Will TextIOWrapper.tell reliably return zero at the beginning of a file?

问题

The docs for TextIOBase.tell, from which TextIOWrapper inherits, state

Return the current stream position as an opaque number. The number does not usually represent a number of bytes in the underlying binary storage.

I am wondering, whether it will always return zero in the specific case that the file pointer is at the beginning of the file?

I wrote a small, non-exhaustive, script to check some obvious scenarios, which returned zero for each case:

import io
import os


rmodes = ['a', 'a+', 'r', 'r+']
wmodes = ['w', 'w+', 'a', 'a+', 'x', 'x+']


for m in wmodes:
    with open('empty.txt', m) as f:
        print(m, f.tell())

    os.system('rm empty.txt')


for m in rmodes:
    open('not-empty.txt', 'w').write('abc\n')

    with open('non-empty.txt', m) as f:
        f.seek(0, io.SEEK_SET)
        print(m, f.tell())

    if m == 'r':continue    # avoid exception

    with open('non-empty.txt', m) as f:
        f.truncate()
        print(m, f.tell())

I believe this code is used to generate the return value of tell

static PyObject *
textiowrapper_build_cookie(cookie_type *cookie)
{
    unsigned char buffer[COOKIE_BUF_LEN];

    memcpy(buffer + OFF_START_POS, &cookie->start_pos, sizeof(cookie->start_pos));
    memcpy(buffer + OFF_DEC_FLAGS, &cookie->dec_flags, sizeof(cookie->dec_flags));
    memcpy(buffer + OFF_BYTES_TO_FEED, &cookie->bytes_to_feed, sizeof(cookie->bytes_to_feed));
    memcpy(buffer + OFF_CHARS_TO_SKIP, &cookie->chars_to_skip, sizeof(cookie->chars_to_skip));
    memcpy(buffer + OFF_NEED_EOF, &cookie->need_eof, sizeof(cookie->need_eof));

    return _PyLong_FromByteArray(buffer, sizeof(buffer),
                                 PY_LITTLE_ENDIAN, 0);
}

Clearly it computes more than just the offset of the file pointer from the beginning of the file. But will this code always return zero at the beginning of a file? What might be the circumstances in which it would not?

I am

aware that the behaviour is implementation-dependent and not guaranteed by the docs
asking specifically about the behaviour in text files, not binary
not asking "how do I know if I am at the beginning of a file in Python?"
not asking for alternative means of obtaining the position of the file pointer

来源：https://stackoverflow.com/questions/61317588/will-textiowrapper-tell-reliably-return-zero-at-the-beginning-of-a-file

标签

python

file-io

cpython

python-internals