What is the relationship between __getattr__ and getattr?

I know this code is right:

class A:   
    def __init__(self):
        self.a = 'a'  
    def method(self):   
        print "method print"  

a = A()   
print getattr(a, 'a', 'default')   
print getattr(a, 'b', 'default')  
print getattr(a, 'method', 'default') 
getattr(a, 'method', 'default')()   

And this is wrong:

# will __getattr__ affect the getattr?

class a(object):
    def __getattr__(self,name):
        return 'xxx'

print getattr(a)

This is also wrong:

a={'aa':'aaaa'}
print getattr(a,'aa')

Where should we use these functions (__getattr__ and getattr) ?