为什么我们不应该在py脚本中使用sys.setdefaultencoding(“ utf-8”)?-Python 实用宝典

为什么我们不应该在py脚本中使用sys.setdefaultencoding(“ utf-8”)?

我在脚本顶部看到了几个使用此脚本的py脚本。在什么情况下应该使用它? import sys reload(sys) sys.setdefaultencoding("utf-8")

问题:为什么我们不应该在py脚本中使用sys.setdefaultencoding(“ utf-8”)?

我在脚本顶部看到了几个使用此脚本的py脚本。在什么情况下应该使用它?

import sys
reload(sys)
sys.setdefaultencoding("utf-8")

I have seen few py scripts which use this at the top of the script. In what cases one should use it?

import sys
reload(sys)
sys.setdefaultencoding("utf-8")

回答 0

根据文档:这允许您从默认的ASCII切换到其他编码,例如UTF-8,Python运行时在必须将字符串缓冲区解码为unicode时将使用该编码。

此功能仅在Python扫描环境时在Python启动时可用。必须在系统范围的模块中调用,sitecustomize.py评估完setdefaultencoding()sys模块后,将从该模块中删除该功能。

实际使用它的唯一方法是通过将属性重新带回的重载hack。

此外,使用sys.setdefaultencoding()一直气馁,它已成为一个无操作的py3k。py3k的编码硬连线到“ utf-8”,更改它会引发错误。

我建议您阅读一些指针:

As per the documentation: This allows you to switch from the default ASCII to other encodings such as UTF-8, which the Python runtime will use whenever it has to decode a string buffer to unicode.

This function is only available at Python start-up time, when Python scans the environment. It has to be called in a system-wide module, sitecustomize.py, After this module has been evaluated, the setdefaultencoding() function is removed from the sys module.

The only way to actually use it is with a reload hack that brings the attribute back.

Also, the use of sys.setdefaultencoding() has always been discouraged, and it has become a no-op in py3k. The encoding of py3k is hard-wired to "utf-8" and changing it raises an error.

I suggest some pointers for reading:


回答 1

tl; dr

答案是永不(除非您真的知道自己在做什么)

在正确理解编码/解码的情况下,可以解决9/10倍的解决方案。

1/10个人的语言环境或环境定义错误,需要设置:

PYTHONIOENCODING="UTF-8"  

在他们的环境中解决控制台打印问题。

它有什么作用?

sys.setdefaultencoding("utf-8")(为了避免重复使用,请删除),更改了Python 2.x需要将Unicode()转换为str()(反之亦然)且未给出编码时使用的默认编码/解码。即:

str(u"\u20AC")
unicode("€")
"{}".format(u"\u20AC") 

在Python 2.x中,默认编码设置为ASCII,并且上面的示例将失败,并显示以下内容:

UnicodeDecodeError: 'ascii' codec can't decode byte 0xe2 in position 0: ordinal not in range(128)

(我的控制台配置为UTF-8,因此"€" = '\xe2\x82\xac',因此为exceptions\xe2

要么

UnicodeEncodeError: 'ascii' codec can't encode character u'\u20ac' in position 0: ordinal not in range(128)

sys.setdefaultencoding("utf-8")将允许这些代码对有用,但对于不使用UTF-8的用户不一定有用。ASCII的默认设置可确保不会将编码假设纳入代码

安慰

sys.stdout.encoding在将字符打印到控制台时,也具有出现fix的副作用。Python使用用户的语言环境(Linux / OS X / Un * x)或代码页(Windows)进行设置。有时,用户的语言环境已损坏,仅需要PYTHONIOENCODING修复控制台编码

例:

$ export LANG=en_GB.gibberish
$ python
>>> import sys
>>> sys.stdout.encoding
'ANSI_X3.4-1968'
>>> print u"\u20AC"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
UnicodeEncodeError: 'ascii' codec can't encode character u'\u20ac' in position 0: ordinal not in range(128)
>>> exit()

$ PYTHONIOENCODING=UTF-8 python
>>> import sys
>>> sys.stdout.encoding
'UTF-8'
>>> print u"\u20AC"
€

sys.setdefaultencoding(“ utf-8”)有什么不好?

人们已经认识到默认的编码是ASCII,因此针对Python 2.x进行了16年的开发。UnicodeError已经编写了异常处理方法来处理发现包含非ASCII的字符串从字符串到Unicode的转换。

来自https://anonbadger.wordpress.com/2015/06/16/why-sys-setdefaultencoding-will-break-code/

def welcome_message(byte_string):
    try:
        return u"%s runs your business" % byte_string
    except UnicodeError:
        return u"%s runs your business" % unicode(byte_string,
            encoding=detect_encoding(byte_string))

print(welcome_message(u"Angstrom (Å®)".encode("latin-1"))

在设置defaultencoding之前,此代码将无法解码ascii编码中的“Å”,然后将进入异常处理程序以猜测编码并将其正确转换为unicode。打印:埃斯特朗(Å®)经营您的业务。将defaultencoding设置为utf-8后,代码将发现byte_string可以解释为utf-8,因此它将处理数据并返回该值:Angstrom(Ů)经营您的业务。

更改应为常数的值将对您依赖的模块产生巨大影响。最好只修复代码中传入和传出的数据。

示例问题

虽然在以下示例中将defaultencoding设置为UTF-8并不是根本原因,但它显示了如何掩盖问题以及如何在输入编码更改时以不明显的方式中断代码: UnicodeDecodeError:'utf8'编解码器可以在位置3131中解码字节0x80:无效的起始字节

tl;dr

The answer is NEVER! (unless you really know what you're doing)

9/10 times the solution can be resolved with a proper understanding of encoding/decoding.

