common in programs meant to be run on unix too treating unrecognized unix directives as errors on windows seems bit extremeespecially given that this is new behavior in and will likely be unexpected why not just ignore unrecognized #lines and run the file with the default python--like every windows python to date hasit' possible that this might be improved in future release (there may be some pushback on this)but today you must change any files using #!/bin/env or other unrecognized patternif you want them to run under the launcher installed with python on windows book examples impact and fix with respect to the book examples ported to this broke roughly dozen scripts that started with #!/bin/env python regrettablythis includes some of the book' userfriendly and top-level demo launcher scripts (pygadgets and pydemosto fixi changed these to use the accepted #!/usr/bin/env python form instead altering your windows file associations to omit the launcher altogether may be another option ( associating py files with python exe instead of py exe)but this negates the launcher' benefitsand seems bit much to ask of usersespecially newcomers one open issue herestrangelypassing any command-line switch to the launchereven python exe argumentseems to negate this effect and fall back on the default python -- py and py py both issue errors on unrecognized #linesbut py - py runs such file with the default python this seems possible launcher bugbut also relies on the defaultthe subject of the next issue pitfall the launcher defaults to oddlythe windows launcher defaults to using an installed python when running scripts that don' select explicitly that isscripts that either have no #directive or use one that names python generically will be run by python by default when launched by icon clicksdirect filename command lines ( py)or launcher command lines that give no version switch (py pythis is true even if is installed after on your machineand has the potential to make many scripts fail initially the implications of this are potentially broad as one exampleclicking the icon of directive-less file just after installing may now failbecause the associated launcher assumes you mean to use by default this probably won' be pleasant first encounter for some python newcomersthis assumes the file has no #directive that provides an explicit python version numberbut most scripts meant to run on windows won' have #line at alland many files coded before the launcher came online won' accommodate its version number expectations most users will be basically compelled to set py_python after installing --hardly usability win program launches that don' give an explicit version number might be arguably ambiguous on unix tooand often rely on symbolic links from python to specific version appendix bthe python windows launcher