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：

aa = int("124")    #Correct
 print "aa = ", aa  #result=124
 bb = int(123.45) #correct
 print "bb = ", bb #result=123
 cc = int("-123.45")  #Error,Can't Convert to int
 print "cc = ",cc
dd = int("34a")    #Error,Can't Convert to int
print "dd = ",dd
ee = int("12.3") #Error,Can't Convert to int
print ee

二、float函数将整数和字符串转换成浮点数。

举例：

aa = float("124")     #Correct
print "aa = ", aa     #result = 124.0 
bb = float("123.45")  #Correct
print "bb = ", bb     #result = 123.45
cc = float(-123.6)    #Correct
print "cc = ",cc      #result = -123.6
dd = float("-123.34") #Correct
print "dd = ",dd      #result = -123.34
ee = float('123v')    #Error,Can't Convert to float
print ee

三、str函数将数字转换成字符

举例：

aa = str(123.4)     #Correct
print aa            #result = '123.4'
bb = str(-124.a)    #SyntaxError: invalid syntax
print bb
cc = str("-123.45") #correct
print cc            #result = '-123.45'
dd = str('ddd')     #correct
print dd            #result = ddd
ee = str(-124.3)    #correct
print ee            #result = -124.3