Python复习笔记—基本语法

 

3年前用过Python，现在忘的差不多了，现在项目又用到它了，重新拾起来，记录笔记以方便以后的重拾。

1. Python脚本

脚本提示符：

 

#! /usr/bin/env python

2. 语言基础

2.1 Numbers

i. Python的整数相除是向上取整

>>> 7/3
2
>>> 7/-3
-3

 ii. 复数用(real+imagj)，或complex(real, imag)创建

2.2 Strings

i. 单引号（'）和双引号（"）均可表示字符串

ii. 多行可以用三个引号（"""）标识开始和结束

iii. 常量字符串前加r表示raw字符串，内容将不会转义

>>> rawstring = r'raw string \n \t \r keeps'
>>> print rawstring
raw string \n \t \r keeps

iv. 字符串可以用加号（+）连接（常量字符串之间可以省略），且可以重复（*）

>>> repeat = 'I' ' Love' * 3 + ' Python'
>>> print repeat
I LoveI LoveI Love Python

v. 字符串切割（slice） 

字符串可以被任意切割，如果范围不正确或超过范围返回空字符串，但下标访问超过范围会抛异常

 

>>> helloworld = 'hello world'
>>> 
>>> helloworld[5:]
' world'
>>> helloworld[6]
'w'
>>> 
>>> helloworld[6:]
'world'
>>> helloworld[3:5]
'lo'
>>> helloworld[-1]
'd'
>>> helloworld[-2:3]
''
>>> helloworld[:-6]
'hello'
>>> helloworld[-0]
'h'
>>> helloworld[30:]
''
>>> helloworld[1:100]
'ello world'
>>> helloworld[6:2]
''
>>> helloworld[30]

Traceback (most recent call last):
  File "<pyshell#77>", line 1, in <module>
    helloworld[30]
IndexError: string index out of range

2.3 Lists

i. Python的List的元素可以不为不同类型

>>> lista = ['egg', 'hurt', 2, 3.14]
>>> lista
['egg', 'hurt', 2, 3.14]

 

ii. List可以像字符串一样随意切割，重复，下标访问

>>> lista[0:2] + ['very'] * 2
['egg', 'hurt', 'very', 'very']

>>> lista[3]
3.14

  iii. List的替换、插入、赋值、清空

>>> lista
['egg', 'hurt', 2, 3.14]
>>> 
>>> lista[0:2]=['foo', 1.72] # replacement
>>> lista
['foo', 1.72, 2, 3.14]
>>> lista[0] = [] #add list as an element
>>> lista
[[], 1.72, 2, 3.14]
>>> lista[0:1] = [] #clear elements between 0 and 1
>>> lista
[1.72, 2, 3.14]
>>> lista[1:1] = ['sec', 'third'] #insert two elements at index 1
>>> lista
[1.72, 'sec', 'third', 2, 3.14]
>>> lista[:0] = lista #make a copy of itself
>>> lista
[1.72, 'sec', 'third', 2, 3.14, 1.72, 'sec', 'third', 2, 3.14]
>>> lista[:] = [] #clear
>>> lista
[]

>>> len(lista)
0

2.4 控制流

i. if...elif..else

>>> def whatx(x) :
	if x > 0 :
		print 'positive'
	elif x == 0 :
		print 'zero'
	else :
		print 'negative'

		
>>> whatx(-3)
negative

   ii. for...in

>>> for i in [1, 2, 3, 4, 5] :
	print i,

	
1 2 3 4 5

 

 和其他语言一样在循环的时候添加删除元素是不安全的，Python可以隐式的拷贝一个List

 

>>> odds = [1, 3, 4, 7]
>>> for i in odds[:] : #make a slice copy of entire list
	if (0 == i % 2) :
		odds.remove(i)
	
>>> odds
[1, 3, 7]

  iii. Range，可指定起始，步长

>>> range(5)
[0, 1, 2, 3, 4]
>>> range(2, 10)
[2, 3, 4, 5, 6, 7, 8, 9]
>>> range(3, 15, 3)
[3, 6, 9, 12]
>>> range(-5, -30, -5)
[-5, -10, -15, -20, -25]

  iv. Pass占位符

Pass什么都不做，只是让语法通过，

 

>>> class Non :
	pass # minimal class for indicate a status

>>> def foo() :
	pass # remember implement this

>>>  

v. 赋值

Python可以一次给多个变量赋值

 

>>> x, y, z = 1, 5, 7
>>> x
1
>>> y
5
>>> z
7

vi. == 和is

==相当于equals，而is确定是否是同一对象。