七月在线爬虫班学习笔记（二）——Python基本语法及面向对象

第二课主要内容如下：

• 代码格式
• 基本语法
• 关键字
• 循环判断
• 函数
• 容器
• 面向对象
• 文件读写
• 多线程
• 错误处理

代码格式

syntax基本语法

 a = 1234
 print(a)
 a = 'abcd'
 print(a)

 try:
     print(b)
 except Exception as e:
     print(e)

 a = [1, 2, 3 , 4]

 def func(a):
     a[0] = 2

 func(a)
 print(a)

 try:
     # Python 2.x 支持
     print(100, 200, 300)
 except Exception as e:
     print(e)

 1234
 abcd
 name 'b' is not defined
 [2, 2, 3, 4]
 100 200 300

condition_and_loop 循环判断

 score = 80
 if score > 90:
     print('A')
 elif score > 70:
     print('B')
 elif score >= 60:
     print('C')
 else:
     print('D')

 total = 0
 i = 1
 while i <= 100:
     total += i
     i += 1  # 没有++i或者--i
 print(total)

 '''
 for循环只作用于容器！！！
 没有这种写法：
   for (i = 0; i < 100; ++i):
       # TODO
 上面这种循环只能用while实现
 '''

 i = 0
 while i < 3:
     j = 0
     while j <= 3:
         if j == 2:
             j += 1
             continue  # 又去了while j <= 3
         print(i, j)
         j += 1
     i += 1

 B
 5050
 0 0
 0 1
 0 3
 1 0
 1 1
 1 3
 2 0
 2 1
 2 3

func函数

def hello(who = 'world'):
    print('hello %s!' % (who))

hello()
hello('sea')

# f(x) = x * 5 + 100
# g(x) = x * 5; f(x) = x + 100
# => f(g(x)) = x * 5 + 100
def g(x):
    return x * 5
def f(gf, x):
    return gf(x) + 100
print(f(g, 100))
print(f(lambda x: x * 5, 100))

def f(gf, x, y):
    return gf(x, y) + 100
print(f(lambda x, y: x * y, 100, 200))

#输出结果
hello world!
hello sea!
600
600
20100

ct_list数组

# list就是数组
li = [1, 2, 3, 4, 5]
# 遍历
for i in li:
    # print(i)
    pass
# 用range模拟for (i = 0; i < x; ++i)
# range(x) => [0, x - 1]
# range(x, y) => [x, y - 1]
# range(x, y, z) => [x, x + z,..., < y]
for i in range(len(li)):
    # print(li[i])
    pass

for i in range(1, 10, 2):
    print(i)

#输出结果
1
3
5
7
9

 # 负数索引
 print(li[-1])
 print(li[-2])

 # 负数step的range => [x, x - z, ..., > z]
 for i in range(3, -1, -1):
     print(i)

 #输出结果
 5
 4
 3
 2
 1
 0

 # 添加元素
 li = []
 li.append(1)
 li.append(2)
 li.append('abc')
 li.append(['d', 'e', 'f'])
 print(li)

 # 按元素添加数组
 li = [1, 2]
 li_2 = [3, 4, 5]
 # 我们想要[1, 2, 3, 4, 5]
 # li.append(li_2) => [1, 2, [3, 4, 5]]
 li.extend(li_2)
 print(li)

 # 删除元素
 li.pop()    # => [1, 2, 3, 4]
 print(li)
 li.pop(2)   # => [1, 2, 4]
 print(li)

 #输出结果
 [1, 2, 'abc', ['d', 'e', 'f']]
 [1, 2, 3, 4, 5]
 [1, 2, 3, 4]
 [1, 2, 4]

 li = [5, 8, 7, 4, 2, 3]
 li.sort()
 print(li)
 # lambda帮助排序
 li = [[5, 2], [3, 8], [2, 11], [7, 6]]
 # li.sort(key = lambda x: x[0]) # 参数名字
 # 与lamda等价写法
 def item_key(x):
     return x[0]
 li.sort(key = item_key)
 print(li)

 #输出结果
 [2, 3, 4, 5, 7, 8]
 [[2, 11], [3, 8], [5, 2], [7, 6]]

ct_tuple只读数组

 # 只读数组
 tp = (1, 2, 3)
 try:
     tp[0] = 100
 except Exception as e:
     print(e)

ct_set没有重复元素的数组

 s = set([1, 2, 2, 3, 3, 4, 5])
 print(s)
 s = set((2, 3, 4, 5, 6, 2, 1, 9))
 print(s)

 #输出
 {1, 2, 3, 4, 5}
 {1, 2, 3, 4, 5, 6, 9}

ct_dict 字典（哈希表）

 # key<->value对应的hash表
 di = {'k1': 'v1', 'k2': 'v2'}
 di['k3'] = 'v3'
 di['k4'] = 'v4'

 for k in di:
     print(di[k])

 for k, v in di.items():
     print(k, v)

