第5章 条件、循环及其他语句

5.1 再谈print和import

5.1.1 打印多个参数

>>> a = 'Hello'
>>> b = ' world!'
>>> print(a,b)
Hello  world!
>>> 

5.1.2 导入时重命名

>>> import math as foobat
>>> foobat.sqrt(4)
2.0
>>> from math import sqrt as foobar
>>> foobar(4)
2.0
>>> 

5.2 赋值魔法

5.2.1 序列解包

>>> x,y,z = 1,2,3
>>> print(x,y,z)
1 2 3
>>> a,b,*c = 1,2,3,4
>>> print(a,b,c)
1 2 [3, 4]
>>> 

5.2.2 链式赋值

x = y = somefunction()

5.2.3 增强赋值

>>> x = 2
>>> x += 1
>>> x *= 2
>>> x
6
>>> 

5.3 代码块：缩进的乐趣

5.4 条件和条件语句

5.4.1 这正是布尔值的用武之地

>>> bool('abc')
True
>>> bool(1)
True
>>> bool('')
False
>>> bool(0)
False
>>> 

5.4.2 有条件执行和if语句

5.4.3 else字句

name = input('what is your name?')
if name.endswith('peter'):
    print('good person')
else:
    print('bad guy')

name = input('again?')
status = 'good person' if name.endswith('peter') else 'bad guy'
print(status)

5.4.4 elif字句

name = input('what is your name?')
if name.endswith('peter'):
    print('good person')
elif name.endswith('a'):
    print('test elif')
else:
    print('bad guy')

5.4.5 代码库嵌套

5.4.6 更复杂的条件

5.4.7 断言
让程序在错误条件出现时立即崩溃好过以后再崩溃。

>>> age = -1
>>> assert age > 0, "bigger zero"
Traceback (most recent call last):
  File "<pyshell#218>", line 1, in <module>
    assert age > 0, "bigger zero"
AssertionError: bigger zero
>>> 

5.5 循环

5.5.1 while循环

5.5.2 for循环

# while循环
x = 1
while x < 10:
    print(x)
    x += 1


# for 循环
words = ["a",'b', 'c']
for word in words:
    print(word)

5.5.3 迭代字典

d = {'x' : '1', 'y' : 2, 'z' : [1,2,4]}
for key in d:
    print(key, " to ", d[key])

for key,value in d.items():
    print(key,' to ',value)

5.5.4 一些迭代工具
1 并行迭代

>>> a = [1,2,3]
>>> b = ['a', 'b', 'c']
>>> for av,bv in zip(a,b):
    print(av,' ', bv)


1   a
2   b
3   c
>>> 

2 迭代时获得索引

a = ['aaa','bbb']
for index,s in enumerate(a):
    print(s,index)
    if 'aa' in s:
        a[index] = 'ccc'
print(a)

5.5.5 跳出循环
break
continue

5.5.6 循环中的else字句

5.6 简单推导
列表推导是一种从其他列表创建列表的方式。

>>> [x * x for x in range(10)]
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
>>> [x * x for x in range(10) if x % 3 == 0]
[0, 9, 36, 81]
>>> [(x,y) for x in range(3) for y in range(3)]
[(0, 0), (0, 1), (0, 2), (1, 0), (1, 1), (1, 2), (2, 0), (2, 1), (2, 2)]
>>> 

5.7 三人行

5.7.1 什么都不做
pass
5.7.2 使用del删除

>>> x = 1
>>> x
1
>>> del x
>>> x
Traceback (most recent call last):
  File "<pyshell#251>", line 1, in <module>
    x
NameError: name 'x' is not defined
>>> 

5.7.3 使用exec和eval执行字符串及计算结果
1 exec
将字符串当作代码执行

>>> exec("print('Hello,world!')")
Hello,world!
>>> 

2 eval
eval用字符串表示表达式的值，并返回结果（exec什么都不返回，因为它本身是条语句）

>>> eval(input("Enter an arithmetic expression: "))
Enter an arithmetic expression: 2+3
5
>>> 

5.8 小结