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__getattr__ and __getattribute__ Compared
To summarize the coding differences between __getattr__ and __getattribute__, the following example uses both to implement three attributes—attr1 is a class attribute, attr2 is an instance attribute, and attr3 is a virtual managed attribute computed when fetched:
class GetAttr:
attr1 = 1
def __init__(self):
self.attr2 = 2
def __getattr__(self, attr): # On undefined attrs only
print('get: ' + attr) # Not attr1: inherited from class
return 3 # Not attr2: stored on instance
X = GetAttr()
print(X.attr1)
print(X.attr2)
print(X.attr3)
print('-'*40)
class GetAttribute(object): # (object) needed in 2.6 only
attr1 = 1
def __init__(self):
self.attr2 = 2
def __getattribute__(self, attr): # On all attr fetches
print('get: ' + attr) # Use superclass to avoid looping here
if attr == 'attr3':
return 3
else:
return object.__getattribute__(self, attr)
X = GetAttribute()
print(X.attr1)
print(X.attr2)
print(X.attr3)
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When run, the __getattr__ version intercepts only attr3 accesses, because it is undefined. The __getattribute__ version, on the other hand, intercepts all attribute fetches and must route those it does not manage to the superclass fetcher to avoid loops:
1
2
get: attr3
3
----------------------------------------
get: attr1
1
get: attr2
2
get: attr3
3
Although __getattribute__ can catch more attribute fetches than __getattr__, in practice they are often just variations on a theme—if attributes are not physically stored, the two have the same effect.
