The downloadable Windows build comes with Python 3.4 and on Linux/UNIX, the installation will share the Python interpreter with system, which makes is easy to switch between Python versions. Any way I can use Python 2.7 without recompiling KLayout, e.g. by setting some environment variables?
Python 3 and 2 are quite different regarding their C API, so there is no way to easily switch between them.
Even on Linux you usually need to recompile when you do a bigger switch like Python 2 to 3. But unlike Windows, KLayout will reference the system installation and hence all the modules that are installed for Python can also be used within KLayout.
On Windows, you'll have to install the Python modules locally together with the KLayout installation.