python 字符串编码

在pythong开发过程中经常遇到编码问题，原因在于没有正确理解编码解码过程

通过以下命令过程帮助理解下，各自体会吧。

宗旨：由Unicode中转进行编码、解码

>>> u = '\uffef'
>>> print u
\uffef
>>> u = u'\uffef'
>>> print u
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
UnicodeEncodeError: 'gbk' codec can't encode character u'\uffef' in position 0: illegal multibyte sequence
>>> str(u)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
UnicodeEncodeError: 'ascii' codec can't encode character u'\uffef' in position 0: ordinal not in range(128)
>>> sys.stdout.encoding
'cp936'
>>> x = u.encode('utf-8')
>>> x
'\xef\xbf\xaf'
>>> x.decode('utf-8')
u'\uffef'
>>> x.encode('utf-8')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
UnicodeDecodeError: 'ascii' codec can't decode byte 0xef in position 0: ordinal not in range(128)
>>> u.decode('utf-8')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "E:\Program Files\Python27\lib\encodings\utf_8.py", line 16, in decode
    return codecs.utf_8_decode(input, errors, True)
UnicodeEncodeError: 'ascii' codec can't encode character u'\uffef' in position 0: ordinal not in range(128)
>>> u.encode('utf-8')
'\xef\xbf\xaf'

>>> print x
锟
>>> str(x)
'\xef\xbf\xaf'
>>> repr(x)
"'\\xef\\xbf\\xaf'"
>>> repr(u)
"u'\\uffef'"
>>> repr(str)
"<type 'str'>"
>>> repr(unicode)
"<type 'unicode'>"

>>> unicode(x)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
UnicodeDecodeError: 'ascii' codec can't decode byte 0xef in position 0: ordinal not in range(128)
>>> unicode(u)
u'\uffef'
>>> isinstance(u, str)
False
>>> isinstance(u, unicode)
True
>>> isinstance(x, str)
True
>>> isinstance(x, unicode)
False

>>> import re
>>> re.search(u'\uff', u)
  File "<stdin>", line 1
SyntaxError: (unicode error) 'unicodeescape' codec can't decode bytes in position 0-XXX escape
>>> re.search(u'ff', u)
>>> re.search(r'\uff', u)
>>> re.search(ur'\uff', u)
  File "<stdin>", line 1
SyntaxError: (unicode error) 'rawunicodeescape' codec can't decode bytes in position\uXXXX

>>> len(None)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: object of type 'NoneType' has no len()
>>> len('aaa')
3
>>> len([])
0
>>> len(())
0
>>> len({})
0