[Python03]-基础知识点

        Python官网有一个基础入门教程，后面还有一个类似考试的题目。这个教程语言很生动，学习起来也比较轻松，感觉很好，这里把学习到的一些基础知识点做一个总结，希望作为一个备忘，感兴趣的同学可以转载。

1.输出

>>> print("This is the first python program.")
This is the first python program.
>>> print "Hello Python"
Hello Python
>>> 

注：这两种方式都支持，但3.0以上版本的python只支持第一种方式。

2.注释

>>> print("not commnet")
not commnet
>>> #print("comment")
>>> 
'''
print("We are in a comment")
print ("We are still in a comment")
'''
print("We are out of the comment")

注：单行注释是#（与powershell一致），多行注释是三个单引号

3.变量

>>> a = 1
>>> print(a)
1
>>> str = "hello"
>>> print str
hello
>>> c = 2
>>> print(a+c)
3
>>> print(type(a))
<type 'int'>
>>> print(type(str))
<type 'str'>
>>> d = "3"
>>> print(a+int(d))
4
>>>

注：python变量的类型是弱类型的，即声明变量时不需要指定类型，这个跟powershell类似。type函数是获取变量类型的，在变量前面加int,str是进行类型强制转换。

4.运算符

>>> print (3 + 4)
7
>>> print (3 - 4)
-1
>>> print (3 / 4)
0
>>> print (3 * 4)
12
>>> print (3 ** 4)
81
>>> print(81//4)
20
>>> a = 0
>>> a += 2
>>> print(a)
2 

注：3**4 是3的4次方，//表示求最大商,+=操作和其他语言相同

5.if...else

a = 20
if a >= 22:
    print("if")
elif a >= 21:
    print("elif")
else:
    print("else")

注：if后面的:不要忘记，主要elif的写法有点特殊

6.函数定义及使用

def someFunction():
	print("foo")
someFunction()
</span><span style="font-family:Microsoft YaHei;">
def someFunction2(a, b):
    print(a+b)
someFunction2(12,451) 

注：不要忘记:，传参数方式跟其他函数相同。

7.循环

for a in range(1,3):
    print (a)
	
a = 1
while a < 10:
    print (a)
    a+=1

注：第一种是for循环，跟常见的形式不太一样，第二种是while循环，比较常见的形式。

8.字符串操作

>>> strVar = "hello"
>>> print(strVar.count('l'))
2
>>> print(strVar.find('h'))
0
>>> print(strVar.upper())
HELLO
>>> print(strVar.lower())
hello
>>> print(strVar.replace('l','m'))
hemmo
>>> print(strVar.strip())
hello
>>>

注：strip()是去除首尾空格

9.列表

>>> sampleList = [1,2,3,4,5,6,7,8]
>>> print(sampleList)
[1, 2, 3, 4, 5, 6, 7, 8]
>>> sampleTuple = ("a","b","c")
>>> print(sampleTuple)
('a', 'b', 'c')
>>> sampleList.append(9)
>>> print(sampleList)
[1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> sampleTuple.append("d")

Traceback (most recent call last):
  File "<pyshell#44>", line 1, in <module>
    sampleTuple.append("d")
AttributeError: 'tuple' object has no attribute 'append'
>>>

注：Python的列表有两种，list 是允许append的，但是tupe是不允许append的。但是如果确实需要append tuple，先将tuple转换成list再调用append即可；

10.字典

>>> myDic = {'username':"Austin",'age':20}
>>> print(myDic["username"])
Austin
>>> print(myDic["age"])
20
>>> for d in myDic:
	print(d,myDic[d])
('username', 'Austin')
('age', 20)
>>>

注：字典属于key/value类型的数据结构，用法比较常见。

11.异常处理

var1 = '1'
try:
    var1 = var1 + 1 # since var1 is a string, it cannot be added to the number 1 
except:
    print(var1, " is not a number") #so we execute this
print(var1) 

var1 = '1'
try:
    var2 = var1 + 1 # since var1 is a string, it cannot be added to the number 1 
except:
    var2 = int(var1) + 1 
print(var2)

注：这里不是catch，而是except，注意书写形式

12.读文件

f = open("test.txt","r") #opens file with name of "test.txt"
print(f.readline())
print(f.readline()) 
f.close()

I am a test file.
Maybe someday, he will promote me to a real file.</span> 

f = open("test.txt","r") #opens file with name of "test.txt"
myList = []
for line in f:
myList.append(line)
print(myList)
f.close()
['I am a test file.\n', 'Maybe someday, he will promote me to a real file.\n', 'Man, I long to be a real file\n', 'and hang out with all my new real file friends.']

注：r 是读的意思，w是写的意思。不要忘记关闭管道！！

13.写文件

f = open("test.txt","w") #opens file with name of "test.txt"
f.close() 

#the file content will be cleared!!!

f = open("test.txt","w") #opens file with name of "test.txt"
f.write("I am a test file.")
f.write("Maybe someday, he will promote me to a real file.")
f.write("Man, I long to be a real file")
f.write("and hang out with all my new real file friends.")
f.close()

f = open("test.txt","a") #opens file with name of "test.txt"
f.write("and can I get some pickles on that")
f.close()

注意：使用w时，open的时候会将文件已有的内容清空！！！

14.类

#ClassOne.py
class Calculator(object):
    #define class to simulate a simple calculator
    def __init__ (self):
        #start with zero
        self.current = 0
    def add(self, amount):
        #add number to current
        self.current += amount
    def getCurrent(self):
        return self.current

calc = Calculator()
print(calc.add(2))
print(calc.getCurrent())

注：类的定义形式跟常见的形式比较不同