Execution Optimization Tools

CPython, Jython, and IronPython all implement the Python language in similar ways: by compiling source code to byte code and executing the byte code on an appropriate virtual machine. Still other systems, including the Psyco just-in-time compiler and the Shedskin C++ translator, instead attempt to optimize the basic execution model. These systems are not required knowledge at this point in your Python career, but a quick look at their place in the execution model might help demystify the model in general.

The Psyco just-in-time compiler

The Psyco system is not another Python implementation, but rather a component that extends the byte code execution model to make programs run faster. In terms of Figure 2-2, Psyco is an enhancement to the PVM that collects and uses type information while the program runs to translate portions of the program's byte code all the way down to real binary machine code for faster execution. Psyco accomplishes this t Jython and IronPython are completely independent implementations of Python that compile Python source for different runtime architectures. It is also possible to access Java and .NET software from standard CPython programs: JPype and Python for .NET systems, for example, allow CPython code to call out to Java and .NET components.

translation without requiring changes to the code or a separate compilation step during development.

Roughly, while your program runs, Psyco collects information about the kinds of objects being passed around; that information can be used to generate highly efficient machine code tailored for those object types. Once generated, the machine code then replaces the corresponding part of the original byte code to speed your program's overall execution. The net effect is that, with Psyco, your program becomes much quicker over time and as it is running. In ideal cases, some Python code may become as fast as compiled C code under Psyco.

Because this translation from byte code happens at program runtime, Psyco is generally known as a just-in-time (JIT) compiler. Psyco is actually a bit different from the JIT compilers some readers may have seen for the Java language, though. Really, Psyco is a specializing JIT compiler—it generates machine code tailored to the data types that your program actually uses. For example, if a part of your program uses different data types at different times, Psyco may generate a different version of machine code to support each different type combination.

Psyco has been shown to speed Python code dramatically. According to its web page, Psyco provides "2x to 100x speed-ups, typically 4x, with an unmodified Python interpreter and unmodified source code, just a dynamically loadable C extension module." Of equal significance, the largest speedups are realized for algorithmic code written in pure Python—exactly the sort of code you might normally migrate to C to optimize. With Psyco, such migrations become even less important.

Psyco is not yet a standard part of Python; you will have to fetch and install it separately. It is also still something of a research project, so you'll have to track its evolution online. In fact, at this writing, although Psyco can still be fetched and installed by itself, it appears that much of the system may eventually be absorbed into the newer "PyPy" project—an attempt to reimplement Python's PVM in Python code, to better support optimizations like Psyco.

Perhaps the largest downside of Psyco is that it currently only generates machine code for Intel x86 architecture chips, though this includes Windows and Linux boxes and recent Macs. For more details on the Psyco extension, and other JIT efforts that may arise, consult http://www.python.org; you can also check out Psyco's home page, which currently resides at http://psyco.sourceforge.net.

The Shedskin C++ translator

Shedskin is an emerging system that takes a different approach to Python program execution—it attempts to translate Python source code to C++ code, which your computer's C++ compiler then compiles to machine code. As such, it represents a platform-neutral approach to running Python code. Shedskin is still somewhat experimental as I write these words, and it limits Python programs to an implicit statically typed constraint that is technically not normal Python, so we won't go into further detail here.

Initial results, though, show that it has the potential to outperform both standard Python and the Psyco extension in terms of execution speed, and it is a promising project. Search the Web for details on the project's current status.

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