Python 中 dir(...) 和 vars(...).keys() 的区别？

Question

Is there a difference between dir(…) and vars(…).keys() in Python?

(I hope there is a difference, because otherwise this would break the "one way to do it" principle... :)

Answer 1

Python objects usually store their instance variables in a dictionary that belongs to the object (except for slots). vars(x) returns this dictionary (as does x.__dict__ ). dir(x) , on the other hand, returns a dictionary of x 's "attributes, its class's attributes, and recursively the attributes of its class's base classes."

When you access an object's attribute using the dot operator, Python does a lot more than just look up the attribute in that objects dictionary. A common case is when x is an instance of class C and you call its method m :

class C:
    def m(self):
        print("m")
    
x = C()
x.m()

The method m is not stored in x.__dict__ . It is an attribute of the class C .

When you call xm() , Python will begin by looking for m in x.__dict__ , but it won't find it. However, it knows that x is an instance of C , so it will next look in C.__dict__ , find it there, and call m with x as the first argument.

So the difference between vars(x) and dir(x) is that dir(x) does the extra work of looking in x 's class (and itsbases ) for attributes that are accessible from it, not just those attributes that are stored in x 's own symbol table. In the above example, vars(x) returns an empty dictionary, because x has no instance variables. However, dir(x) returns

['__class__', '__delattr__', '__dict__', '__dir__', '__doc__', '__eq__', 
'__format__', '__ge__', '__getattribute__', '__gt__', '__hash__', 
'__init__', '__init_subclass__', '__le__', '__lt__', '__module__', 
'__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', 
'__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__', 
'm']

Answer 2

The documentation has this to say about dir :

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.

And this about vars :

Without arguments, return a dictionary corresponding to the current local symbol table. With a module, class or class instance object as argument (or anything else that has a __dict__ attribute), returns a dictionary corresponding to the object's symbol table.

If you don't see the difference, maybe this will show you more (grouped for easier reading):

>>> dir(list)
['__add__', '__class__', '__class_getitem__', '__contains__', '__delattr__', 
'__delitem__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', 
'__getattribute__', '__getitem__', '__gt__', '__hash__', '__iadd__', '__imul__', 
'__init__', '__init_subclass__', '__iter__', '__le__', '__len__', '__lt__', 
'__mul__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', 
'__reversed__', '__rmul__', '__setattr__', '__setitem__', '__sizeof__', 
'__str__', '__subclasshook__', 'append', 'clear', 'copy', 'count', 'extend', 
'index', 'insert', 'pop', 'remove', 'reverse', 'sort']
>>> vars(list).keys()
dict_keys(['__repr__', 
'__hash__', 
'__getattribute__', 
'__lt__', '__le__', '__eq__', '__ne__', '__gt__', '__ge__', 
'__iter__', 
'__init__', 
'__len__', 
'__getitem__', '__setitem__', '__delitem__', 
'__add__', '__mul__', '__rmul__', '__contains__', '__iadd__', '__imul__', 
'__new__', 
'__reversed__', '__sizeof__', 
'clear', 'copy', 'append', 'insert', 'extend', 'pop', 'remove', 'index', 'count', 'reverse', 'sort', 
'__class_getitem__', 
'__doc__'])

If you don't feel like reading through that, dir includes these attributes while vars does not:

>>> set(dir(list)) - vars(list).keys()
{'__class__', '__delattr__', '__dir__', '__format__', '__init_subclass__', 
'__reduce__', '__reduce_ex__', '__setattr__', '__str__', '__subclasshook__'}

Note also that dir() 's output is sorted alphabetically, whereas vars() 's output is sorted by the order the attributes were defined in.

Answer 3

Apart from Answers given, I would like to add that, using vars() with instances built-in types will give error, as instances builtin types do not have __dict__ attribute.

eg.

In [96]: vars([])
---------------------------------------------------------------------------

TypeError Traceback (most recent call last)
<ipython-input-96-a6cdd8d17b23> in <module>()
      ----> 1 vars([])
TypeError: vars() argument must have __dict__ attribute