Python 入门（三）整数和浮点数+布尔类型

整数和浮点数

Python支持对整数和浮点数直接进行四则混合运算，运算规则和数学上的四则运算规则完全一致。

基本的运算：

1 + 2 + 3   # ==> 6

4 * 5 - 6   # ==> 14

7.5 / 8 + 2.1   # ==> 3.0375

使用括号可以提升优先级，这和数学运算完全一致，注意只能使用小括号，但是括号可以嵌套很多层：

(1 + 2) * 3    # ==> 9

(2.2 + 3.3) / (1.5 * (9 - 0.3))    # ==> 0.42145593869731807

和数学运算不同的地方是，Python的整数运算结果仍然是整数，浮点数运算结果仍然是浮点数：

1 + 2    # ==> 整数 3

1.0 + 2.0    # ==> 浮点数 3.0

但是整数和浮点数混合运算的结果就变成浮点数了：

1 + 2.0    # ==> 浮点数 3.0

为什么要区分整数运算和浮点数运算呢？这是因为整数运算的结果永远是精确的，而浮点数运算的结果不一定精确，因为计算机内存再大，也无法精确表示出无限循环小数，比如 0.1 换成二进制表示就是无限循环小数。

那整数的除法运算遇到除不尽的时候，结果难道不是浮点数吗？我们来试一下：

11 / 4    # ==> 2

令很多初学者惊讶的是，Python的整数除法，即使除不尽，结果仍然是整数，余数直接被扔掉。不过，Python提供了一个求余的运算 % 可以计算余数：

11 % 4    # ==> 3

如果我们要计算 11 / 4 的精确结果，按照“整数和浮点数混合运算的结果是浮点数”的法则，把两个数中的一个变成浮点数再运算就没问题了：

11.0 / 4    # ==> 2.75

任务

请计算 2.5 + 10 / 4 ,并解释计算结果为什么不是期望的 5.0 ?

请修复上述运算，使得计算结果是 5.0

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布尔类型

我们已经了解了Python支持布尔类型的数据，布尔类型只有True和False两种值，但是布尔类型有以下几种运算：

与运算：只有两个布尔值都为 True 时，计算结果才为 True。

True and True   # ==> True

True and False   # ==> False

False and True   # ==> False

False and False   # ==> False

或运算：只要有一个布尔值为 True，计算结果就是 True。

True or True   # ==> True

True or False   # ==> True

False or True   # ==> True

False or False   # ==> False

非运算：把True变为False，或者把False变为True：

not True   # ==> False

not False   # ==> True

布尔运算在计算机中用来做条件判断，根据计算结果为True或者False，计算机可以自动执行不同的后续代码。

在Python中，布尔类型还可以与其他数据类型做 and、or和not运算，请看下面的代码：

a = True

print a and 'a=T' or 'a=F'

计算结果不是布尔类型，而是字符串 'a=T'，这是为什么呢？

因为Python把0、空字符串''和None看成 False，其他数值和非空字符串都看成 True，所以：

True and 'a=T' 计算结果是 'a=T'

继续计算 'a=T' or 'a=F' 计算结果还是 'a=T'

要解释上述结果，又涉及到 and 和 or 运算的一条重要法则：短路计算。

1. 在计算 a and b 时，如果 a 是 False，则根据与运算法则，整个结果必定为 False，因此返回 a；如果 a 是 True，则整个计算结果必定取决与 b，因此返回 b。

2. 在计算 a or b 时，如果 a 是 True，则根据或运算法则，整个计算结果必定为 True，因此返回 a；如果 a 是 False，则整个计算结果必定取决于 b，因此返回 b。

所以Python解释器在做布尔运算时，只要能提前确定计算结果，它就不会往后算了，直接返回结果。

任务

请运行如下代码，并解释打印的结果：

a = 'python'

print 'hello,', a or 'world'

b = ''

print 'hello,', b or 'world'

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" alt="" />