Convert Utf-8 with Bom to Utf-8 with No Bom in Python

Convert UTF-8 with BOM to UTF-8 with no BOM in Python

Simply use the "utf-8-sig" codec:

fp = open("file.txt")
s = fp.read()
u = s.decode("utf-8-sig")

That gives you a unicode string without the BOM. You can then use

s = u.encode("utf-8")

to get a normal UTF-8 encoded string back in s. If your files are big, then you should avoid reading them all into memory. The BOM is simply three bytes at the beginning of the file, so you can use this code to strip them out of the file:

import os, sys, codecs

BUFSIZE = 4096
BOMLEN = len(codecs.BOM_UTF8)

path = sys.argv[1]
with open(path, "r+b") as fp:
    chunk = fp.read(BUFSIZE)
    if chunk.startswith(codecs.BOM_UTF8):
        i = 0
        chunk = chunk[BOMLEN:]
        while chunk:
            fp.seek(i)
            fp.write(chunk)
            i += len(chunk)
            fp.seek(BOMLEN, os.SEEK_CUR)
            chunk = fp.read(BUFSIZE)
        fp.seek(-BOMLEN, os.SEEK_CUR)
        fp.truncate()

It opens the file, reads a chunk, and writes it out to the file 3 bytes earlier than where it read it. The file is rewritten in-place. As easier solution is to write the shorter file to a new file like newtover's answer. That would be simpler, but use twice the disk space for a short period.

As for guessing the encoding, then you can just loop through the encoding from most to least specific:

def decode(s):
    for encoding in "utf-8-sig", "utf-16":
        try:
            return s.decode(encoding)
        except UnicodeDecodeError:
            continue
    return s.decode("latin-1") # will always work

An UTF-16 encoded file wont decode as UTF-8, so we try with UTF-8 first. If that fails, then we try with UTF-16. Finally, we use Latin-1 — this will always work since all 256 bytes are legal values in Latin-1. You may want to return None instead in this case since it's really a fallback and your code might want to handle this more carefully (if it can).

Is utf-8-sig suitable for decoding both UTF-8 and UTF-8 BOM?

The utf-8-sig codec will decode both utf-8-sig-encoded text and text encoded with the standard utf-8 encoding

>>> s = 'Straße'
>>> utf8_sig = s.encode('utf-8-sig')
>>> utf8 = s.encode('utf')
>>> print(utf8_sig.decode('utf-8-sig'))
Straße
>>> print(utf8.decode('utf-8-sig'))
Straße

From the codecs docs:

Before any of the Unicode characters is written to the file, a UTF-8 encoded BOM (which looks like this as a byte sequence: 0xef, 0xbb, 0xbf) is written ... On decoding utf-8-sig will skip those three bytes if they appear as the first three bytes in the file.

The utf-8-sig encoding in most common in Windows environments. If you're sharing files with users on mac or *nix systems, the standard utf-8 encoding is what they would expect to receive.

Convert UTF-16 to UTF-8 and remove BOM?

Just use str.decode and str.encode:

with open(ff_name, 'rb') as source_file:
  with open(target_file_name, 'w+b') as dest_file:
    contents = source_file.read()
    dest_file.write(contents.decode('utf-16').encode('utf-8'))

str.decode will get rid of the BOM for you (and deduce the endianness).

Reading Unicode file data with BOM chars in Python

There is no reason to check if a BOM exists or not, utf-8-sig manages that for you and behaves exactly as utf-8 if the BOM does not exist:

# Standard UTF-8 without BOM
>>> b'hello'.decode('utf-8')
'hello'
>>> b'hello'.decode('utf-8-sig')
'hello'

# BOM encoded UTF-8
>>> b'\xef\xbb\xbfhello'.decode('utf-8')
'\ufeffhello'
>>> b'\xef\xbb\xbfhello'.decode('utf-8-sig')
'hello'

In the example above, you can see utf-8-sig correctly decodes the given string regardless of the existence of BOM. If you think there is even a small chance that a BOM character might exist in the files you are reading, just use utf-8-sig and not worry about it

How to generate XML, UTF-8 with BOM using Python Element Tree?

Peek into sources of ElementTree.write shows that prolog is hardcoded there (https://github.com/python/cpython/blob/main/Lib/xml/etree/ElementTree.py or permalink https://github.com/python/cpython/blob/ee0ac328d38a86f7907598c94cb88a97635b32f8/Lib/xml/etree/ElementTree.py). Therefore probably using internals of ET is the only option (other than monkey-pathing module), to write required preamble and keep BOM in the file:

import xml.etree.ElementTree as ET
qnames, namespaces = ET._namespaces(tree._root, None)
with open(lang_resx_fpath,'w',encoding='utf-8-sig') as f:
    f.write("<?xml version='1.0' encoding='utf-8'?>\n"     )
    ET._serialize_xml(f.write,
                        tree._root, qnames, namespaces,
                       short_empty_elements=False)

Probably it is not more elegant than your solution (and maybe it is even less elegant). The only advantage is that it does not require writing file twice, which would be minor benefit besides some huge XML files.

How am I suppposed to handle the BOM while text processing using sys.stdin in Python 3?

As a complement to the existing answer, it is possible to filter the UTF8 BOM from stdin with the codecs module. Simply you must use sys.stdin.buffer to access the underlying byte stream and decode it with a StreamReader

import sys
import codecs

# trick to process sys.stdin with a custom encoding
fin = codecs.getreader('utf_8_sig')(sys.stdin.buffer, errors='replace')

if '-i' in sys.argv: # For command line option "-i <infile>"
    fin = open(sys.argv[sys.argv.index('-i') + 1], 'rt',
               encoding='utf_8_sig', errors='replace')

for line in fin:
    ...Processing here...

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