but in pythonthe compiler is always present at runtime and is part of the system that runs programs this makes for much more rapid development cycle there is no need to precompile and link before execution may beginsimply type and run the code this also adds much more dynamic flavor to the language--it is possibleand often very convenientfor python programs to construct and execute other python programs at runtime the eval and exec built-insfor instanceaccept and run strings containing python program code this structure is also why python lends itself to product customization--because python code can be changed on the flyusers can modify the python parts of system onsite without needing to have or compile the entire system' code at more fundamental levelkeep in mind that all we really have in python is runtime-there is no initial compile-time phase at alland everything happens as the program is running this even includes operations such as the creation of functions and classes and the linkage of modules such events occur before execution in more static languagesbut happen as programs execute in python as we'll seethis makes for much more dynamic programming experience than that to which some readers may be accustomed execution model variations now that we've studied the internal execution flow described in the prior sectioni should note that it reflects the standard implementation of python today but is not really requirement of the python language itself because of thatthe execution model is prone to changing with time in factthere are already few systems that modify the picture in figure - somewhat before moving onlet' briefly explore the most prominent of these variations python implementation alternatives strictly speakingas this book edition is being writtenthere are at least five implementations of the python language--cpythonjythonironpythonstacklessand pypy although there is much cross-fertilization of ideas and work between these pythonseach is separately installed software systemwith its own developers and user base other potential candidates here include the cython and shed skin systemsbut they are discussed later as optimization tools because they do not implement the standard python language (the former is python/ mixand the latter is implicitly statically typedin briefcpython is the standard implementationand the system that most readers will wish to use (if you're not surethis probably includes youthis is also the version used in this bookthough the core python language presented here is almost entirely the same in the alternatives all the other python implementations have specific purexecution model variations