Finally, note that the runtime execution model sketched here is really an artifact of the current implementation, and not the language itself. For instance, it’s not impossible that a full, traditional compiler for Python source to machine code may appear during the shelf life of this book (although one has not in over a decade). New byte code formats and implementation variants may also be adopted in the future. For instance:

• The emerging Parrot project aims to provide a common byte code format, virtual machine, and optimization techniques, for a variety of programming languages (see http://www.python.org).

• The Stackless Python system is a standard CPython implementation variant, which does not save state on the C language call stack. This makes Python more easily ported to small stack architectures, and opens up novel programming possibilities such as co-routines.

• The new PyPy project is an attempt to reimplement the PVM in Python itself, in order to enable new implementation techniques.

Although such future implementation schemes may alter the runtime structure of Python somewhat, it seems likely that the byte code compiler will still be the standard for some time to come. The portability and runtime flexibility of byte code are important features to many Python systems. Moreover, adding type constraint declarations to support static compilation would break the flexibility, conciseness, simplicity, and overall spirit of Python coding. Due to Python’s highly dynamic nature, any future implementation will likely retain many artifacts of the current PVM. 

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