What's the difference between dir() and __dir__?

Question

in python what\'s the difference between dir() function and __dir__ attribute in python?

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Answer 1

dir calls __dir__ internally:

In [1]: class Hello():
   ...:     def __dir__(self):
   ...:         return [1,2,3]
   ...:     

In [2]: dir(Hello())
Out[2]: [1, 2, 3]