Some class attributes computed prematurely

>>> import screenshot
>>> screenshot
<module 'screenshot' from 'scripts/modules/screenshot.py'>
>>> bpy.data.filepath
''
>>> s1 = screenshot.Screenshot()
>>> vars(s1)
{}
>>> bpy.data.filepath
'/home/satishg/blender/testing/270/screenshots/screenshots_bugfix_module.blend'
>>> s2 = screenshot.Screenshot()
>>> vars(s2)
{'filename': 'screenshots_bugfix_module', 'dirname': '/home/satishg/blender/testing/270/screenshots'}