2. Next we create an instance of this class. We pass it the name of the module or
package. This is needed so that Flask knows where to look for templates, static
files, and so on.
3. We then use the route() decorator to tell Flask what URL should trigger our
function.
4. The function is given a name which is also used to generate URLs for that partic-
ular function, and returns the message we want to display in the user’s browser.
5. Finally we use the run() function to run the local server with our application.
The if __name__ == ’__main__’: makes sure the server only runs if the script is
executed directly from the Python interpreter and not used as imported module.
To stop the server, hit control-C.
Externally Visible Server
If you run the server you will notice that the server is only accessible from your own
computer, not from any other in the network. This is the default because in debugging
mode a user of the application can execute arbitrary Python code on your computer.
If you have debug disabled or trust the users on your network, you can make the server
publicly available simply by changing the call of the run() method to look like this:
app.run(host=’0.0.0.0’)
This tells your operating system to listen on all public IPs.
3.2 Debug Mode
The run() method is nice to start a local development server, but you would have to
restart it manually after each change to your code. That is not very nice and Flask can
do better. If you enable debug support the server will reload itself on code changes,
and it will also provide you with a helpful debugger if things go wrong.
There are two ways to enable debugging. Either set that flag on the application object:
app.debug = True
app.run()
Or pass it as a parameter to run:
app.run(debug=True)
Both methods have the exact same effect.
Attention
Even though the interactive debugger does not work in forking environments (which
makes it nearly impossible to use on production servers), it still allows the execution
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