Use setattr() to set the value of the instance attribute by string:
>>> class A():
... def __init__(self):
... self.a = 200
... self.y = 400
...
>>> a = A()
>>> l = ['x', 'y', 'z']
>>> for item in l:
... setattr(a, item, 1000)
...
>>> a.x
1000
>>> a.y
1000
>>> a.z
1000