 #输出
 v3
 v4
 v2
 v1
 k3 v3
 k4 v4
 k2 v2
 k1 v1

slice数组切片

 # [1, 2, 3, 4, 5]
 #  => [1, 2, 3]
 #  => [3, 4]
 li = [1, 2, 3, 4, 5]
 li_0_2 =li[0:3] # 0 <= ? < 3
 # 等价li[:3]
 print(li_0_2)
 # [start, end, step] => [start, start + step, ..., < end]
 # start默认是0，end默认-1，step默认1
 li_last_3 = li[-1:-4:-1]
 print(li_last_3)

 # 直接用切片反转数组
 print(li[::-1])
 print(li[-2::-1])

 # 切片是复制
 li_0_2[-1] = 100
 print(li)

 #输出
 [1, 2, 3]
 [5, 4, 3]
 [5, 4, 3, 2, 1]
 [4, 3, 2, 1]
 [1, 2, 3, 4, 5

list_string字符串与数组的关系

 s = 'abcdefg'
 try:
     str[0] = 'x'
 except Exception as e:
     print(e)

 # 修改字符串
 li = list(s)
 # print(li)
 li[0] = 'x'
 s = ''.join(li)
 print(s)
 s = '-'.join(li)
 print(s)

 # 切割
 s = 'abc,def,ghi'
 p1, p2, p3 = s.split(',')
 print(p1, p2, p3)

 # 下标访问和切片
 s = 'abcdefg'
 print(s[0], s[-1])
 print(s[2:5])

 #输出
 'type' object does not support item assignment
 xbcdefg
 x-b-c-d-e-f-g
 abc def ghi
 a g
 cde

obj面向对象

 # 用type查看对象类型
 print(type([1, 2, 3, 4]))
 print(type('abcd'))
 print(type({1:2, 2:3}))

 # 用dir查看属性和方法
 print(dir(list))

 <class 'list'>
 <class 'str'>
 <class 'dict'>
 ['__add__', '__class__', '__contains__', '__delattr__', '__delitem__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__getitem__', '__gt__', '__hash__', '__iadd__', '__imul__', '__init__', '__iter__', '__le__', '__len__', '__lt__', '__mul__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__reversed__', '__rmul__', '__setattr__', '__setitem__', '__sizeof__', '__str__', '__subclasshook__', 'append', 'clear', 'copy', 'count', 'extend', 'index', 'insert', 'pop', 'remove', 'reverse', 'sort']

 class Clazz(object):
     # self参考C++的this指针！
     def __init__(self, x, y):
         self.x = x
         self.y = y

     # 声明成员函数的时候，第一个参数一定是self，不要忘记！
     def display(self):
         print(self.x, self.y)

 print(type(Clazz))
 clz = Clazz(100, 200)
 clz.display()  # => display(clz)

 class Base:
     def run(self):
         print('Base::run')

 class Tom(Base):
     def run(self):
         print('Tom::run')

 t = Tom()
 print(isinstance(t, Base))
 t.run()

<class 'type'>
100 200
True
Tom::run

 def run(runner):
     runner.run()

 class R1:
     def run(self):
         print('R1::run')

 class R2:
     def run(self):
         print('R2::run')

 run(R1())
 run(R2())

 #输出
 R1::run
 R2::run

file_rw文件读写

 # 打开文件操作可能失败,异常处理后不会造成资源泄露等情况
 with open('text.txt', 'w') as f:
     f.write('''1234
     abcd
     nefgh''')

 with open('text.txt', 'rb') as f:
     print(f.read())

 with open('text.txt') as f:
     for line in f.readlines():
         print(line.strip())

 #输出
 b'1234\r\n    abcd\r\n    nefgh'
 1234
 abcd
 nefgh

threads多线程

 import threading

 def thread_func(x):
     # 自己加sleep和其它复杂操作看效果
     print('%d\n' % (x * 100))

 threads = []
 for i in range(5):
     threads.append(threading.Thread(target = thread_func, args = (100, )))

 for thread in threads:
     thread.start()

 for thread in threads:
     thread.join()

 #输出
 10000

 10000

 10000

 10000

 10000

error_handling错误和异常处理

 # 错误处理

 try:
     r = 10 / 0
 except ZeroDivisionError as e:
     print(type(e))
     print(e)
 finally:
     # 主要防止资源泄露(服务端。客户端一般不会资源泄露)！
     print('Always come here.')

 #输出
 <class 'ZeroDivisionError'>
 division by zero
 Always come here.

 import logging
 logging.basicConfig(filename='example.log',level=logging.DEBUG)
 logging.debug('This message should go to the log file')
 logging.info('So should this')
 logging.warning('And this, too')