Is it bad practice to write a whole Flask application in one file?
As it is a micro framework, you should not rely on it to build full blown applications since it's not designed for it. 
As long a you keep your project small (a few forms, a few tables and mostlys static contents) you will be fine. But if you want to have a bigger application, you might "out program" the capacities of the framework in terms of modularity and reuse of code. In that case you might want to migrate toward a full blown framework where everything is separated in their own module.
I'm currently writing a web application in Python using the Flask web framework. I'm really getting used to just putting everything in the one file, unlike many other projects I see where they have different directories for classes, views, and stuff. However, the Flask example just stuff everything into the one file, which is what I seem to be going with.
Is there any risks or problems in writing the whole web app in the one single file, or is it better to spread out my functions and classes across separate files?