (with the virtual environment activated), run pip uninstall flask -y
• Then run pip install flask to get the latest stable version, or specify a known-good version like pip install flask==2.0.3

内容的提问来源于stack exchange，提问作者user9353487