为什么打印到utf-8文件失败？

Question

So I ran into a problem this afternoon, I was able to solve it, but I don't quite understand why it worked.

this is related to a problem I had the other week: python check if utf-8 string is uppercase

basically, the following will not work:

#!/usr/bin/python

import codecs
from lxml import etree

outFile = codecs.open('test.xml', 'w', 'utf-8') #cannot use codecs.open()

root = etree.Element('root')
sect = etree.SubElement(root,'sect')


words = (   u'\u041c\u041e\u0421\u041a\u0412\u0410', # capital of Russia, all uppercase
            u'R\xc9SUM\xc9',    # RESUME with accents
            u'R\xe9sum\xe9',    # Resume with accents
            u'R\xe9SUM\xe9', )  # ReSUMe with accents

for word in words:
    print word
    if word.encode('utf8').decode('utf8').isupper(): #.isupper won't function on utf8 
        title = etree.SubElement(sect,'title')
        title.text = word
    else:
       item = etree.SubElement(sect,'item')
       item.text = word 

print>>outFile,etree.tostring(root,pretty_print=True,xml_declaration=True,encoding='utf-8')

it fails with the following:

Traceback (most recent call last):
File "./temp.py", line 25, in
print >>outFile,etree.tostring(root,pretty_print=True,xml_declaration=True,encoding='utf-8')
File "/usr/lib/python2.7/codecs.py",
line 691, in write
return self.writer.write(data) File "/usr/lib/python2.7/codecs.py",
line 351, in write
data, consumed = self.encode(object, self.errors)
UnicodeDecodeError: 'ascii' codec
can't decode byte 0xd0 in position 66:
ordinal not in range(128)

but if I open the new file without codecs.open('test.xml', 'w', 'utf-8') and instead use outFile = open('test.xml', 'w') it works perfectly.

So whats happening??

• since encoding='utf-8' is specified in etree.tostring() is it encoding the file again?

• if I leave codecs.open() and remove encoding='utf-8' the file then becomes an ascii file. Why? becuase etree.tostring() has a default encoding of ascii I persume?

• but etree.tostring() is simply being written to stdout, and is then redirect to a file that was created as a utf-8 file??

  • is print>> not workings as I expect? outFile.write(etree.tostring()) behaves the same way.

Basically, why wouldn't this work? what is going on here. It might be trivial, but I am obviously a bit confused and have a desire to figure out why my solution works,

Answer 1

You've opened the file with UTF-8 encoding, which means that it expects Unicode strings.

tostring is encoding to UTF-8 (in the form of bytestrings, str), which you're writing to the file.

Because the file is expecting Unicode, it's decoding the bytestrings to Unicode using the default ASCII encoding so that it can then encode the Unicode to UTF-8.

Unfortunately, the bytestrings aren't ASCII.

EDIT: The best advice to avoid this kind of problem is to use Unicode internally, decoding on input and encoding on output.

Answer 2

Using print>>outFile is a little strange. I don't have lxml installed, but the built-in xml.etree library is similar (but doesn't support pretty_print ). Wrap the root Element in an ElementTree and use the write method.

Also, if you using a # coding line to declare the encoding of the source file, you can use readable Unicode strings instead of escape codes:

#!/usr/bin/python
# coding: utf8

import codecs
from xml.etree import ElementTree as etree

root = etree.Element(u'root')
sect = etree.SubElement(root,u'sect')


words = [u'МОСКВА',u'RÉSUMÉ',u'Résumé',u'RéSUMé']

for word in words:
    print word
    if word.isupper():
        title = etree.SubElement(sect,u'title')
        title.text = word
    else:
       item = etree.SubElement(sect,u'item')
       item.text = word 

tree = etree.ElementTree(root)
tree.write('text.xml',xml_declaration=True,encoding='utf-8')

Answer 3

In addition to MRABs answer some lines of code:

import codecs
from lxml import etree

root = etree.Element('root')
sect = etree.SubElement(root,'sect')

# do some other xml building here

with codecs.open('test.xml', 'w', encoding='utf-8') as f:
    f.write(etree.tostring(root, encoding=unicode))