Write to UTF-8 file in Python

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")?

Solution 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.

Solution 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.

Solution 3:

It is very simple just use this. Not any library needed.

with open('text.txt', 'w', encoding='utf-8') as f:
    f.write(text)