What is the difference between class and instance attributes?

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There is a significant semantic difference (beyond performance considerations):

  • when the attribute is defined on the instance (which is what we usually do), there can be multiple objects referred to. Each gets a totally separate version of that attribute.
  • when the attribute is defined on the class, there is only one underlying object referred to, so if operations on different instances of that class both attempt to set/(append/extend/insert/etc.) the attribute, then:
    • if the attribute is a builtin type (like int, float, boolean, string), operations on one object will overwrite (clobber) the value
    • if the attribute is a mutable type (like a list or a dict), we will get unwanted leakage.

For example:

>>> class A: foo = []
>>> a, b = A(), A()
>>> a.foo.append(5)
>>> b.foo
[5]
>>> class A:
...  def __init__(self): self.foo = []
>>> a, b = A(), A()
>>> a.foo.append(5)
>>> b.foo    
[]

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