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

It's worth pointing out that the __dir__ on many objects cannot be customized, so if you call it directly there is no opportunity to place a shim/wrapper in there.

It's easy to replace the builtin dir and give it some special powers if you need to. Tricks like this can be very useful when debugging.

Answer 2

dir calls __dir__ internally:

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

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

Answer 3

The docs explain it:

If the object has a method named __dir__(), this method will be called and must return the list of attributes. This allows objects that implement a custom __getattr__() or __getattribute__() function to customize the way dir() reports their attributes.

If the object does not provide __dir__(), the function tries its best to gather information from the object’s __dict__ attribute, if defined, and from its type object. The resulting list is not necessarily complete, and may be inaccurate when the object has a custom __getattr__().

Answer 4

dir calls __dir__ method if it is present, from python documentation :

dir([object])¶ Without arguments, return the list of names in the current local scope. With an argument, attempt to return a list of valid attributes for that object.

If the object has a method named __dir__(), this method will be called and must return the list of attributes. This allows objects that implement a custom __getattr__() or __getattribute__() function to customize the way dir() reports their attributes.

If the object does not provide __dir__(), the function tries its best to gather information from the object’s __dict__ attribute, if defined, and from its type object. The resulting list is not necessarily complete, and may be inaccurate when the object has a custom __getattr__().