When comparing web2py vs Phalcon, the Slant community recommends Phalcon for most people. In the question“What are the best backend web frameworks?” Phalcon is ranked 10th while web2py is ranked 24th. The most important reason people chose Phalcon is:
Since the framework is and extension built in C it's extremely fast and efficient. It's actually one of the fastest PHP frameworks, and according to some benchmarks it's the fastest framework out there.
Specs
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Pros
Pro Documentation is written in form of a book which is good for beginners
web2py documentation does not follow the common pattern of using Sphinx, MkDocs or ReadTheDocs which is goos for exeperienced developers. Although documentation in form of a book is very easy and good for beginners. Turning web2py the most easy and comprehensive framework to learn and also to teach.
Pro Web2py apps run on GAE, AWS, VPNs, PythonAnywhere, etc
Web2py apps are designed to be portable. With some minor restrictions web2py apps can run on any VPS on SQL databases and/or Mongo, as well as on Google App Engine with the Google Datastore. It is truly code ones and run everywhere. For example at Camio.com we use web2py internally to access a GAE datastore which contains more images than Instagram.
Pro User support
The web2py community is open and friendly and it gives concrete support to newbies and old timers. It's not difficult to get answers from the BDFL Massimo Di Pierro himself.
Pro Maintainable over time
One really positive aspect of web2py application is their maintainability over the years. Old code works even if the framework is updated to the latest version. Not only that, if code is written well it is very short and a new team can pick it up over in little time.
Pro PyCharm supports web2py
While web2py has its own web based IDE which is convenient, it works with WinIDE, PyCharm, and Eclipse. The first two explicitly support web2py. The latter requires some configuration.
Pro Easy to learn without losing any power
web2py is very easy to learn for beginners, yet it has a great deal of power and flexibility as application needs become more complex. It includes an impressively comprehensive set of features, making development very productive without the need to integrate a lot of third-party libraries.
Pro Can be tuned to be really fast in production
The framework is really fast in production after some optimization and fine tuning which can minimize the memory footprint in order to make it run on a really small VPS or slice.
Pro Easily extendable
Allows users to easily extend functionality by using external libraries.
Pro No need to import the API to access the models and controllers context at every request
Models and controllers live in the context of the HTTP request. So the developer does not have to import the API to access this context at every request.
In other words, the models, controllers and templates in web2py use a domain specific language which uses pure web2py syntax and allows to import any module but exposes a few additional objects.
Pro Includes a web-based IDE for creating and managing applications
web2py includes an "admin" app that serves as a web-based IDE for web2py applications. It includes many features, such as application creation, compiling, and packaging; an error ticketing system; a code editor; a debugger; a controller doctest runner; Git and Mercurial integration; and one-click deployment to PythonAnywhere, Google App Engine, and OpenShift.
It is not intended as a full desktop IDE replacement, but it includes some helpful web2py specific functionality and can be convenient for basic editing and debugging tasks and quick prototyping, even for those who primarily work with a more full-featured desktop IDE or editor.
Pro web2py supports the classic editor Vim
Pro Extremely fast
Since the framework is and extension built in C it's extremely fast and efficient. It's actually one of the fastest PHP frameworks, and according to some benchmarks it's the fastest framework out there.
Pro Open source
Phalcon is open source and is available under the BSD License.
Pro Uses Volt template engine
The Volt template engine, which is embedded into Phalcon itself takes it's inspiration from the Jinja template engine and as such it's nice to look at, with a clear and understandable syntax.
Volt also compiles very fast, like Phalcon itself, so it avoids being a bottleneck for the framework's overall speed.
Pro Very flexible project structure
You can set up the base project as you want. It's very flexible
Pro Clean and light project code
Since the framework code is not in the project directory, the code is light and clean.
Pro Customizable with Zephir
Zephir is a high-level language designed to create PHP extensions easily by PHP programmers with no knowledge in C.
Zephir does this by compiling directly to C and then the C program is in turn compiled to be run as a PHP extension. This, coupled with the fact that Zephir's syntax is very similar to PHP makes it a perfect way for PHP developers to use it for customizing Phalcon.
Pro Loosely coupled components
Some components can be used as standalone packages like models, views, etc..
Cons
Con Abandoned framework with (almost) no community
Con The web IDE is not a full-featured IDE
web2py includes an "admin" app that serves as a web-based IDE for web2py applications. It includes many features, such as application creation, compiling, and packaging; an error ticketing system; a code editor; a debugger; a controller doctest runner; Git and Mercurial integration; and one-click deployment to PythonAnywhere, Google App Engine, and OpenShift. However, particularly with regard to code editing and debugging and version control integration, it is not as full-featured as some of the more popular desktop IDEs such as PyCharm. So, developers expecting a PyCharm-like experience may be somewhat disappointed. In any case, use of the web-based IDE is completely optional.
Con Not for shared hosting
Phalcon needs root access to install the PHP extension which is written in C. Developers who plan on using Phalcon must use VPS or Cloud Hosting with root access available.
Con Require good programming skills
Not so easy to use if you want to gain the best from it.
Con Debugging requires knowledge of C
You need to be a C programmer to debug Zephir or C code. Or if Phalcon is not maintained anymore and you have a problem and don't have much skills in C, you will be hard-pressed to find a C programmer to fix it.