Print to UTF-8 encoded file, with platform-depende

问题:

In Python, what is the best way to write to a UTF-8 encoded file with platform-dependent newlines? the solution would ideally work quite transparently in a program that does a lot of printing in Python 2. (Information about Python 3 is welcome too!)

In fact, the standard way of writing to a UTF-8 file seems to be codecs.open('name.txt', 'w'). However, the documentation indicates that

(…) no automatic conversion of '\n' is done on reading and writing.

because the file is actually opened in binary mode. So, how to write to a UTF-8 file with proper platform-dependent newlines?

Note: The 't' mode seems to actually do the job (codecs.open('name.txt', 'wt')) with Python 2.6 on Windows XP, but is this documented and guaranteed to work?

回答1:

Presuming Python 2.7.1 (that's the docs that you quoted): The 'wt' mode is not documented (the ONLY mode documented is 'r'), and does not work -- the codecs module appends 'b' to the mode, which causes it to fail:

>>> f = codecs.open('bar.txt', 'wt', encoding='utf8')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\python27\lib\codecs.py", line 881, in open
    file = __builtin__.open(filename, mode, buffering)
ValueError: Invalid mode ('wtb')

Avoid the codecs module and DIY:

f = open('bar.text', 'w')
f.write(unicode_object.encode('utf8'))

Update about Python 3.x:

It appears the codecs.open() has the same deficiency (won't write platform-specific line terminator). However built-in open(), which has an encoding arg, is happy to do it:

[Python 3.2 on Windows 7 Pro]
>>> import codecs
>>> f = codecs.open('bar.txt', 'w', encoding='utf8')
>>> f.write('line1\nline2\n')
>>> f.close()
>>> open('bar.txt', 'rb').read()
b'line1\nline2\n'
>>> f = open('bar.txt', 'w', encoding='utf8')
>>> f.write('line1\nline2\n')
12
>>> f.close()
>>> open('bar.txt', 'rb').read()
b'line1\r\nline2\r\n'
>>>

Update about Python 2.6

The docs say the same as the 2.7 docs. The difference is that the "bludgeon into binary mode" hack of appending "b" to the mode arg failed in 2.6 because "wtb" wasn't detected as as an invalid mode, the file was opened in text mode, and appears to work as you wanted, not as documented:

>>> import codecs
>>> f = codecs.open('fubar.txt', 'wt', encoding='utf8')
>>> f.write(u'\u0a0aline1\n\xffline2\n')
>>> f.close()
>>> open('fubar.txt', 'rb').read()
'\xe0\xa8\x8aline1\r\n\xc3\xbfline2\r\n' # "works"
>>> f.mode
'wtb' # oops
>>>

回答2:

Are you looking for os.linesep? http://www.python.org/doc//current/library/os.html#os.linesep

回答3:

In Python 2, why not encode explicitly?

with open('myfile.txt', 'w') as f:
    print >> f, some_unicode_text.encode('UTF-8')

Both embedded newlines, and those emitted by print, will be converted to the appropriate platform newline.