how can i know if a python object have a attribute such as 'attr1'?

[thinke365]

for example, i may define a python class:
class A:
     def sayHello():
          print 'hello'

a = A()
a.attr1 = 'hello'
a.attr2 = 'bb'

b = A()
a.attr2 = 'aa'

how can i know whether an object have an attribute named attr1?

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