March 2022
Intermediate to advanced
522 pages
13h 52m
Chinese
Content preview from Python语言及其应用（第2版）
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24
｜
第
2
章
•
发现中间的
=
；
•
知道这是赋值；
•
计算右侧的值（获取
x
所引用对象的值并与
12
相加）；
•
将结果赋给左侧的变量
y
。
在交互式解释器中输入变量
y
的名称，就会打印出该变量的值。
如果你的程序一开始就是
y = x + 12
，则
Python
会产生一个
异常
（错误），因为变量
x
还
没有值：
>>> y = x + 12
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'x' is not defined
第
9
章会详细介绍异常。用计算机语言来说，
x
尚未初始化。
在代数中，可以逆向求解，将值代入
y
来计算
x
。在
Python
中，需要先在右侧得到字面值
和初始化过的变量，然后再给左侧的
x
赋值。
>>> y = 5
>>> x = 12 - y
>>> x
7
2.7
变量是名称
，
不是位置
现在是时候说明
Python
中关于变量的一个重要事实了：
变量仅仅是名称而已
。不同于很多
其他计算机语言，这是了解
Python
的关键
，尤其是当使用像列表这样的可变对象时。赋值
操作并不会复制值，而只是为包含数据的对象附加一个名称。名称是对事物的引用，而非
事物本身。把名称想象成一个带有细绳的标签，绳子的一头系着位于计算机内存某处的对
象盒子（参见图
2-3
）。
图 2-3：指向对象的名称（变量
a
指向值为
7
的整数对象）
数据：类型、值、变量和名称
｜
25
在其他语言中，变量本身具有类型并与内存位置绑定。你可以修改该位置上的值 ...
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I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.