Trying to access a non-existing attribute raises an AttributeError:
>>> x.energy
Traceback (most recent call last):
File "<stdin>", line 1, in
AttributeError: 'Robot' object has no attribute 'energy'
>>>
This exception can be avoided by using getattr, especially with the third argument as the default value for the attribute::
>>> getattr(x, 'energy', 100)
100
>>>