Write to UTF-8 file in Python

Question

I'm really confused with the codecs.open function. When I do:

file = codecs.open("temp", "w", "utf-8")
file.write(codecs.BOM_UTF8)
file.close()

It gives me the error

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

If I do:

file = open("temp", "w")
file.write(codecs.BOM_UTF8)
file.close()

It works fine.

Question is why does the first method fail? And how do I insert the bom?

If the second method is the correct way of doing it, what the point of using codecs.open(filename, "w", "utf-8")?

Answer 1

I believe the problem is that codecs.BOM_UTF8 is a byte string, not a Unicode string. I suspect the file handler is trying to guess what you really mean based on "I'm meant to be writing Unicode as UTF-8-encoded text, but you've given me a byte string!"

Try writing the Unicode string for the byte order mark (i.e. Unicode U+FEFF) directly, so that the file just encodes that as UTF-8:

import codecs

file = codecs.open("lol", "w", "utf-8")
file.write(u'\ufeff')
file.close()

(That seems to give the right answer - a file with bytes EF BB BF.)

EDIT: S. Lott's suggestion of using "utf-8-sig" as the encoding is a better one than explicitly writing the BOM yourself, but I'll leave this answer here as it explains what was going wrong before.

Answer 2

Read the following: http://docs.python.org/library/codecs.html#module-encodings.utf_8_sig

Do this

with codecs.open("test_output", "w", "utf-8-sig") as temp:
    temp.write("hi mom\n")
    temp.write(u"This has ♭")

The resulting file is UTF-8 with the expected BOM.

Answer 3

@S-Lott gives the right procedure, but expanding on the Unicode issues, the Python interpreter can provide more insights.

Jon Skeet is right (unusual) about the codecs module - it contains byte strings:

>>> import codecs
>>> codecs.BOM
'\xff\xfe'
>>> codecs.BOM_UTF8
'\xef\xbb\xbf'
>>> 

Picking another nit, the BOM has a standard Unicode name, and it can be entered as:

>>> bom= u"\N{ZERO WIDTH NO-BREAK SPACE}"
>>> bom
u'\ufeff'

It is also accessible via unicodedata:

>>> import unicodedata
>>> unicodedata.lookup('ZERO WIDTH NO-BREAK SPACE')
u'\ufeff'
>>> 

Answer 4

I use the file *nix command to convert a unknown charset file in a utf-8 file

# -*- encoding: utf-8 -*-

# converting a unknown formatting file in utf-8

import codecs
import commands

file_location = "jumper.sub"
file_encoding = commands.getoutput('file -b --mime-encoding %s' % file_location)

file_stream = codecs.open(file_location, 'r', file_encoding)
file_output = codecs.open(file_location+"b", 'w', 'utf-8')

for l in file_stream:
    file_output.write(l)

file_stream.close()
file_output.close()