[Python] python flask can't find '__main__' module in ''

Hi All I am just learning about flask. I have used pip to install it. Then when I run this basic code I get an error. Basically I see its working then abruptly exits with the following error. This maybe looks to be some environment issue but I'm not sure. The strange thing this was working the other day now it's not.

from flask import Flask
app = Flask(__name__)

@app.route('/')
def hello_world():
return 'Hello, World!'


if __name__ == '__main__':
app.run(debug=True, port=8000, host='0.0.0.0')

* Running on http://0.0.0.0:8000/ (Press CTRL+C to quit)
* Restarting with stat
/Library/Frameworks/Python.framework/Versions/3.4/bin/python3: can't find '__main__' module in ''

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