assign_once.py

class AssignOnce:
    ''' Descriptor, which prevents accidential overwriting of attributes:
    >>> class T:
    ...     a = AssignOnce('a')
    ... 
    >>> t = T()
    >>> t.a
    >>> t.a = 3
    >>> t.a = 4
    Traceback (most recent call last):
        ...
    ValueError: Overwriting a
    >>> t.a = None
    >>> t.a = 10
    '''
    
    def __init__(self, name):
        self.name = '_'+name
    
    def __get__(self, obj, owner=None):
        return getattr(obj, self.name, None)
    
    def __set__(self, obj, val):
        if val != None and getattr(obj, self.name, None) != None:
            raise ValueError("Overwriting "+self.name[1:])
        setattr(obj, self.name, val)