Remove very last character in file

Accepted answer of Martijn is simple and kind of works, but does not account for text files with:

• UTF-8 encoding containing non-English characters (which is the default encoding for text files in Python 3)
• one newline character at the end of the file (which is the default in Linux editors like vim or gedit)

If the text file contains non-English characters, neither of the answers provided so far would work.

What follows is an example, that solves both problems, which also allows removing more than one character from the end of the file:

import os


def truncate_utf8_chars(filename, count, ignore_newlines=True):
    """
    Truncates last `count` characters of a text file encoded in UTF-8.
    :param filename: The path to the text file to read
    :param count: Number of UTF-8 characters to remove from the end of the file
    :param ignore_newlines: Set to true, if the newline character at the end of the file should be ignored
    """
    with open(filename, 'rb+') as f:
        last_char = None

        size = os.fstat(f.fileno()).st_size

        offset = 1
        chars = 0
        while offset <= size:
            f.seek(-offset, os.SEEK_END)
            b = ord(f.read(1))

            if ignore_newlines:
                if b == 0x0D or b == 0x0A:
                    offset += 1
                    continue

            if b & 0b10000000 == 0 or b & 0b11000000 == 0b11000000:
                # This is the first byte of a UTF8 character
                chars += 1
                if chars == count:
                    # When `count` number of characters have been found, move current position back
                    # with one byte (to include the byte just checked) and truncate the file
                    f.seek(-1, os.SEEK_CUR)
                    f.truncate()
                    return
            offset += 1

How it works:

• Reads only the last few bytes of a UTF-8 encoded text file in binary mode
• Iterates the bytes backwards, looking for the start of a UTF-8 character
• Once a character (different from a newline) is found, return that as the last character in the text file

Sample text file - bg.txt:

Здравей свят

How to use:

filename = 'bg.txt'
print('Before truncate:', open(filename).read())
truncate_utf8_chars(filename, 1)
print('After truncate:', open(filename).read())

Outputs:

Before truncate: Здравей свят
After truncate: Здравей свя

This works with both UTF-8 and ASCII encoded files.

Use fileobject.seek() to seek 1 position from the end, then use file.truncate() to remove the remainder of the file:

import os

with open(filename, 'rb+') as filehandle:
    filehandle.seek(-1, os.SEEK_END)
    filehandle.truncate()

This works fine for single-byte encodings. If you have a multi-byte encoding (such as UTF-16 or UTF-32) you need to seek back enough bytes from the end to account for a single codepoint.

For variable-byte encodings, it depends on the codec if you can use this technique at all. For UTF-8, you need to find the first byte (from the end) where bytevalue & 0xC0 != 0x80 is true, and truncate from that point on. That ensures you don't truncate in the middle of a multi-byte UTF-8 codepoint:

with open(filename, 'rb+') as filehandle:
    # move to end, then scan forward until a non-continuation byte is found
    filehandle.seek(-1, os.SEEK_END)
    while filehandle.read(1) & 0xC0 == 0x80:
        # we just read 1 byte, which moved the file position forward,
        # skip back 2 bytes to move to the byte before the current.
        filehandle.seek(-2, os.SEEK_CUR)

    # last read byte is our truncation point, move back to it.
    filehandle.seek(-1, os.SEEK_CUR)
    filehandle.truncate()

Note that UTF-8 is a superset of ASCII, so the above works for ASCII-encoded files too.