Python中表达式x += y和x = x+y 的区别详解

2020-01-04 16:52:15

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前言

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直接看下面代码:

x +=y

In [66]: id(a)Out[66]: 4476839480In [67]: id(b)Out[67]: 4476839480In [68]: a = [1, 2, 3]In [69]: b = aIn [70]: id(a)Out[70]: 4477149984In [71]: id(b)Out[71]: 4477149984In [72]: a += [4, 5, 6]In [73]: aOut[73]: [1, 2, 3, 4, 5, 6]In [74]: bOut[74]: [1, 2, 3, 4, 5, 6]In [75]: id(a)Out[75]: 4477149984In [76]: id(b)Out[76]: 4477149984

x = x + y

In [77]: a = [1, 2, 3]In [78]: b = aIn [79]: id(a)Out[79]: 4476686208In [80]: id(b)Out[80]: 4476686208In [81]: a = a + [4, 5, 6]In [82]: aOut[82]: [1, 2, 3, 4, 5, 6]In [83]: bOut[83]: [1, 2, 3]In [84]: id(a)Out[84]: 4477187640In [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_VALUEIn [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: