Learning Python

Odds and Ends

In this section, we introduce a few module-related ideas that seem important enough to stand on their own (or obscure enough to defy our organizational skills).

Module Compilation Model

As currently implemented, the Python system is often called an interpreter, but it’s really somewhere between a classic interpreter and compiler. As in Java, Python programs are compiled to an intermediate form called bytecode, which is then executed on something called a virtual machine. Since the Python virtual machine interprets the bytecode form, we can get away with saying that Python is interpreted, but it still goes through a compile phase first.

Luckily, the compile step is completely automated and hidden in Python. Python programmers simply import modules and use the names they define; Python takes care to automatically compile modules to bytecode when they are first imported. Moreover, Python tries to save a module’s bytecode in a file, so it can avoid recompiling in the future if the source code hasn’t been changed. In effect, Python comes with an automatic make system to manage recompiles.^[38]

Here’s how this works. You may have noticed .pyc files in your module directories after running programs; these are the files Python generates to save a module’s bytecode (provided you have write access to source directories). When a module M is imported, Python loads a M.pyc bytecode file instead of the corresponding M.py source file, as long as the M.py file hasn’t been changed since ...