A web application framework is a code library that make a developer's life easier when building reliable, scalable and maintainable web applications.
Web frameworks encapsulate what developers have learned over the past twenty years while building dynamic web applications. Frameworks make it easier to reuse code for common HTTP operations and to structure your code so that it is maintainable.
Frameworks provide functionality in their code or through extensions to perform common operations required to run web applications. These common operations include:
Not all web frameworks include code for all of the above functionality. Frameworks fall somewhere between simply executing a single use case and attempting to be everything to every developer with increased complexity. Some frameworks take the "batteries-included" approach where everything possible comes bundled with the framework while others have a minimal code library that plays well with extensions.
For example, the Django web application framework includes an Object-Relational Mapping (ORM) layer that abstracts relational database read, write, query, and delete operations. However, Django's ORM cannot work without significant modification on non-relational databases such MongoDB. Some other web frameworks such as Flask and Pyramid are easier to use with non-relational databases by incorporating external Python libraries. There is a spectrum between minimal functionality with easy extensibility and including everything in the framework with tight integration.
"What is a web framework?" by Jeff Knupp is a in-depth explanation of what a web framework is and their relation to web servers.
Pyramid, Falcon, web.py are the most common Python web frameworks other than Django, Flask and Bottle.
This roundup of 14 minimal Python frameworks contains both familiar and less known Python libraries.
The web micro-framework battle presentation goes over Bottle, Flask, and many other lesser known Python web frameworks.
