Programming Python, Second Edition

Pipes

Pipes, another cross-program communication device, are made available in Python with the built-in os.pipe call. Pipes are unidirectional channels that work something like a shared memory buffer, but with an interface resembling a simple file on each of two ends. In typical use, one program writes data on one end of the pipe, and another reads that data on the other end. Each program only sees its end of the pipes, and processes it using normal Python file calls.

Pipes are much more within the operating system, though. For instance, calls to read a pipe will normally block the caller until data becomes available (i.e., is sent by the program on the other end), rather than returning an end-of-file indicator. Because of such properties, pipes are also a way to synchronize the execution of independent programs.

Anonymous Pipe Basics

Pipes come in two flavors -- anonymous and named. Named pipes (sometimes called “fifos”) are represented by a file on your computer. Anonymous pipes only exist within processes, though, and are typically used in conjunction with process forks as a way to link parent and spawned child processes within an application -- parent and child converse over shared pipe file descriptors. Because named pipes are really external files, the communicating processes need not be related at all (in fact, they can be independently started programs).

Since they are more traditional, let’s start with a look at anonymous pipes. To illustrate, the script in Example 3-15 ...