1/10 people have an incorrectly defined locale or environment and need to set:

PYTHONIOENCODING="UTF-8"  

in their environment to fix console printing problems.

What does it do?

sys.setdefaultencoding("utf-8") (struck through to avoid re-use) changes the default encoding/decoding used whenever Python 2.x needs to convert a Unicode() to a str() (and vice-versa) and the encoding is not given. I.e:

str(u"\u20AC")
unicode("€")
"{}".format(u"\u20AC") 

In Python 2.x, the default encoding is set to ASCII and the above examples will fail with:

UnicodeDecodeError: 'ascii' codec can't decode byte 0xe2 in position 0: ordinal not in range(128)

(My console is configured as UTF-8, so "€" = '\xe2\x82\xac', hence exception on \xe2)

or

UnicodeEncodeError: 'ascii' codec can't encode character u'\u20ac' in position 0: ordinal not in range(128)

sys.setdefaultencoding("utf-8") will allow these to work for me, but won't necessarily work for people who don't use UTF-8. The default of ASCII ensures that assumptions of encoding are not baked into code

Console

also has a side effect of appearing to fix sys.stdout.encoding, used when printing characters to the console. Python uses the user's locale (Linux/OS X/Un*x) or codepage (Windows) to set this. Occasionally, a user's locale is broken and just requires PYTHONIOENCODING to fix the console encoding.

Example:

$ export LANG=en_GB.gibberish
$ python
>>> import sys
>>> sys.stdout.encoding
'ANSI_X3.4-1968'
>>> print u"\u20AC"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
UnicodeEncodeError: 'ascii' codec can't encode character u'\u20ac' in position 0: ordinal not in range(128)
>>> exit()

$ PYTHONIOENCODING=UTF-8 python
>>> import sys
>>> sys.stdout.encoding
'UTF-8'
>>> print u"\u20AC"
€

What's so bad with sys.setdefaultencoding("utf-8")?

People have been developing against Python 2.x for 16 years on the understanding that the default encoding is ASCII. UnicodeError exception handling methods have been written to handle string to Unicode conversions on strings that are found to contain non-ASCII.

From https://anonbadger.wordpress.com/2015/06/16/why-sys-setdefaultencoding-will-break-code/

def welcome_message(byte_string):
    try:
        return u"%s runs your business" % byte_string
    except UnicodeError:
        return u"%s runs your business" % unicode(byte_string,
            encoding=detect_encoding(byte_string))

print(welcome_message(u"Angstrom (Å®)".encode("latin-1"))

Previous to setting defaultencoding this code would be unable to decode the “Å” in the ascii encoding and then would enter the exception handler to guess the encoding and properly turn it into unicode. Printing: Angstrom (Å®) runs your business. Once you’ve set the defaultencoding to utf-8 the code will find that the byte_string can be interpreted as utf-8 and so it will mangle the data and return this instead: Angstrom (Ů) runs your business.

Changing what should be a constant will have dramatic effects on modules you depend upon. It's better to just fix the data coming in and out of your code.

Example problem

While the setting of defaultencoding to UTF-8 isn't the root cause in the following example, it shows how problems are masked and how, when the input encoding changes, the code breaks in an unobvious way: UnicodeDecodeError: 'utf8' codec can't decode byte 0x80 in position 3131: invalid start byte


回答 2

#!/usr/bin/env python
#-*- coding: utf-8 -*-
u = u'moçambique'
print u.encode("utf-8")
print u

chmod +x test.py
./test.py
moçambique
moçambique

./test.py > output.txt
Traceback (most recent call last):
  File "./test.py", line 5, in <module>
    print u
UnicodeEncodeError: 'ascii' codec can't encode character 
u'\xe7' in position 2: ordinal not in range(128)

在shell上工作时,不发送到sdtout,因此这是写stdout的一种解决方法。

我做了另一种方法,如果未定义sys.stdout.encoding,或者换句话说,需要先导出PYTHONIOENCODING = UTF-8才能写入stdout,否则该方法将不运行。

import sys
if (sys.stdout.encoding is None):            
    print >> sys.stderr, "please set python env PYTHONIOENCODING=UTF-8, example: export PYTHONIOENCODING=UTF-8, when write to stdout." 
    exit(1)


因此,使用相同的示例:

export PYTHONIOENCODING=UTF-8
./test.py > output.txt

将工作

#!/usr/bin/env python
#-*- coding: utf-8 -*-
u = u'moçambique'
print u.encode("utf-8")
print u

chmod +x test.py
./test.py
moçambique
moçambique

./test.py > output.txt
Traceback (most recent call last):
  File "./test.py", line 5, in <module>
    print u
UnicodeEncodeError: 'ascii' codec can't encode character 
u'\xe7' in position 2: ordinal not in range(128)

on shell works , sending to sdtout not , so that is one workaround, to write to stdout .

