Python中表達(dá)式x += y和x = x+y 的區(qū)別詳解

2020-01-04 16:52:15

字體：大中小

供稿：網(wǎng)友

前言

本文主要給大家介紹的是關(guān)于Python中表達(dá)式x += y和x = x+y 區(qū)別的相關(guān)內(nèi)容，分享出來(lái)供大家參考學(xué)習(xí)，下面來(lái)看看詳細(xì)的介紹：

直接看下面代碼:

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

可以看出對(duì)于可變對(duì)象而言 x = x + y 這種形式已經(jīng)改變了x的指向，而不是"inplace"的修改。

使用dis模塊進(jìn)行一下驗(yàn)證。

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

可以看到字節(jié)碼是不同的: