gistfile1.py

def check_attr(obj, attr_name):
    """
    *Example*:

    if `obj` has 'one__two__three' attribute and
    `attr_name` is 'one' this will return 'one',
    if `attr_name` is 'one__two__three__four' will return
    'one__two__three'

    >>> class C(object): pass
    >>> c = C()
    >>> c.some__super = 1
    >>> c.some__super__duper = 2
    >>> c.some = C()
    >>> c.some.shit = 3
    >>> c.tag__tag = C()
    >>> c.tag__tag.url = 'url'
    >>> check_attr(c, 'some__super__bla')
    'some__super'
    >>> check_attr(c, 'some__super__duper')
    'some__super__duper'
    >>> check_attr(c, 'some__super123__bla')
    'some'
    >>> check_attr(c, 'some__shit')
    'some'
    >>> check_attr(c, 'some')
    'some'
    >>> check_attr(c, 'wtf')
    >>> check_attr(c, 'wtf__shit')
    >>> check_attr(c, 'tag__tag__url')
    'tag__tag'
    """
    def proccess(attr1, attr2):
        attr = '__'.join([attr1, attr2])
        if hasattr(obj, attr):
            return attr
        return attr1

    if hasattr(obj, attr_name):
        return attr_name
    splited_attr = attr_name.split('__')
    attr = reduce(proccess, splited_attr)
    if not hasattr(obj, attr):
        return None
    return attr

if __name__ == '__main__':
    import doctest
    doctest.testmod()