This is not clear if the compiler actually does not run or if it runs
and takes a long time and then gives an error?

What error (if any) do you get from python?

Try if it will run a "hello world" python program located in the same
place as the compiler, see if the -vv option provides any info.

Thank you.

The compiler did run (I could see it in the task manager) but it was not exiting for a minute or so. After I restarted my computer, everything started to work as expected. (Not sure why.)

I read GNU coding standards:

http://www.gnu.org/prep/standards/standards.html#Errors

I can make my custom compiler use these kind of messages:

sourcefile:line1.column1-line2.column2: message

Here are some more questions:

The above format cannot be used under windows, because the source file name can have a colon in it. (E.g. C:\temp\text.txt:12:14). I really want to use absolute file names, because the compiler that I write is able to read source files from different drives automatically, and because the files are organized in packages (so two file can have the same name if they are in different packages). How to overcome that problem?
Unfortunately, the message can also contain a colon. What to do with that?
How to distinguish errors from warnings? I was reading the [build-menu] but I could only find some examples. It was not clear how to do it.
Another question: can I make Geany open a file if it is not opened yet? E.g. a file is listed in an error message, but it is not opened in Geany yet. I would like Geany to open that file when I click or double click the error message.

Thanks,

Laszlo