code file in the original directoryor something similar in __pycache__ subdirectory there? scripts if your platform supports itadd the #line to the top of your module py module filegive the file executable privilegesand run it directly as an executable what does the first line need to contain#usually only has meaning on unixlinuxand unix-like platforms such as mac os xif you're working on windowsinstead try running your file by listing just its name in command prompt window without the word "pythonbefore it (this works on recent versions of windows)via the start-run dialog boxor similar if you are using python or the windows launcher that installs with itexperiment with changing your script' #line to launch different python versions you may have installed on your computer (or equivalentlywork through the tutorial in appendix errors and debugging experiment with typing mathematical expressions and assignments at the python interactive command line along the waytype the expressions * and and reference an undefined variable name as we did early on in this what happensyou may not know it yetbut when you make mistakeyou're doing exception processinga topic we'll explore in depth in part vii as you'll learn thereyou are technically triggering what' known as the default exception handler--logic that prints standard error message if you do not catch an errorthe default handler does and prints the standard error message in response exceptions are also bound up with the notion of debugging in python when you're first starting outpython' default error messages on exceptions will probably provide as much error-handling support as you need--they give the cause of the erroras well as showing the lines in your code that were active when the error occurred for more about debuggingsee the sidebar "debugging python codeon page breaks and cycles at the python command linetypel [ append(ll make -item list append as single item to itself print la cyclic/circular object what happensin all recent versions of pythonyou'll see strange output that we'll describe in the solutions appendixand which will make more sense when we study references in the next part of the book if you're using python version older than ctrl- key combination will probably help on most platforms why do you think your version of python responds the way it does for this codeif you do have python older than release ( hopefully rare scenario today!)make sure your machine can stop program with ctrl- key combination of some sort before running this testor you may be waiting long time how you run programs