Python中表达式x += y和x = x+y 的区别详解

前言

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直接看下面代码:

x +=y

In [66]: id(a)
Out[66]: 4476839480

In [67]: id(b)
Out[67]: 4476839480

In [68]: a = [1, 2, 3]

In [69]: b = a

In [70]: id(a)
Out[70]: 4477149984

In [71]: id(b)
Out[71]: 4477149984

In [72]: a += [4, 5, 6]

In [73]: a
Out[73]: [1, 2, 3, 4, 5, 6]

In [74]: b
Out[74]: [1, 2, 3, 4, 5, 6]

In [75]: id(a)
Out[75]: 4477149984

In [76]: id(b)
Out[76]: 4477149984

x = x + y

In [77]: a = [1, 2, 3]

In [78]: b = a

In [79]: id(a)
Out[79]: 4476686208

In [80]: id(b)
Out[80]: 4476686208

In [81]: a = a + [4, 5, 6]

In [82]: a
Out[82]: [1, 2, 3, 4, 5, 6]

In [83]: b
Out[83]: [1, 2, 3]

In [84]: id(a)
Out[84]: 4477187640

In [85]: id(b)
Out[85]: 4476686208

可以看出对于可变对象而言 x = x + y 这种形式已经改变了x的指向，而不是"inplace"的修改。

使用dis模块进行一下验证。

In [86]: def foo(x, y):
 ...:  x += y
 ...:  return x
 ...:

In [87]: def foo(x, y):
 ...:  x += y
 ...:

In [88]: def foo2(x, y):
 ...:  x = x + y
 ...:

In [89]: dis.dis(foo)
 2   0 LOAD_FAST    0 (x)
    3 LOAD_FAST    1 (y)
    6 INPLACE_ADD
    7 STORE_FAST    0 (x)
    10 LOAD_CONST    0 (None)
    13 RETURN_VALUE

In [90]: dis.dis(foo2)
 2   0 LOAD_FAST    0 (x)
    3 LOAD_FAST    1 (y)
    6 BINARY_ADD
    7 STORE_FAST    0 (x)
    10 LOAD_CONST    0 (None)
    13 RETURN_VALUE

可以看到字节码是不同的:

• += 对应 INPLACE_ADD
• 对应BINARY_ADD
  

注意：以上仅适用于CPython实现。

总结

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