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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数字
｜
35
>>> 2 + 3 * 4
如果先执行加法运算，那么
2 + 3
等于
5
，
5 * 4
等于
20
。但如果先执行乘法运算，则
3 * 4
等于
12
，
2 + 12
等于
14
。和大多数语言一样，在
Python
中，乘法的
优先级
高于加法，所
以第二种执行顺序才是你会看到的：
>>> 2 + 3 * 4
14
从哪里能知道优先级规则呢？附录
E
中有一个表格列出了所有的优先级，但是我发现自己
在实践中根本就没查过这些规则。按照你希望的计算执行顺序添加圆括号要容易得多：
>>> 2 + (3 * 4)
14
下面这个幂运算的例子
>>> -5 ** 2
-25
等同于
>>> - (5 ** 2)
-25
这可能不是你要的结果。加上圆括号会更清晰：
>>> (-5) ** 2
25
按照这种方法，不管是谁阅读代码，都不用去猜测代码意图或是查找优先级规则。
3.2.5
基数
在
Python
中，整数默认使用十进制数（基数为
10
），除非你使用前缀明确指定了其他基
数。也许你永远都不会在自己的代码中用到其他基数，但有可能在别人的
Python
代码中见
到它们。
通常我们都有
10
根手指和
10
根脚趾
，所以计数方式是
0
、
1
、
2
、
3
、
4
、
5
、
6
、
7
、
8
、
9
。
接下来，一位数字不够用了，于是将
1
放到“十位”，将
0
放到“个位”：
10
代表“
1
个十
加
0
个一”
。和罗马数字符号不同，阿拉伯数字没有表示“
10
”的单个符号。接下来就是
11
、
12...
一直到
19
，再按照之前的方法进位成
20
（
2
个十加
0
个一），以此类推。
基数是指在需要进位前可以使用的数位数量。基数为
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