Python基础复习_Key Points
1. 弱类型：不需要为变量指定类型。（C语言是强类型，必须给变量指定类型。如：int，string等。强类型的好处：方便做语法检查）
1 a = 12342 print(a)3 a = ‘abcd’4 print(a)
#outcomes:
1234abcd
2. 变量必须先赋值。Python中变量要先赋值再使用。而C语言中，定义一个变量（int i)，i是有一个初始值。
3. Python是通过引用传递变量的。引用变量的地址空间。
4. Python2 和Python3的常见差异
- xrange 在Python2中，xrange主要是解决python循环中的效率问题。与range相比，xrange使用generator来解决空间效率问题。而在python3中，xrange和range的效率是一样的。
- print 在python2中，无需加括号，但在python3中，要加括号。
- 数据类型统一，取消unicode和long
- utf8：python3代码默认utf8
1. none是True和False之外的一种状态。可以等价为false。
2. del是从容器里删除对象。
3. python中只有三种操作：与、或、非。没有&&，||等。
4. 函数是没有返回值的，要返回值，需加return
默认参数：
1 def hello(who = 'world'):2 print('hello %s!' %(who))3 4 hello()5 hello('sea') #Outcome:
hello world!hello sea!
Lambda VS Normal defunction:
1 def g(x):2 return x*53 def f(gf,x): # gf has got the memory address of g(x), no matter what it is called.4 return gf(x)+100 # gf(x) call g(x) and pass the argument x to g(x).5 print(f(g,100)) # g -> pass the memory address of g(x) to the argument. 6 print(f(lambda x:x*110,100)) 7 # process: 1. lambda generate a memory address for itself (x*110)8 #2. call f(g,100) and pass the memory address of lambda to the argument gf.9 #3. gf(x) call lambda x*110 and pass 100 to x, so the result of lambda equals 11100
#Outcome:
60011100
1 def f(gf,x,y):2 return gf(x,y)+1003 print(f(lambda x,y:x*y,100,200))
#Outcome:
20100
5. while & for
1 totoal = 02 i = 13 while i <= 100:4 total += i5 i += 1 #没有++i或者--i6 print（total）
#outcome：
5050
for循环只作用用于容器！！
没有这种写法：
for (i = 0; i<100; ++i) pass
上面这种循环只能用while实现。
1 i = 02 while i < 3:3 j = 0 4 while j <=3:5 if j == 2:6 break / continue #只退出当前循环，循环继续执行j=3.。。7 print(i,j)8 j += 19 i +=1
#outcomes:
Break:0 00 11 0 1 12 02 1
Continue：0 00 1
1. List
用lambda对第一元素做排序
切片
2. String
Mode
‘r’ – Read mode which is used when the file is only being read ‘w’ – Write mode which is used to edit and write new information to the file (any existing files with the same name will be erased when this mode is activated) 'b' - 'b' appended to the mode ('rb','rb+','wb','wb+') opens the file in binary mode‘a’ – Appending mode, which is used to add new data to the end of the file; that is new information is automatically amended to the end ‘r+’ – Special read and write mode, which is used to handle both actions when working with a file Open a file
>>> f = open('workfile', 'w') Open a file with 'with'
>>> with open('workfile') as f: #If you’re not using the keyword, then you should call f.close() to close the file and immediately free up any system resources used by it.... read_data = f.read()>>> f.closed #If you don’t explicitly close a file, Python’s garbage collector will eventually destroy the object and close the open file for you, but the file may stay open for a while. #with as equals: try: f = open('workfile','r') for line in f.readlines(): print(line) except: ... finnally: f.close() Read a file:
>>> f.read() #Read a whole file.#Outcome:'This is the entire file.\n'>>> f.readline() #Read a line of the file#Outcome:'This is the first line of the file.\n'>>> f.readline()#Outcome:'Second line of the file\n' >>> for line in f: #Read a file line-by-line... print(line, end='')#outcomes:This is the first line of the file.Second line of the file
Write a file:
f.write('This is a test\n')
#错误处理
1 import logging #在生产环境中，最有效的调试方式。 2 3 ''' 4 作业，自己实现将不同的等级的信息写到不同日志文件。 5 logging.info() 6 logging.debug() 7 ''' 8 9 try:10 r=10/011 except ZeroDivisionError as e: #捕捉异常12 print(type(e))13 print(e)14 finally: #主要是防止服务端资源泄漏！15 print('always come here.')