UTF-8 HTML and CSS files with BOM (and how to remove the BOM with Python)

StackOverflow https://stackoverflow.com/questions/2456380

Question

First, some background: I'm developing a web application using Python. All of my (text) files are currently stored in UTF-8 with the BOM. This includes all my HTML templates and CSS files. These resources are stored as binary data (BOM and all) in my DB.

When I retrieve the templates from the DB, I decode them using template.decode('utf-8'). When the HTML arrives in the browser, the BOM is present at the beginning of the HTTP response body. This generates a very interesting error in Chrome:

Extra <html> encountered. Migrating attributes back to the original <html> element and ignoring the tag.

Chrome seems to generate an <html> tag automatically when it sees the BOM and mistakes it for content, making the real <html> tag an error.

So, using Python, what is the best way to remove the BOM from my UTF-8 encoded templates (if it exists -- I can't guarantee this in the future)?

For other text-based files like CSS, will major browsers correctly interpret (or ignore) the BOM? They are being sent as plain binary data without .decode('utf-8').

Note: I am using Python 2.5.

Thanks!

Was it helpful?

Solution

Since you state:

All of my (text) files are currently stored in UTF-8 with the BOM

then use the 'utf-8-sig' codec to decode them:

>>> s = u'Hello, world!'.encode('utf-8-sig')
>>> s
'\xef\xbb\xbfHello, world!'
>>> s.decode('utf-8-sig')
u'Hello, world!'

It automatically removes the expected BOM, and works correctly if the BOM is not present as well.

OTHER TIPS

Check the first character after decoding to see if it's the BOM:

if u.startswith(u'\ufeff'):
  u = u[1:]

The previously-accepted answer is WRONG.

u'\ufffe' is not a character. If you get it in a unicode string somebody has stuffed up mightily.

The BOM (aka ZERO WIDTH NO-BREAK SPACE) is u'\ufeff'

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

Read this (Ctrl-F search for BOM) and this and this (Ctrl-F search for BOM).

Here's a correct and typo/braino-resistant answer:

Decode your input into unicode_str. Then do this:

# If I mistype the following, it's very likely to cause a SyntaxError.
UNICODE_BOM = u'\N{ZERO WIDTH NO-BREAK SPACE}'
if unicode_str and unicode_str[0] == UNICODE_BOM:
    unicode_str = unicode_str[1:]

Bonus: using a named constant gives your readers a bit more of a clue to what is going on than does a collection of seemingly-arbitrary hexoglyphics.

Update Unfortunately there seems to be no suitable named constant in the standard Python library.

Alas, the codecs module provides only "a snare and a delusion":

>>> import pprint, codecs
>>> pprint.pprint([(k, getattr(codecs, k)) for k in dir(codecs) if k.startswith('BOM')])
[('BOM', '\xff\xfe'),   #### aarrgghh!! ####
 ('BOM32_BE', '\xfe\xff'),
 ('BOM32_LE', '\xff\xfe'),
 ('BOM64_BE', '\x00\x00\xfe\xff'),
 ('BOM64_LE', '\xff\xfe\x00\x00'),
 ('BOM_BE', '\xfe\xff'),
 ('BOM_LE', '\xff\xfe'),
 ('BOM_UTF16', '\xff\xfe'),
 ('BOM_UTF16_BE', '\xfe\xff'),
 ('BOM_UTF16_LE', '\xff\xfe'),
 ('BOM_UTF32', '\xff\xfe\x00\x00'),
 ('BOM_UTF32_BE', '\x00\x00\xfe\xff'),
 ('BOM_UTF32_LE', '\xff\xfe\x00\x00'),
 ('BOM_UTF8', '\xef\xbb\xbf')]
>>>

Update 2 If you have not yet decoded your input, and wish to check it for a BOM, you need to check for TWO different BOMs for UTF-16 and at least TWO different BOMs for UTF-32. If there was only one way each, then you wouldn't need a BOM, would you?

Here verbatim unprettified from my own code is my solution to this:

def check_for_bom(s):
    bom_info = (
        ('\xFF\xFE\x00\x00', 4, 'UTF-32LE'),
        ('\x00\x00\xFE\xFF', 4, 'UTF-32BE'),
        ('\xEF\xBB\xBF',     3, 'UTF-8'),
        ('\xFF\xFE',         2, 'UTF-16LE'),
        ('\xFE\xFF',         2, 'UTF-16BE'),
        )
    for sig, siglen, enc in bom_info:
        if s.startswith(sig):
            return enc, siglen
    return None, 0

The input s should be at least the first 4 bytes of your input. It returns the encoding that can be used to decode the post-BOM part of your input, plus the length of the BOM (if any).

If you are paranoid, you could allow for another 2 (non-standard) UTF-32 orderings, but Python doesn't supply an encoding for them and I've never heard of an actual occurrence, so I don't bother.

You can use something similar to remove BOM:

import os, codecs
def remove_bom_from_file(filename, newfilename):
    if os.path.isfile(filename):
        # open file
        f = open(filename,'rb')

        # read first 4 bytes
        header = f.read(4)

        # check if we have BOM...
        bom_len = 0
        encodings = [ ( codecs.BOM_UTF32, 4 ),
            ( codecs.BOM_UTF16, 2 ),
            ( codecs.BOM_UTF8, 3 ) ]

        # ... and remove appropriate number of bytes    
        for h, l in encodings:
            if header.startswith(h):
                bom_len = l
                break
        f.seek(0)
        f.read(bom_len)

        # copy the rest of file
        contents = f.read() 
        nf = open(newfilename)
        nf.write(contents)
        nf.close()
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