http://cnblogs.com/dabiao/archive/2010/03/07/1680096.html
一、int函数能够
（1）把符合数学格式的数字型字符串转换成整数
（2）把浮点数转换成整数，但是只是简单的取整，而非四舍五入。
举例：
1 aa = int("124") #Correct
2 print "aa = ", aa #result=124
3 bb = int(123.45) #correct
4 print "bb = ", bb #result=123
5 cc = int("-123.45") #Error,Can't Convert to int
6 print "cc = ",cc
7 dd = int("34a") #Error,Can't Convert to int
8 print "dd = ",dd
9 ee = int("12.3") #Error,Can't Convert to int
10 print ee
11
2 print "aa = ", aa #result=124
3 bb = int(123.45) #correct
4 print "bb = ", bb #result=123
5 cc = int("-123.45") #Error,Can't Convert to int
6 print "cc = ",cc
7 dd = int("34a") #Error,Can't Convert to int
8 print "dd = ",dd
9 ee = int("12.3") #Error,Can't Convert to int
10 print ee
11
二、float函数将整数和字符串转换成浮点数。
举例：
1 aa = float("124") #Correct
2 print "aa = ", aa #result = 124.0
3 bb = float("123.45") #Correct
4 print "bb = ", bb #result = 123.45
5 cc = float(-123.6) #Correct
6 print "cc = ",cc #result = -123.6
7 dd = float("-123.34") #Correct
8 print "dd = ",dd #result = -123.34
9 ee = float('123v') #Error,Can't Convert to float
10 print ee
2 print "aa = ", aa #result = 124.0
3 bb = float("123.45") #Correct
4 print "bb = ", bb #result = 123.45
5 cc = float(-123.6) #Correct
6 print "cc = ",cc #result = -123.6
7 dd = float("-123.34") #Correct
8 print "dd = ",dd #result = -123.34
9 ee = float('123v') #Error,Can't Convert to float
10 print ee
三、str函数将数字转换成字符
举例：
1 aa = str(123.4) #Correct
2 print aa #result = '123.4'
3 bb = str(-124.a) #SyntaxError: invalid syntax
4 print bb
5 cc = str("-123.45") #correct
6 print cc #result = '-123.45'
7 dd = str('ddd') #correct
8 print dd #result = ddd
9 ee = str(-124.3) #correct
10 print ee #result = -124.3
2 print aa #result = '123.4'
3 bb = str(-124.a) #SyntaxError: invalid syntax
4 print bb
5 cc = str("-123.45") #correct
6 print cc #result = '-123.45'
7 dd = str('ddd') #correct
8 print dd #result = ddd
9 ee = str(-124.3) #correct
10 print ee #result = -124.3