__str __（）是否在幕后调用encode（）方法？

Question

It seems to me that built-in functions __repr__ and __str__ have an important difference in their base definition.

>>> t2 = u'\u0131\u015f\u0131k'
>>> print t2
ışık
>>> t2
Out[0]: u'\u0131\u015f\u0131k'

t2.decode raises an error since t2 is a unicode string.

>>> enc = 'utf-8'
>>> t2.decode(enc)
------------------------------------------------------------
Traceback (most recent call last):
  File "<ipython console>", line 1, in <module>
  File "C:\java\python\Python25\Lib\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 0-2: ordin
al not in range(128)

__str__ raises an error as if decode() function is being called:

>>> t2.__str__()
------------------------------------------------------------
Traceback (most recent call last):
  File "<ipython console>", line 1, in <module>
UnicodeEncodeError: 'ascii' codec can't encode characters in position 0-2: ordin
al not in range(128)

but __repr__ works without problem:

>>> t2.__repr__()
Out[0]: "u'\\u0131\\u015f\\u0131k'"

Why does __str__ produce an error whereas __repr__ work properly?

This small difference seems to cause a bug in one django application that I am working on.

Answer 1

Basically, __str__ can only output ascii strings. Since t2 contains unicode codepoints above ascii, it cannot be represented with just a string. __repr__ , on the other hand, tries to output the python code needed to recreate the object. You'll see that the output from repr(t2) (this syntax is preferred to t2.__repr_() ) is exactly what you set t2 equal to up on the first line. The result from repr looks roughly like ['\\', 'u', '0', ...], which are all ascii values, but the output from str is trying to be [chr(0x0131), chr(0x015f), chr(0x0131), 'k'], most of which are above the range of characters acceptable in a python string. Generally, when dealing with django applications, you should use __unicode__ for everything, and never touch __str__ .

More info in the django documentation on strings .

Answer 2

In general, calling str.__unicode__() or unicode.__str__() is a very bad idea, because bytes can't be safely converted to Unicode character points and vice versa. The exception is ASCII values, which are generally the same in all single-byte encodings. The problem is that you're using the wrong method for conversion.

To convert unicode to str , you should use encode() :

>>> t1 = u"\u0131\u015f\u0131k"
>>> t1.encode("utf-8")
'\xc4\xb1\xc5\x9f\xc4\xb1k'

To convert str to unicode , use decode() :

>>> t2 = '\xc4\xb1\xc5\x9f\xc4\xb1k'
>>> t2.decode("utf-8")
u'\u0131\u015f\u0131k'

Answer 3

To add a bit of support to John's good answer:

To understand the naming of the two methods encode() and decode() , you just have to see that Python considers unicode strings of the form u'...' to be in the reference format . You encode going from the reference format into another format (eg utf-8), and you decode from some other format to come to the reference format. The unicode format is always considered the "real thing" :-).

Answer 4

请注意，在Python 3中，unicode是默认设置，而__str__()应该始终为您提供unicode。