Your class doesn't have a __init__(), so by the time it's instantiated, the attribute atoms is not present. You'd have to do C.setdata('something') so C.atoms becomes available.
>>> C = Residues()
>>> C.atoms.append('thing')
Traceback (most recent call last):
File "<pyshell#84>", line 1, in <module>
B.atoms.append('thing')
AttributeError: Residues instance has no attribute 'atoms'
>>> C.setdata('something')
>>> C.atoms.append('thing') # now it works
>>>
Unlike in languages like Java, where you know at compile time what attributes/member variables an object will have, in Python you can dynamically add attributes at runtime. This also implies instances of the same class can have different attributes.
To ensure you'll always have (unless you mess with it down the line, then it's your own fault) an atoms list you could add a constructor:
def __init__(self):
self.atoms = []