第四章 Python外壳：代码结构

Python的独特语法：

• 不使用分号结束语句，而是回车；
• 通过代码缩进来区分代码块；
• if、while、for等，都不用括号，但不能没有冒号（：）。

如何将一行命令分为多行？

>>> myNameIs = "LiZhiXin."
>>> myNameIs
‘LiZhiXin.‘

4.1 条件语句和循环语句

如何使用if、elif和else进行比较？

>>> a = 1
>>> if a < 2:
    print ("yes")
elif a > 2:
    print ("no")
else:
    print ("chaos")

yes

如何连接条件表达式？

python中没有！、&& 和 || 运算符，分别用 not、and 和 or代替。

>>> not 1
False
>>> 5 < 6 and 1 < 2
True
>>> 5 < 6 or 1 > 2
True

python中有哪些假值？

>>> False
False
>>> None
>>> 0
0
>>> 0.0
0.0
>>> ‘‘
‘‘
>>> []
[]
>>> ()
()
>>> {}
{}
>>> set()
set()

如何使用while进行循环？

>>> count = 1
>>> while count <= 5:
    print(count)
    count += 1

1
2
3
4
5

如何跳出循环？

>>> while True:
    stuff = input("string to calpitalize [type q to quit]:")
    if stuff == ‘q‘:
        break
    print(stuff.capitalize())

string to calpitalize [type q to quit]:li zhi xin
Li zhi xin
string to calpitalize [type q to quit]:q
>>>

如何跳到循环开始（用于跳过特定条件的循环）？

>>> while True:
    value = input("Interger, please [q to quit]: ")
    if value == ‘q‘:
        break
    number = int(value)
    if number % 2 == 0:
        continue
    print(number, "squared is ", number*number)

Interger, please [q to quit]: 1
1 squared is  1
Interger, please [q to quit]: 2
Interger, please [q to quit]: 3
3 squared is  9
Interger, please [q to quit]: 4
Interger, please [q to quit]: q

循环外的else是如何使用的？

>>> numbers = [1,3,5]
>>> position = 0
>>> while position < len(numbers):
    number = numbers[position]
    if number % 2 == 0:
        print(‘Found even number‘, number)
        break
    position += 1
else:
    print("No even number found.")

No even number found.

*如何使用for进行迭代（很独特）？

python中的for很牛逼，可以在数据结构和具体实现未知的情况下，遍历整个数据结构：

>>> rabbits = [‘a‘, ‘b‘, ‘c‘, ‘d‘]
>>> current = 0
>>> while current < len(rabbits):
    print(rabbits[current])
    current += 1

a
b
c
d
>>> for rabbit in rabbits:
    print(rabbit)

a
b
c
d

>>> word = ‘cat‘
>>> for letter in word:
    print(letter)

c
a
t

>>> a = {‘a‘:‘b‘, ‘c‘:‘d‘, ‘e‘:‘f‘}
>>> for key in a:
    print(key)

a
c
e

>>> for value in a.values():
    print(value)

b
d
f

>>> for item in a.items():
    print(item)

(‘a‘, ‘b‘)
(‘c‘, ‘d‘)
(‘e‘, ‘f‘)

>>> for key, value in a.items():
    print(key, value)

a b
c d
e f

如何用zip（）函数进行迭代？

对多个序列使用并行迭代，就是一起迭代，等价于将多个序列有序合并了。

>>> c = 0.1, 0.2, 0.3
>>> a = (1, 2, 3)
>>> b = [‘a‘, ‘b‘, ‘c‘, ‘d‘]
>>> c = 0.1, 0.2, 0.3, 0.4, 0.5,
>>> for e, f, g in zip(a, b, c):
    print(e, ‘ ‘, f, ‘ ‘, g)

1   a   0.1
2   b   0.2
3   c   0.3

list（）、zip（）和 dict（）可以灵活运用。

如何生成特定区间的自然数序列？

range（）的使用方法类似切片，但是却是使用‘，’分隔，而不是‘：’。

>>> for x in range(0,3):
    print(x)

0
1
2
>>> list(range(0,3))
[0, 1, 2]

4.2 推导式（python特有）

从一个或多个迭代器创建数据结构，可以将循环和条件结合。

列表推导式

>>> number_list = [number for number in range(1,6)]
>>> number_list
[1, 2, 3, 4, 5]

>>> a_list = [number for number in range(1,6) if number % 2 ==1]
>>> a_list
[1, 3, 5]

>>> rows = range(1,4)
>>> cols = range(1,3)
>>> cells = [(row, col) for row in rows for col in cols]
>>> for cell in cells:
    print(cell)

(1, 1)
(1, 2)
(2, 1)
(2, 2)
(3, 1)
(3, 2)

字典推导式

>>> word = ‘letters‘
>>> letter_counts = {letter:word.count(letter) for letter in word}
>>> letter_counts
{‘l‘: 1, ‘t‘: 2, ‘r‘: 1, ‘s‘: 1, ‘e‘: 2}

函数

python中参数有哪几种使用方法？

>>> def func(a, b, c):
    print(‘a is ‘, a, ‘b is ‘, b, ‘c is ‘, c)

>>> func(1, 2, 3)
a is  1 b is  2 c is  3

>>> func(c = 1, b = 2, a = 3)
a is  3 b is  2 c is  1

>>> d = [1, 2, 3]
>>> func(*d)
a is  1 b is  2 c is  3

>>> e = {‘c‘:1, ‘b‘:2, ‘a‘:3}
>>> func(**e)
a is  3 b is  2 c is  1

生成器

装饰器

命名空间和作用域

异常