[GRiker]

Here's a template that lets you display a simple dialog:

import os
from calibre.customize import FileTypePlugin

class HelloWorld(FileTypePlugin):

    name                = 'HelloWorld'
    description         = 'A shell for creating plugins'
    supported_platforms = ['windows', 'osx', 'linux']
    author              = 'your_name_here'
    version             = (1, 0, 0)
    file_types          = set(['your_file_type_here'])
    on_import           = True 

    def run(self, path_to_ebook):
        from calibre.gui2 import is_ok_to_use_qt
        from PyQt4.Qt import QMessageBox

        if is_ok_to_use_qt():
            QMessageBox.question( None, 
                        'Hello World Plugin', 
                        "path_to_ebook: %s" % path_to_ebook,
                         QMessageBox.Ok)

        return path_to_ebook

I believe if you return None calibre understands that the import failed.

G