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)
4397797440
>>> id(y)
4397797440
>>>
>>> x += [3]
>>>
>>> ｘ
[1, 2, 3]
>>> y #Also added to y
[1, 2, 3]
>>>
>>> id(x) #Same object as before assignment
4397797440
>>> id(y)
4397797440
a = a + b
>>> x = y = [1, 2] #y is the same object as x before assignment
>>>
>>> id(x)
4397797440
>>> id(y)
4397797440
>>>
>>> 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
4395937472
>>> id(y)
4397797440
Recommended Posts