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An Introduction to Python
by Guido van Rossum and Fred L. Drake, Jr.
Paperback (6"x9"), 124 pages
ISBN 0954161769
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6.1.2 "Compiled" Python files

As an important speed-up of the start-up time for short programs that use a lot of standard modules, if a file called ‘spam.pyc’ exists in the directory where ‘spam.py’ is found, this is assumed to contain an already-"byte-compiled" version of the module ‘spam’. The modification time of the version of ‘spam.py’ used to create ‘spam.pyc’ is recorded in ‘spam.pyc’, and the ‘.pyc’ file is ignored if these don't match.

Normally, you don't need to do anything to create the ‘spam.pyc’ file. Whenever ‘spam.py’ is successfully compiled, an attempt is made to write the compiled version to ‘spam.pyc’. It is not an error if this attempt fails; if for any reason the file is not written completely, the resulting ‘spam.pyc’ file will be recognized as invalid and thus ignored later. The contents of the ‘spam.pyc’ file are platform independent, so a Python module directory can be shared by machines of different architectures.

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