01.python基础知识快速入门

代码格式

强制缩进

• Python开发者有意让违反了缩进规则的程序不能通过编译，以此来强制程序员养成良好的编程习惯。并且Python语言利用缩进表示语句块的开始和退出（Off-side规则），而非使用花括号或者某种关键字。增加缩进表示语句块的开始，而减少缩进则表示语句块的退出。缩进成为了语法的一部分。
• 根据PEP的规定，必须使用4个空格来表示每级缩进。

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基本语法

• 弱类型
• 变量必须先赋值再使用
• 小心引用
• 2和3的常见差异
  
  • xrange
  • print
  • 数据类型统一，取消unicode和long
  • utf8: 3.x代码默认utf-8

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# 变量必须先赋值再使用
a = 1234
print(a)
a = 'abcd'
print(a)

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

#传值使用的是引用
a = [1, 2, 3 , 4]
def func(a):
    a[0] = 2

func(a)
print(a)

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关键字

• 常量

  True False None
  

• 对象与容器

  class import from del
  

• 逻辑操作

  and or not
  

• 函数

  def return
  

• 判断与循环控制

  if elif else
  is
  in
  assert
  for while continue break
  

• 异常

  raise try except finally
  with as
  

• 作用域

  global nonlocal
  

• 匿名函数与协程

  yield lambda
  

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循环判断

• 注意加“:”
• 没有do-while循环
• 没有switch
• break是中断整个循环
• continue是结束本次迭代进入下一个

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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

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函数

• def定义函数
• 默认参数
• 名字参数
• 函数也是对象
• 函数式编程简介：map/reduce/lambda

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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))

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容器

list：数组

# list就是数组

li = [1, 2, 3, 4, 5]
# 遍历
for i in li:
    print(i)
print("--------------------")

# 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])
print("--------------------")

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

print("--------------------")

# 负数索引
print(li[-1])
print(li[-2])
print("--------------------")

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

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

# 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)
print("--------------------")

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

li = [5, 8, 7, 4, 2, 3]
li.sort()
print(li)
print("--------------------")

# 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)

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tuple：只读数组

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

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set：没有重复元素的数组

# 去重的数组
s = set([1, 2, 2, 3, 3, 4, 5])
print(s)
s = set((2, 3, 4, 5, 6, 2, 1, 9))
print(s)

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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)

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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)

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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])

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面向对象

• 一切皆对象
• 获取对象信息：type和dir
• class声明对象
• 重要的self
• 继承与多态（什么是鸭子类型？）

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

# 用dir查看属性和方法
print(dir(list))
print("------------------")

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))
print("------------------")

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))
print("------------------")
t.run()


# 鸭子类型
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())

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文件读写

• 文本文件读写

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

with open('text.txt') as f:
    while True:
        line = f.readline()
        if not line:
            break
        print(line)

• 二进制文件读写

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

• string与bytes

s = 'abcdefg'
b = bytes(s)
print(b)

• 文件和目录操作

import os
#遍历文件夹下的所有文件
for x in os.listdir("D:\\蚂蚁众包\\20180609"):
    file_path = "%s\\%s"%("D:\\蚂蚁众包\\20180609",x)
    #判断是否是文件夹
    if os.path.isdir(file_path):
        print(file_path,"is a dir.")
    #判断是否是文件
    elif os.path.isfile(file_path):
        print(file_path,"is a file.")
    else:
        print(file_path,"is not a dir or file.")

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多线程

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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()

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错误和异常处理

# 错误处理

import logging

try:
    r = 10 / 0
except ZeroDivisionError as e:
    print(type(e))
    print(e)
finally:
    # 主要防止资源泄露！
    print('Always come here.')

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