勘误：
缩进和换行的问题，因为我刚刚接触，理解得太肤浅，在上一篇笔记中写错了。
def结束要换行，if转elif和else不用换行，for循环结束要换行。
路曼曼其修远兮，吾将上下而求索。
Functions：
def printArea(width = 1, height = 2):
>>>printArea()
>>>printArea(4,2.5)
>>>printArea(height = 5, width = 3)
>>>printArea(width = 1.2)
>>>printArea(height = 6.2)
width. height.
1. 2.
4. 2.5
3. 5
1.2. 2
1. 6.2
str
s1 = str() 等价于s1 = ""，定义一个空字符串。
s[-1] == s[-1 + len(s)]
s1 = "Welcome"
s2 = s1 * 3 # s2 = "WelcomeWelcomeWelcome"
>>>"come" in s1
True
>>>"come" not in s1
False
s = "welcome to python"
>>>s.isalnum()
False
# isalnum() returns True if s is alphanumeric, 文数字，只含大写字母、小写字母或数字。
>>>"welcome".isalpha()
True
# isalpha() returns True if s is a alphabetic string
>>>"2012".isdigit()
True
>>>"first".isidentifier()
False
# isidentifier() returns True if s is a Python identifier.
>>>s.islower()
>>>s.isupper()
>>>s.isspace()
>>>s.endswith("thon")
True
>>>s.startswith("good")
False
>>>s.find("become")
-1
>>>s.rfind("o")
17 # returns the highest index where "o" starts in the string s, or -1 if "o" is not found in s.
>>>s.count("o")
3
>>>s1=s.capitalize()
>>>s1
Welcome to python
>>>s2=s.upper()
>>>s2
WELCOME TO PYTHON
>>>s3=s.lower()
>>>s3
welcome to python
>>>s4=s.title()
>>>s4
Welcome To Python
>>>s5=s.swapcase()
>>>s5
wELCOME tO pYTHON
>>>s6=s.replace("pYTHON","HEAVEN")
>>>s6
wELCOME tO HEAVEN
>>>s
welcome to python
>>>s=" welcome to python\t"
>>>s1=s.lstrip()
'welcome to python\t'
>>>s2=s.rstrip()
' welcome to python'
>>>s3=s.strip()
'welcome to python'
>>>s="welcome"
>>>s1=s.center(11)
' welcome '
#center(11) places s in the center of a string with 11 characters
>>>s2=s.ljust(11)
'welcome '
>>>s3=s.rjust(11)
' welcome'
>>>r1=Rational.Rational(4,2)
>>>r1
2
>>>r2=Rational.Rational(3,2)
>>>r2
3/2
