When you execute your code, python creates a compiled pyc file. This file is the one executed in posterior runs if you do not modify your code
From here:
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 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 is platform
independent, so a Python module directory can be shared by machines of
different architectures.