文件内容
a="中国"
print a

#报错
➜  ~ python a.py
  File "a.py", line 1
SyntaxError: Non-ASCII character '\xe4' in file b.py on line 1, but no encoding declared; see http://python.org/dev/peps/pep-0263/ for details

编译会读取变量a存到内存，因为包含中文，转化为str(ascii默认编码)报错

磁盘字节串utf-8-->无法转为内存字节串str(ascii)

 

>>> sys.getdefaultencoding()
'ascii'
>>> a="中国"
>>> a.encode("gbk")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
UnicodeDecodeError: 'ascii' codec can't decode byte 0xe4 in position 0: ordinal not in range(128)
>>> b=u"中国"
>>> b.encode("gbk")
'\xd6\xd0\xb9\xfa'

变量a的类型是str(utf-8字节串)，会自动调用decode(默认编码ascii)，转为unicode，再调encode

按照默认编码去decode变量utf-8编码字节串会报错

 

 

>>> "中国".decode("utf-8").encode("ascii")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
UnicodeEncodeError: 'ascii' codec can't encode characters in position 0-1: ordinal not in range(128)
>>> "中国".decode("utf-8").encode("gbk")
'\xd6\xd0\xb9\xfa'
>>> "a".decode("utf-8").encode("ascii")
'a'

 

ascii是utf-8的子集合，小集合转换大集合没有问题

大集合转为小集合就报错不支持

 

>>> a = "中国"
>>> b = a.decode("utf-8")
>>> b
u'\u4e2d\u56fd'
>>> b.decode("utf-8")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib64/python2.7/encodings/utf_8.py", line 16, in decode
    return codecs.utf_8_decode(input, errors, True)
UnicodeEncodeError: 'ascii' codec can't encode characters in position 0-1: ordinal not in range(128)

 

理解编码错误时，首先要判断str时以什么编码存放在内存中的，然后判断是否符合转换公式(str-(decode)->unicode-(encode)->str)。

判断方法，在python2中用len(str)或者bytes(str)可以判断，中文utf-8占3个字节，gbk占2个字节，ascii占一个字节。

 

此生必看的科学实验-水知道答案

http://v.youku.com/v_show/id_XMjgzMzcwNDk4OA