Reading Unicode file data with BOM chars in Python

Question

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

BOM characters should be automatically stripped when decoding UTF-16, but not UTF-8, unless you explicitly use the utf-8-sig encoding. You could try something like this:

import io
import chardet
import codecs

bytes = min(32, os.path.getsize(filename))
raw = open(filename, 'rb').read(bytes)

if raw.startswith(codecs.BOM_UTF8):
    encoding = 'utf-8-sig'
else:
    result = chardet.detect(raw)
    encoding = result['encoding']

infile = io.open(filename, mode, encoding=encoding)
data = infile.read()
infile.close()

print(data)

I've composed a nifty BOM-based detector based on Chewie's answer. It's sufficient in the common use case where data can be either in a known local encoding or Unicode with BOM (that's what text editors typically produce). More importantly, unlike chardet, it doesn't do any random guessing, so it gives predictable results:

def detect_by_bom(path, default):
    with open(path, 'rb') as f:
        raw = f.read(4)    # will read less if the file is smaller
    # BOM_UTF32_LE's start is equal to BOM_UTF16_LE so need to try the former first
    for enc, boms in \
            ('utf-8-sig', (codecs.BOM_UTF8,)), \
            ('utf-32', (codecs.BOM_UTF32_LE, codecs.BOM_UTF32_BE)), \
            ('utf-16', (codecs.BOM_UTF16_LE, codecs.BOM_UTF16_BE)):
        if any(raw.startswith(bom) for bom in boms):
            return enc
    return default

chardet detects BOM_UTF8 automatically since 2.3.0 version released on Oct 7, 2014:

#!/usr/bin/env python
import chardet # $ pip install chardet

# detect file encoding
with open(filename, 'rb') as file:
    raw = file.read(32) # at most 32 bytes are returned
    encoding = chardet.detect(raw)['encoding']

with open(filename, encoding=encoding) as file:
    text = file.read()
print(text)

Note: chardet may return 'UTF-XXLE', 'UTF-XXBE' encodings that leave the BOM in the text. 'LE', 'BE' should be stripped to avoid it -- though it is easier to detect BOM yourself at this point e.g., as in @ivan_pozdeev's answer.

To avoid UnicodeEncodeError while printing Unicode text to Windows console, see Python, Unicode, and the Windows console.