General file reading in Panda is handled by the Virtual File System.
Although it presents the files and directories that it provides access to as a single, unbroken file system, it can in fact include files from multiple sources (such as Multifiles) in the hierarchy, regardless of the underlying structure.
This has the advantage of allowing one to access files and directories without worrying overmuch about where they actually reside, and even access Multifile archives as a directory hierarchy.
NB: While present in C++, on the Python side Panda does not offer file writing functionality, with the exception of writing to certain specialized file types. For general file writing, however, Python itself offers file-handling functionality, including potentially-useful features such as reading to the end of the line in a single call.
Thread-safe file I/O
In versions 1.6.0 and above, Panda3D offers a thread-safe replacement for the Python file module. You can find it in direct.stdpy.file. The interface is exactly the same as Python's, so it's safe to put this import above all the files where you want to use the "file" or "open" functions:
from direct.stdpy.file import *
This module reimplements Python's file I/O mechanisms using Panda constructs. This enables Python to interface more easily with Panda's virtual file system, and it also better-supports Panda's SIMPLE_THREADS model, by avoiding blocking all threads while waiting for I/O to complete.
Note: This section is incomplete.