These guidelines are informal descriptions of the sort of thinking that goes into the design of Pidgin, libpurple, and family. These are not hard-and-fast rules, but they are conscious decisions made by the Pidgin developers which will be violated only after careful consideration. (Or by mistake, and corrected shortly thereafter!)
While Pidgin is a multiprotocol IM client, the goal is to hide protocols from the user as much as possible. Obviously users have to know about individual protocols when they create or modify accounts, but in day-to-day communication and usage, the intent is that users don't have to think about protocols at all. The workflow in Pidgin is intended to be "I would like to chat with Sean about wibbles", not "I would like to create an XMPP conversation with seanegan@…". The focus is on the goal, not the process. In reaching toward this focus, we have chosen to paper over the differences between the various protocols and features as much as possible (without crippling or needlessly complicating things). This has lead to decisions such as the removal of protocol icons from the buddy list and the implementation of contact-aware chats and logs. Sean has elaborated on this theme: http://www.pidgin.im/~seanegan/cgi-bin/pyblosxom.cgi/identity.html
Simpler Is Better
Perfection is achieved, not when there is nothing more to add, but when there is nothing left to take away. --Antoine de Saint-Exupery
In general we try to keep the code and the user interface simple. Especially when it comes to preferences in the UI. Pidgin should be as streamlined as possible. It is an IM client, so messaging someone should be easy. See this essay by Havoc Pennington for an explanation of some ideas that we try to follow.
It's important for the code to be simple because this is an open source project and developers come and go. You never know who's going to be looking at your code next. If you write a function that's hard to follow then the next person that comes along will end up rewriting it, and that's counterproductive.
In plain language, this means that the protocol-specific code goes in the protocol plugin ('prpl'), and that libpurple exists, and is cleanly separated from the user interface. There are practical implications to this. While all of our code depends on glib, only the Pidgin specific parts depend on GTK+. To implement, for example, file transfer, there are 3 steps. First, the protocol(s) have to support it. By themselves, however, the protocols can do nothing. So the "core," libpurple, has to support it also (the second step). We do not want massive amounts of very similar code in libpurple, so the implementation of file transfer at the libpurple level has to abstract away from how individual protocols handle it, so as to be able to use the same calls from all file transfer supporting protocols. Last, but not least, before the user can actually send or receive a file, the UI (Pidgin, Finch or Adium) must support it. These interfaces know nothing about the protocol, and have only limited contact with the core. This helps to enforce the desire for uniformity explained above. It also makes it easier for the only sort of duplication we encourage: many interfaces. The core implementation cannot assume too much about what the UI will do, because the GTK+ UI (Pidgin) might need to handle a file transfer somewhat differently than the ncurses based UI (Finch). Patches that voilate this layering will be rejected. In practice, this means that there is more work involved to introduce a new class of functionality, say file transfer, white-boarding, voice, or video. On the other hand, it means less work to implement any given class of functionality for a new protocol or for a new UI.
Our source code is very much a 'live' document. It should reflect what is currently needed, not what used to be needed or what might be needed in the future. Old code should be removed if it isn't being used (of course, you can only remove public functions and structures when the major version number increases)--it'll always be in the source code repository if anyone needs it. The code should contain documentation about what it does and why.
Pidgin and libpurple are single threaded. That means that the network code runs in the same process as the user interface. Network code must be non-blocking, otherwise the UI will be unresponsive. Code should be event-driven. Long running tasks should be asynchronous. File descriptors that need to be watched for changes should be added to the event loop.