Bug report

I've noticed unexpectedly poor performance of calling _io.BufferedReader.tell() in a Python script I've been working on, when compared to calling _io.BytesIO.tell(). From looking at the source in https://github.com/python/cpython/blob/main/Modules/_io/bufferedio.c it looks to me like _io.BufferedReader.tell() may be slow because it ignores the cached stream position, instead re-getting the position from the raw stream on every call:

cpython/Modules/_io/bufferedio.c/#_io__Buffered_tell_impl appears to always get the position by calling _buffered_raw_tell(). The implementation notes specify that the absolute position of the raw stream is cached if possible, so I'm suspecting that _io__Buffered_tell_impl should be using the RAW_TELL macro instead of _buffered_raw_tell so that it uses the cached value when available.

It may be that the cached value cannot be relied upon in _io__Buffered_tell_impl, and that calling _buffered_raw_tell() is necessary here, but I couldn't tell if that was the case from a quick look at the source and git blame, so I have made this bug report.

The script below does some simple timing of the _io.BufferedReader.tell() and _io.BytesIO.tell() functions.
Example output from the script:

BufferedReader.tell() time:
        0.5720981000049505s
BytesIO.tell() time:
        0.050327699980698526s

I would have expected both functions to be within the same order of magnitude.

import timeit
from io import BytesIO
import _io
import tempfile
import os

timing_count = 1_000_000

print()
print()

temp_file_path = None
try:
    handle, temp_file_path = tempfile.mkstemp()
    # Use `open` so we get an _io.BufferedReader
    with open(handle, 'rb') as f:
        assert isinstance(f, _io.BufferedReader)
        tell = f.tell
        t = timeit.timeit("tell()", globals=globals(), number=timing_count)
        # About 0.6s on my system.
        print(f"BufferedReader.tell() time:\n\t{t}s")
finally:
    # Remove the temporary file
    if temp_file_path is not None:
        os.remove(temp_file_path)

# I would expect _io_BufferedReader.tell() to have similar performance to BytesIO.tell(),
# but it takes about 10 times the duration compared to BytesIO.tell().
with BytesIO(b"") as f:
    assert isinstance(f, _io.BytesIO)
    tell = f.tell
    t = timeit.timeit("tell()", globals=globals(), number=timing_count)
    # About 0.05s on my system.
    print(f"BytesIO.tell() time:\n\t{t}s")

Your environment

I am working on an addon script for Blender 4.0 alpha with its bundled Python version, which reports the Python version as 3.10.11 (main, May 9 2023, 12:29:22) [MSC v.1928 64 bit (AMD64)]

• CPython versions tested on: 3.10.11
• Operating system and architecture: Windows-10-10.0.19045-SP0