I made other approach, which is not run if sys.stdout.encoding is not define, or in others words , need export PYTHONIOENCODING=UTF-8 first to write to stdout.

import sys
if (sys.stdout.encoding is None):            
    print >> sys.stderr, "please set python env PYTHONIOENCODING=UTF-8, example: export PYTHONIOENCODING=UTF-8, when write to stdout." 
    exit(1)


so, using same example:

export PYTHONIOENCODING=UTF-8
./test.py > output.txt

will work


回答 3

  • 第一个危险在于reload(sys)

    重新加载模块时,实际上在运行时中获得了该模块的两个副本。旧模块是一个Python对象,就像其他所有模块一样,只要存在对它的引用,它就会保持活动状态。因此,一半的对象将指向旧模块,而另一半则指向新模块。进行更改时,当某些随机对象看不到更改时,您将永远看不到它:

    (This is IPython shell)
    
    In [1]: import sys
    
    In [2]: sys.stdout
    Out[2]: <colorama.ansitowin32.StreamWrapper at 0x3a2aac8>
    
    In [3]: reload(sys)
    <module 'sys' (built-in)>
    
    In [4]: sys.stdout
    Out[4]: <open file '<stdout>', mode 'w' at 0x00000000022E20C0>
    
    In [11]: import IPython.terminal
    
    In [14]: IPython.terminal.interactiveshell.sys.stdout
    Out[14]: <colorama.ansitowin32.StreamWrapper at 0x3a9aac8>
  • 现在,sys.setdefaultencoding()适当的

    。现在,这utf-8是地球上最聪明的编码(向后兼容ASCII和所有语言),现在转换“正常”了,可能出什么问题了吗?

    好吧,什么都可以。那就是危险。

    • 可能有些代码依赖于UnicodeError为非ASCII输入抛出的代码,或者使用错误处理程序进行代码转换,这现在会产生意外结果。而且,由于所有代码都是使用默认设置进行测试的,因此您在此处严格处于“不受支持”的范围,并且没人能保证它们的代码将如何运行。
    • 如果系统上并非所有组件都使用UTF-8,则转码可能会产生意外或无法使用的结果,因为Python 2实际上具有多个独立的“默认字符串编码”。(请记住,程序必须在客户的设备上为客户工作。)
      • 同样,最糟糕的是您永远不会知道,因为转换是隐式的 -您实际上并不知道转换的时间和地点。(Python Zen,koan 2 ahoy!)您将永远不知道为什么(如果)代码可以在一个系统上运行而在另一个系统上中断。(或者更好的是,可以在IDE中工作,并且可以在控制台中中断。)
  • The first danger lies in reload(sys).

    When you reload a module, you actually get two copies of the module in your runtime. The old module is a Python object like everything else, and stays alive as long as there are references to it. So, half of the objects will be pointing to the old module, and half to the new one. When you make some change, you will never see it coming when some random object doesn't see the change:

    (This is IPython shell)
    
    In [1]: import sys
    
    In [2]: sys.stdout
    Out[2]: <colorama.ansitowin32.StreamWrapper at 0x3a2aac8>
    
    In [3]: reload(sys)
    <module 'sys' (built-in)>
    
    In [4]: sys.stdout
    Out[4]: <open file '<stdout>', mode 'w' at 0x00000000022E20C0>
    
    In [11]: import IPython.terminal
    
    In [14]: IPython.terminal.interactiveshell.sys.stdout
    Out[14]: <colorama.ansitowin32.StreamWrapper at 0x3a9aac8>
    
  • Now, sys.setdefaultencoding() proper

    . Now, utf-8 is the sanest encoding on the planet (backward-compatible with ASCII and all), the conversion now "just works", what could possibly go wrong?

    Well, anything. And that is the danger.

    • There may be some code that relies on the UnicodeError being thrown for non-ASCII input, or does the transcoding with an error handler, which now produces an unexpected result. And since all code is tested with the default setting, you're strictly on "unsupported" territory here, and no-one gives you guarantees about how their code will behave.
    • The transcoding may produce unexpected or unusable results if not everything on the system uses UTF-8 because Python 2 actually has multiple independent "default string encodings". (Remember, a program must work for the customer, on the customer's equipment.)
      • Again, the worst thing is you will never know that because the conversion is implicit -- you don't really know when and where it happens. (Python Zen, koan 2 ahoy!) You will never know why (and if) your code works on one system and breaks on another. (Or better yet, works in IDE and breaks in console.)

本文由 Python 实用宝典 作者:Python实用宝典 发表,其版权均为 Python 实用宝典 所有,文章内容系作者个人观点,不代表 Python 实用宝典 对观点赞同或支持。如需转载,请注明文章来源。
7

抱歉,评论已关闭!