Python a + = b and a = a + b are different

Some sites say that the operator ʻa + = b is the same as ʻa = a + b, but it is not exactly the same for mutable objects. Be careful if you have variables that point to the same object. (Especially for those who use NumPy!)


In a nutshell, there are the following differences. For mutable objects,

--In ʻa + = b, the object pointed to by ʻa does not change before and after the assignment. --When ʻa = a + b, the object pointed to by ʻa changes before and after the assignment.

However, for immutable objects, it changes in both cases (@shiracamus's comment has an example).


Let's experiment with adding [3] to [1, 2] to create [1, 2, 3]. y refers to the object of x before assignment. You can get the ID of an object with the built-in function ʻid (x) `.

a += b

>>> x = y = [1, 2]  #y is the same object as x before assignment
>>> id(x)
>>> id(y)
>>> x += [3]
>>> x
[1, 2, 3]
>>> y  #Also added to y
[1, 2, 3]
>>> id(x)  #Same object as before assignment
>>> id(y)

a = a + b

>>> x = y = [1, 2]  #y is the same object as x before assignment
>>> id(x)
>>> id(y)
>>> x = x + [3]
>>> x
[1, 2, 3]
>>> y  #Not added to y
[1, 2]
>>> id(x)  #Point to an object different from the one before the assignment
>>> id(y)

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